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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorarthur
    • CommentTimeAug 8th 2008
    I've read the various threads on solar hot water with a combi boiler and am still wondering about it. Our 1.5 bed flat has an old combi boiler that could do with replacing and a roof (with a nice south facing section) that is also reaching the end of its life which made me think it might be worth adding solar. We also have an old tank in the loft which could presumably be reconnected. However, we're used to using a combi and although I know some of you think them very inefficient, I think using it sensibly as we do, its appropriate at the moment. We avoid turning on hot taps for small amounts of hot water and try to wash-up before or after showering to avoid heating up too often (we also collect the water we run while waiting for it to heat in a can for the garden).

    As I understand it, a combi boiler is possible with solar if:
    1) its a model capable of modulating to deal with the incoming hot feed
    2) Pre-stored water is kept below 25 or above 50 degrees or subsequently heated to 60 degrees to prevent legionairres risk

    But, since the solar heat needs to be stored anyway, is there any point in a combi system - I'll still need a tank?
    Presumably I coul still have mains pressure hot water with a solar and conventional boiler system?

    Currently we use around 5-6kWh a day of gas for hot water (with an old, inefficient boiler with a permanent pilot light which seems to use a surprising amount of energy). I wouldn't want to install solar and discover we're using more or the same due to the need to heat a whole tank up to 60degrees or something when the sun isn't shining. There's also electricity consumption. Would a combi boiler's electrical consumption be more/less/the same as the pumps needed for a solar storage system (I've posted elsewhere about how variable the electrical consumption of boilers seems to be).
    • CommentAuthortony
    • CommentTimeAug 8th 2008
    Is the roof yours to put a solar panel on? Yes you can have mains hot water and yes you can still have a combi.
    • CommentAuthorarthur
    • CommentTimeAug 8th 2008
    I'm thinking we could maybe buy the roof from other freeholders in exchange for renovating it.

    Would there be any point in having a combi with solar though? If its for pre-heating how do you avoid storing water below 50 degrees in the water? Or would you run the mains water through a heat exchanger in a solar storage tank which is part of a separate circulation?
    Some people really like combis and solar. Like you I cannot really see the point and would not recommend it. Thousands of people on mainland Europe do it, though.
    • CommentAuthortony
    • CommentTimeAug 8th 2008
    Think of the solar as a pre-heat system -- sometimes it will not need further heating rather thermally mixing to a safe usable temperature other times needing hotting up a touch.

    The less you heat it the more you save.
    • CommentAuthorarthur
    • CommentTimeAug 17th 2008
    But even if pre-heating you'd still need to store it wouldn't you? Solar wouldn't be able to preheat it instantly would it?
    There's some interesting diagrams involving this on Green Spec:




    You can also get Weather compensators that may (although I'm not sure) turn on different aspects of your system depending on the weather available. For instance, when there's less sunlight available, they can cause the boiler to kick in to maintain the 50 C needed way before the temp dips...........
    • CommentAuthorJackyR
    • CommentTimeAug 22nd 2008
    I've this week had an Alpha combi and solar panel system fitted, and I'm spitting feathers. I only discovered during installation that storing warm water is a legionnaire's risk, and the water has to be heated further to pasteurize it.

    So for the last few days, intermittent sun has provided me with a tank-full of beautiful 48 degree water (42 is enough to bathe, 55 to wash up), which the combi then heats through an additional 22 degrees to reach 70. Given mains is about 15 degrees at this time of year, a combi-only system would only have to heat the water through 30 degrees to be perfectly comfortable.

    The tank temp at 11.30 this morning was 30 degrees: 40 degree rise required with the solar system; only 30 degrees with combi-only. And that's before counting electricity usage by the pump, or the financial and environmental cost of the system.

    If I were doing it again, I'd use a non-combi boiler, set the storage tank at 45-50 degrees and simply pasteurize the tank at 60 degrees once a week (apparently some do it automatically, and heating from below is more reliable than from the side to really hit those legionnella farms at the bottom of the tank). That, or get a system with a second heat exchanger, so the mains water remains separate from the stored solar water.

    If anyone can reassure me that I've somewhere got these calculations wrong, I'd be delighted. I'm sick as a parrot at the moment.
    • CommentAuthorarthur
    • CommentTimeAug 22nd 2008
    Sorry to hear of your disapointment Jacky. This was the kind of thing I was wondering about.
    However, don't most people have their system boilers set to 60 degrees or more anyway to avoid legionairres. I'd not heard about raising the temperature weekly and didn't realise some systems could do this automatically.

    The Atmos system sends the mains combi water through a heat exchanger in the solar preheat tank so isn't contaminated by the lukewarm water. This seems a better arrangment to me. I would have thought that this kind of combi set-up would in fact be more efficient than a system-boiler solar-arrangment in certain situations (small flats with little hot water use) due to not needing to heat a whole tank to 60 degrees at any stage.

    If its any reassurance, The Yellow House website talks about tank temperature and is a bit sceptical about the legionnaire risk:
    • CommentAuthorarthur
    • CommentTimeAug 22nd 2008
    Compare how the Alpha solar works to the Atmos system:
    What would the disadvantages of the atmos system be?
    I have just started a new thread, (About all year round solar) that indirectly is dealing with the risk of legionella (and/or any other bacteria) - the problem with these solar systems seems to be this problem of being essentially "underpowered/useless" in low light situations.
    Besides the point is solar that dont work all year round will be useless once Russia cuts the gas,...
    Personally I would rather have a cold shower in summer than winter haha!
    • CommentAuthorJackyR
    • CommentTimeAug 22nd 2008
    From that info, Arthur, the Atmos would be vastly better.

    Advantages of Atmos
    1) The key point is the diverter kicks in at 45 degrees (Alpha: 60 degrees), so you get to use your solar-heated water for only the pump cost.
    2) The Atmos stores water up to 85 degrees (Alpha: 70 degrees), so you can store more energy.

    Disadvantages of Atmos
    1) The legionella-free claim is not absolutely true, if I understand aright, as it breaks the "keep cold water cold, keep hot water hot" rule. But water sitting at 20 to 40 degrees in the mains pipes within the tank will not be there for long (unless you go away on hol, in which case advice is to run taps before showering on return), so the risk is that much smaller.
    2) Do you want tap water at more than 45 degrees on demand?
    3) Are you happy with the flow rates they describe? Don't know what effects, if any, arise from heating from a lower temperature source. You could get a plumber's flow cup with thermometer on eBay, to measure your current system (or someone else's!) and decide what you're really looking for.

    I have no idea about the robustness and serviceability of Atmos kit. My heating engineer seems pretty good, so I'll ask him next week. (OK, so he was the one who suggested the Alpha, but it was his first Alpha installation and he doesn't yet know my figures...)

    I'm still livid about the whole thing, but perhaps you can get something useful from my disaster and I'll be Saving the World by proxy :-/

    Re legionella-scepticism: my tank looks likely to spend quite a lot of it's time in the 20 - 40 degree ideal legionnella range. Since dust and colds reduce my seriously asthmatic Other Half to a pitiful state ("Next time you're that bad, stop dithering and dial 999", said the doctor irately), so although we might've risked the Atmos, I don't think we'll be turning down the Alpha... Oh well.
    • CommentAuthorJackyR
    • CommentTimeAug 23rd 2008
    OK, here's an excellent document on legionella, with specific reference to solar thermal systems. It originates in the Netherlands, where there seems to be a lot of research/installation. In fact, Atmos solar kit comes from the Netherlands.
    • CommentAuthorJackyR
    • CommentTimeAug 23rd 2008
    And it would help if I included the link :-)
    • CommentAuthorJackyR
    • CommentTimeAug 23rd 2008
    And it includes an analysis of the Atmos MonoSolar and MultiSolar systems, which are made by Daalderop.
    • CommentAuthorarthur
    • CommentTimeAug 24th 2008
    Thanks for that document Jacky. Its interesting that it seems to say that even setting the combi to 60degrees is not good enough with solar preheating: "In instanteous after-heaters, set at 60C, the residence time is too short for the bacteria to be killed...". That suggests that you ought to have your combi set to 65 or 70 degrees!

    I'm also puzzled that the Atmos pdf says it also heats all water to 60 degrees if the solar water is less than 45 degrees. This would seem unnecessary due to the use of the heat exchanger for indirect heating. As they say: "No risk of Legionella as stored water transfers heat through copper pipe heat exchanger". I would have thought the small quantity of water stored in the heat exchanger would be no more risk than from the warm water left in the pipes or exchanger with a normal combi system and therefore heating to 60degrees wouldn't be necessary.
    • CommentAuthorJackyR
    • CommentTimeAug 26th 2008
    Yeah, I'm on the trail, asking Atmos about that and another contradiction in their literature (*now* I put in all the research...).

    Mains water left in the MonoSolar heat exchanger would stay at a nice cuddly solar water temp until it is drawn off, whereas that in a combi would start to cool as soon as the flames die. But according to p36 of the doc above, keeping water at 25 to 45 degrees for less than 2 days is considered neutral legionella risk. You can also see the massively increased risk after 1 week.

    NB The Atmos pdf states that hot solar water input => increased flow rates with their recommended MonoSolar combi, NOT modulated gas burn (as with my Alpha, which today I watched clicking on and off as my Other Half showered). It could mean a certain amount of tap-jockeying for the showerer, according to the day's weather, and would be bounded by the minimum flow for triggering the combi.

    Will report back any response from Atmos.
    • CommentAuthorwookey
    • CommentTimeAug 26th 2008
    Jacky - I think you are obsessing rather about legionnella risk. With a UK mains water supply the risk is negligible unless you are particularly infirm. You have to combine an a lot of factors to get infected: Initial infection in the water (with a pathogenic strain), suitable temperature, sufficietly long storage period in undisturbed water, something for the bugs to eat (they don't live on water alone), delivery in an aerosol (water itself won't do you any harm), breathing in said aerosol (and it getting into your lungs which normally won't happen unless you are a smoker or otherwise have lung disease) and finally actually succumbing to the infection (about 5% of people).

    Obvisouly if you are running a nursing home or hospital then you need to be careful (and have complicated plumbing to be careful with) but in a household of normal people you can pretty-much forget about it - you have much more serious risks to worry about (like falling down the stairs, burning the house down, and driving to work). About 15 people a year die of legionaires in the UK (and about 1,000,000 of other causes).

    It is something to consider, but try to keep it in perspective. There is a lot of emphasis of the risks by people selling expensive plumbing systems, because scaring people is always a good way to sell them stuff :-) The thermal store people are right that a thermal store reduces the risk significantly compared to a hot-water tank, but the point is that the risk wasn't very high in the first place. The energy use of systems is a much more significant consideration IMHO. As you say the task26 document is a thorough treatment (task 26 have good documents on everything to do with solar thermal systems).

    Now, on to the much more interesting subject of combining solar with Combi boilers.

    As arthur originally pointed out, because a solar system always involves storage (due to low/intermittent input power) it is easier to combine it with a system or open vent boiler than with a combi. Note that one approach to this is simply to ignore the 'combi' part of your boiler and treat it like a system boiler. This is very simple to do and is often the cheapest approach to the issue (keeps your existing boiler, simple plumbing/control).

    It is difficult to compare the efficiencies of a combi-based solar system with a conventional tank+boiler system. I have spent some time doing this with inconclusive results it's basically a tradeoff between the inefficiencies of the combi's stop-start operation against the losses of standing heat storage. However the short version is that it depends on your hot water usage profile: the lower the hot water use use the more likely it is that a combi-based system will need less energy to run, although if there are a lot of very short water deliveries then that pushes things in favour of the tank again. Despite combis being generally dis-recommended for use

    As to methods of doing it there are various solutions. The Atmos system (very well designed, but an expensive store for the volume) has already been mentioned. There are other suppliers who provide combis that will take warm/hot water as input - get one of these if considering adding solar. If you ask suppliers about max input temp you have to phrase your question carefully so as to avoid getting an unhelpful legionnalla-paranoia-inspired answer of '25C'. Pop over to the Navitron forum for lists of combi boiler suited to use with solar.

    The cheapest option is simply to have a pair of gate valves to switch between summer (water from solar tank) and winter (water from Combi) modes: http://mark-ju.net/solar_water/index.htm it is difficult to automatically switch between these modes because of the pressure differential between the mains combi and low-pressure unvented hot water tank.

    A recent approach which has the benefit of simplicity is the Grant combisol - a valve that passes water straight through if above 60C, but otherwise mixres it down (with cold) to 25C and passes it through to the Combi. That is GBP 120+VAT and works with any combi.

    As disussed above there is a choice of using a conventional vented hot water tank, an unvented hot water tank, or a thermal store/heat bank. All are possible but the pressure-difference makes the unvented option tricky unless you use the low-tech option above.

    Jacky- I feel you are being a little harsh on your solar/combi system, and your sums are missing one point about mixing. Yes, most combis are very stupid and will heat the water approx +20C whatever temp you stick it in at, but if you are filling a bath, having a shower or filling a washing-up bowl then you will be mixing with cold water to the desired temp so you will use a lower volume of hotter water, thus causing exactly the same amount of energy to be used in heating your water. Your complaint is that there are times when it could be 48C all-solar water but in fact will be 70C 1/3rd gas-heated water. However you are doing much better than if the water was entirely heated by gas using the combi to 48C. If you want to maximise the percentage of that energy which comes from solar then adding an electronic interlock so that the combi does not fire if the tank temp is over, say, 50C would be one way to get it working more like how you feel it ought to work. Another would be to add a combi-bypass valve like the atmos (actually a conventional mixing valve plumbed in backwards with the check-valves removed) so the combi is entirely bypassed.

    I thought about this for a long time before picking a system and eventually decided against a combi, although it was a close thing. I intend to use a thermal store system eventually, but so for only have the boiler and solar panels installed. This set-up is using the existing (very old) hot water tank with a good thick layer of scale and sediment, and it has regularly spent a week or three in the 20-45C range this year. We make no particular effort to get it over 60, just wait for the sun to do it. No-one has died yet and we use the water for cooking as well as washing.

    And finally, if you want to look at the way things are going in the future, read the 'rebus' report on solar hot water systems. That is a Danish Phd-designed system which makes really good use of a system boiler (used in 'semi combi' mode) and stratified solar storage controlled with electronics to provide hot water and heating in an efficient and versatile way. It's pretty radical for plumbing: http://www.physics.uio.no/energy/rebus/downloads/REBUS_final_01-2007.pdf
    • CommentAuthorJackyR
    • CommentTimeAug 27th 2008
    Thanks wookey. Actually, my other half _does_ have chronic, life-threatening lung disease and loves his aerosol-generating shower first thing. The figures aren't exactly high (though of course they'd go up if everyone overrode installed prevention measures!), but in our case worth making an informed decision on. (Actually, I really *am* more likely to die falling down stairs, but that's another story.)

    All of which is, sadly, irrelevant, as the real obstacle is that the plumber hard-wiring the system feels he has building regs to meet.

    Back to combis.

    I have a hot-input, modulating combi, which is as intelligent as one could reasonably hope. The system does indeed have a 60 degree diverter to the taps. I'm contemplating various work-arounds that will still keep the plumber happy.

    But you seem to think my gas-usage fears are unfounded with the system as it stands: if you can convince me, I'll be so pleased...

    Your arguments:
    <blockquote>you will use a lower volume of hotter water, thus causing exactly the same amount of energy to be used in heating your water</blockquote>
    Yes, modulo heat-loss, but surely the whole point is: better that energy be solar than gas? We don't come close to using the whole 90l tank, so I have a higher volume of cooler (48C) water FOR FREE; vice versa requires additional energy input.

    <blockquote>in fact will be 70C 1/3rd gas-heated water. However you are doing much better than if the water was entirely heated by gas using the combi to 48C</blockquote>
    • 1/3? My mains T is 13 degrees, not 0.
    • Heating water entirely by gas from 13 to 45 degrees (as I would with combi-only), delta T is 32 degrees;
    heating water by solar plus gas from 13 to 38 to 70 degrees, delta T for the gas phase is also 32 degrees.
    Energy required to raise the same vol of water the same number of degrees is theoretically equal, regardless where in the scale this happens, no? Tho in practice 38 to 70 degrees would take more energy (heat loss, temp gradients, etc).

    Obviously, with a lower mains T in winter, or a different T in the solar tank, the whole gas-consumption changes. But my Other Half showers in the morning, and this week the tank has been at less than the even-stevens T of 38 degrees for three mornings. So if I'm right, it would be more efficient to turn off the solar feed to the combi, turn it right down, and give it mains water on those occasions.

    As I say, I'd be delighted if you can come up with a hole in this reasoning - which I don't think you have yet, but maybe I just haven't understood. Meanwhile the damn panel is looking at best like a greyish-white elephant and at worst like a liability.

    Sorry Arthur, I seem to have hijacked your thread...
    • CommentAuthorarthur
    • CommentTimeAug 27th 2008
    <blockquote><cite>Posted By: wookey</cite>Jacky - but if you are filling a bath, having a shower or filling a washing-up bowl then you will be mixing with cold water to the desired temp so you will use a lower volume of hotter water, thus causing exactly the same amount of energy to be used in heating your water. </blockquote>

    Is that correct? I imagined there to be diminishing returns or something at higher temperatures making it less efficient to heat above target temperature and then cool with cold. But I don't know anything about that really.

    I also don't understand your point about pressure difference "between these modes because of the pressure differential between the mains combi and low-pressure unvented hot water tank." I thought an unvented system was high pressure? Or am I confused?

    Also, do you know if the Grant combisol can be set at a lower temperature than 60degrees - more like 45? Its a bit frustrating if solar heated water at 59degrees is mixed with cold to 25 degrees and then reheated by the combi to tap temperature.

    Thanks for your useful comments and perspective on legionnaires.

    For our style of usage I feel fairly sure that a combi system must be more efficient unless the legionairres issue means it has to be heated to a higher temperature (60 or 65) than with a storage system (50).

    The solar storage tank in the Atmos system is effectively a thermal store and seems a good solution to me but as mentioned it would be nice to know if it also insists on heating to 60 degrees.
    • CommentAuthorarthur
    • CommentTimeAug 27th 2008
    This diagram of the atmos system indicates that the to 50 degrees (see the notes underneath) which isn't too bad a compromise I suppose.
    • CommentAuthorwookey
    • CommentTimeAug 27th 2008
    Jacky. Yes my 1/3rd was approximate. Let's use your worked example. Does the combi really heat 38->70 not 38->60? Do you not have a knob on the front to turn it down a bit? But using that example, yes the two temp raises use the same amount of gas/energy (modulo pipe losses), but the thing you are missing is that the shower temp is fixed at 42C. So you will need a much lower volume of 70degree water than 45 degree water for a given output of 42C water.

    This is straightforward in the case of baths and basins (which mix to a volume), but for showers it is not so simple and will depend on the plumbing. In practice you may just get a shower at the same temp and nearly twice the flowrate with the 70C water, rather than a shower at the same flowrate using less hot water - I'm not at all sure how the dynamics of the various parts work out in your case. I suggest you measure it with a bucket and see. (just turn off the combi to get the straight-through measurement).

    But you should be able to see that the break-even point is not 38C. It's higher. It's too early to do the sums right but gut feel says around 52C?

    Arthur: Yes that really is correct. Raising the temp of a lump of water 1C uses a constant amount of energy - it's doesn't matter how hot it already is. The only real-life thing that makes higher temps worse is that it increases the temp diff between pipe and air or tank and outside, so losses there increase in proportion. i.e 70C water will ose energy at 5/3rds the rate of 50C water when the surrounding air temp is 20C. So for the same losses if runing at the higher temp better insulation is needed.

    Yes, sorry that should have been 'vented' not 'unvented' - well spotted - you are reading all this stuff then :-)

    I don't know any details about the combisol, no. Ask grant. I bet they restrict it to 60C for legionella-paranoia. (And to sell in some other EU contries there is a legal limit of either 55 or 60 so it's probably easier for them to stick it at 60 and leave it). As this thread makes clear - people worrying about legionella is going to be the cause of a great deal of energy inefficiency.
    • CommentAuthorarthur
    • CommentTimeAug 27th 2008
    Thanks Wookey, yes we're reading avidly!
    Looking through the Navitron forum I noticed you estimated (insulated) tank losses at 2kWh/24 hours and losses from boiler startup (more often with a combi) at around 0.4kWh each firing which is very interesting. Did your further research confirm or modify these estimates?

    I also noticed someone raising the problem that a diverter valve in a combi set up (such as the Atmos system) will not stop the combi initially firing (because the water in the valve will have cooled) before the hot water from the solar tank comes through and the combi realises. This is unfortunate. I can't see that the Atmos system has a solution to it. Suggested solution on the forum was turning off the combi in summer (no central heating needed) and disconnecting the flow in winter when you believe the solar water is hot enough but want central heating (don't know how easy this would be). Do you have this problem with your Alpha Jacky when your solar water is above 60degrees.
    • CommentAuthorwookey
    • CommentTimeAug 27th 2008
    Arthur - you have been dilgent in finding such relevant posts from some while back :-)

    Yes the 2kWh/24 hrs for tanks is typical for reasonably-insulated 300l, but there is quite a range (factor of 4?) depending on design/insulation/number of connections/pipedetailing. No, I never got any better numbers for Combi start/stop losses.

    Best way to fix the combi false-start problem is an electrical interlock. A tank sensor which inhibits the combi from firing if it is above some temp (e.g. 45C). The only problem here is that your combi may not necessarily make this easy by providing a suitable input.
    • CommentAuthorJackyR
    • CommentTimeSep 1st 2008 edited
    Arthur, yes my Alpha does indeed fire initially, even when the solar water is hot. However, Tests Have Shown it's possible to trickle away the cold in our 50cm of feed pipe without triggering the combi - delightfully turning one of my combi pet hates (non-firing at low flow rates) into an actual benefit!

    The 90l insulated tank, in a room at about 20 degrees, dropped from 64 to 45 degrees over 10pm to 10am, which is about 4kWh/24 hrs (no?). Pipes are yet to be insulated, but that's basically the figure this system yields.

    Also, while the sun shines, water seems to be pumped round the solar panel much faster when someone showers. So the system does seem fairly intelligent.

    Wookey! Aha! I get what you're saying! Putting water through a rise of 32 degrees but to a higher output temp will mean only half-a-bath of gas-heated water (plus half-a-bath of cold), instead of a full bath of gas-heated water. So my Cassandra-like figures are wrong! Although sadly I still can't fully use a 45 degree tank without further tinkering. Will sit down when I have enough brain cells and do the sums, as well as more experiments with combi setting, output temp, etc. But thank you!

    Getting back to Arthur's original dilemma, there was a comment on another thread that almost all solutions proposed there were optimal for _some_ situation - although often not for the proposer's own! I think this may be a similar case, and with such low water usage, edge effects may well outweigh the intended effects.

    • There are the stop-start losses with the combi.
    • In practice heat loss _will_ be greater if the water is at a higher temp, not just from pipes and tank but from the boiler itself (heat transfer to hotter water will be less efficient than to colder).
    • A household which uses more hot water than its solar tank capacity each day, will be optimised differently from one which uses a fraction of the tank.
    • Is an intelligent, integrated system noticeably more efficient than a Heath-Robinson one? For what circumstances?

    Still a way to go, but I'm beginning to feel the Alpha is less of a lemon than at first appeared...
    • CommentAuthorarthur
    • CommentTimeSep 15th 2008
    I look forward to seeing it JackyR.
    • CommentAuthorJackyR
    • CommentTimeOct 5th 2008 edited
    OK, numbers.
    Refs to Alpha brochure are to http://www.alpha-innovation.co.uk/products/downloads/solarsmart/alpha_SolarSmart_booklet_brochure.pdf
    Refs to Atmos brochure are to http://www.atmos.uk.com/core_files/productDoc(137).pdf
    Refs to Peter Emblin are emails from the sales manager at Atmos, who has been very helpful.
    NB I have no connection with either of these companies other than as a (potential) customer.

    My Alpha SolarSmart
    • Min temp to access solar water: approx 50 degrees? Lowest temp observed on tank with diverter valve operating was 52 degrees. When tank showing 49 degrees, diverter valve does not operate and combi comes on. (my obs)

    • Max storage temp approx 70 degrees (brochure)

    • Combi output seems to aim for 60 degrees (not 70 as in brochure, so not much use for Legionnaire's control). When combi dial is at dot before Mark 1 (scale 1-9), still triggers so that input 19 degrees -> output 35 degrees. At higher input temps, on all dial settings, combi modulates to keep output temp ≤ 60-ish. (my obs)

    • Max flow rate is 18 l/min (brochure)

    With these sums I've only noticed three days when delta T through the combi was greater with solar than without (and combi-energy was still less on those days, per wookey).

    Atmos/Daalderop MonoSolar

    • Min temp to access solar water: "I can confirm categorically that the Solar Diverter Valve is set to 45 degrees C" (Peter Emblin)

    • Max storage temp: 85 degrees (brochure, Peter Emblin)

    • Combi output temp is 60 degrees (brochure)

    • Flow rate varies from 12.5 to 20 l/min at 45 degrees, depending on input temp (brochure)

    • On temperature of water output from solar tank: "The answer is a little complex and indefinate.

    "If the tank is up to temperatiure then the initial draw of of water will certainly be at the tank temperature. There are two factors which will mean that the exit temperature of the hot water being drawn off will reduce.

    "1. After the first few litres the tank temperature will be dropping as it loses heat to the mains water being drawn. This will be mitigated to some extent if it is a hot sunny day by the circulation of the thermal store water gathering more heat, but this is at a lower flow rate than the hot water and so will not keep up.

    "2. The faster the flow of cold water through the coil, the less heat it can pick up. So the exit temperature is variable according to the pressure and flow rate of water.

    "Given these two factors it is impossible to give any particular measurement of exactly how hot the exit temperature will be, [...] it could be as high as 85 degrees.

    "...Overall we can safely say that the MonoSolar will provide approximately 50% of your domestic hot water demand." (Peter Emblin)

    • On Legionnaire's:
    "The thermal store water in the tank circulates to the panel and back, and never comes in contact with the mains water. Inside the MonoSolar Tank is a coil which is fed directly from the mains. Cold mains water enters the coil at the bottom and goes through the coil heat exchanger picking up heat from the thermal store water surrounding the coil. This heated mains pressure water then goes to the Solar Diverter Valve, thence to the boiler if less than 45 [degrees] C or direct to the TMV if greater.

    “The big worry with all systems is where you have a store of water in some way that could sit at ambient temperatures thus giving the environment for legionella. Given that the heated water is always instantaneous as in a combi and that this DHW never comes in contact with the stored thermal store water; there is no stored water for Legionella to breed in. I should also point out that the coil in the MonoSolar Tank is a large bore single continous coil so there are no places for Legionella to hide." (Peter Emblin)

    • CommentAuthorJackyR
    • CommentTimeOct 5th 2008
    None of this takes account of performance of collector, pumps, tank insulation, etc, but all these being equal I'd go for Atmos if I had my time again - the range of storable/usable temps is just so much better.
    • CommentAuthorJackyR
    • CommentTimeOct 6th 2008
    Or of course you can do a separate components system, as described in many places on this forum.

    For us an integrated, off-the-shelf system was always going to be the best option. Without the time/energy to become sufficiently competent in plumbing/leccy, we had to trust someone: an interested and on-the-ball Corgi might have done, but the manufacturer will still be around next year (touch wood) and the system's already tuned.

    Still going to ask Corgi for solar-only T-off to kitchen tap, tho!
    • CommentAuthorarthur
    • CommentTimeOct 6th 2008
    Thanks Jacky. Interesting. The Atmos does seem to be a carefully thought out system, although I believe some of the solar buffs are not impressed by the drainback system.
    Do you know if the Atmos system has any integrated solution to the false firing of the combi boiler (due to cold in the pipes despite a warm tank).
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