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Green Building Bible, Fourth Edition
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    Hi all,

    My name is Karl, this is my first post but I’ve been reading and researching using this superb forum for quite a while now.
    Thank you all for sharing your obviously hard earned knowledge, it really is appreciated and has help develop my knowledge and interst in renewables and heat engineering.

    I have a question which I can’t quite find a definitive answer to, having searched for a while.

    Is it ok to you use one coil for multiple solar thermal inputs?

    Some background and sorry if this is too much info but I have seen from general responses here that the more detail, the better able people are to provide input.
    I have an existing 3 coil 1000ltr thermal store supplying my DHW via a full height stainless steel 7.5m2 coil, works brilliantly.
    Direct connection (store is open vented) of a 24kw wood burner via laddomatt and digital stove control. Rarely used except for depths of winter when the family really enjoy the live flame experience and I get that smug satisfaction knowing that we are loading the thermal store with heat too.
    Upper coil, 2m2 supplied by 15kw pellet boiler, tops up my DHW on less dull days and via 2 port valve inputs heat to the UFH when needed (mainly early hours of morning (2-6 am) all very efficient.
    Lower coil 3m2 supplied by 6.6m2 flat plate solar thermal, provides 100% DHW production for c.8 months of the year. Remaining 4 months it contributes c.50% DHW and some (c.40%) space heating as UFH take off is 60% up the store so once the temp sensor at 60% hits 40c the heat is pumped to the UFH until the temp at 60% drops to 35c. So the UFH gets 40c input from solar most days during heating season, average is about 1 hour recovery from 35c to 40c for the store for 15 mins supply to the UFH all whilst keeping the upper 40% at DHW temp, very happy with the stratification.
    I now have an additional 50 EV tubes to add to the system but no more coils available. I’d like to T into the lower solar coil and use it for both solar arrays, (existing flat plate and new EV tubes) obviously running through separate pump stations with non return valves and separate controllers. I have some direct connection ports left so running through a PHE could be another option but would involve more pumps and more plumbing. Not my preferred option.

    Could I ask what people think about sharing a coil for two heat inputs?
    I have seen it done before for oil CH when a boiler stove has been added so I know it’s technically possible and accepted practice in some places. I have never seen or heard of it done with solar though.

    Many thanks in advance for all replies,
    • CommentAuthorowlman
    • CommentTimeSep 9th 2018
    Like you my Solar Thermal (ST) generally provides more than adequate DHW in Summer, in the shoulder months I top it up for a short while with electric immersion from PV.
    In Winter I use it to pre-heat my solar cylinder DHW which I then divert to a PHE fed from my accumulator tank.

    My gut reaction would be that an extra 50 tubes would "overface " your cylinder in Summer and you have to ask if it's worth it for the extra winter gain? Think constant stagnation in Summer.
    Regarding your question about connecting two ST supplies into one coil. It may well be possible, I seem to remember the phrase "Neutral Point" in the field of back boilers etc.. Whether that would apply to two pumped Glycol sources I don't know.

    Maybe a new simple 2 coil solar cylinder with a single pumped flow and return connection to your existing spare coil may be the answer,- I suppose therefore a "Neutral Point".
    I think the 'Du--ley Neutraliser (not to advertise! ...but you may want to think solid fuel stoves) was designed for just this. Haven't come across one in over 10 years, though, I think.
    Thanks owlman and Nick.

    Regarding the point of overfacing, I still think it’s worth it as as I have left provision for the install, all pipe work is in place, pump station mounted etc. I’m not starting from scratch, if I were I’d look to PV as you have.
    I was initially concerned about summer stagnation but then remembered in my particular build I have a 200m2 concrete basement with more than sufficient space to act as a heat dump.
    The family room there never really gets warm (without heat) even in summer so dumping heat there will work nicely for us.
    If I found summer gains really to be too much, I can cover or remove some tubes from the array as they will be mounted vertically on a large south facing wall at ground level.
    That or I could be tempted to buy an outdoor hot tub and redirect the extra gains there.
    The winter gains won’t be as much as summer clearly but I have a well insulated house which is currently c.40% heated by flat plate solar so if we could bring this to c.75% or more than we would be delighted, considering that the general opinion seems to be that EV tubes perform better than flat plate.
    Luckily for me the 50 EV tube system cost very little, I spotted and picked up from a builders liquidation sale.

    I actually have a 450ltr dual coil cylinder which could be used as a neutral point so I’ll think on that, would be a little replumbing to do that approach but doable.
    I’ll also check out that neutraliser to see if it’s suitable.

    Still interested to know if anyone else has experience pairing sources to a single coil though so please chip in if any additional advice or suggestions please.

    So I have taken a look at the type C and type R neutralisers and I actually have something similar to the type R in the heat distribution side of my system.
    It’s made by a different company in Ireland for “system linking” and works well, though it looses a lot of heat as it’s uninsulated. I must get that sorted.
    Using something similar to twin the solar inputs would definitely work but seems overkill to be honest and I can’t see, for my purpose anyway, how it would offer any benefit over simply T’ing the two solar inputs close to the coil at the thermal store. Both solar circuits are pressurised, pumped and will run through a pressurised coil. I’m not mixing open and pressurised systems. Plus both are protected against thermo syphion too so all the key benefits offered by a neutralizer are already addressed.
    Maybe I’m missing something...?
    No, maybe you are not. I am not a plumber, but is there a major difference between, say, a 70-80 tube array feeding in via one pipe, and a 50-tube array + 6.6m2 FP teed in close to one another? If you are looking to dump heat to the basement in the summer then presumably fail-safe controls are important, but that isn't rocket science (or maybe rocket science *does* have fail-safe controls. I'm going to stop musing now.
    • CommentAuthorowlman
    • CommentTimeSep 9th 2018
    If there is a danger It may be that you've got two pumps on different circuits pushing, or pulling, heated glycol at quite possibly differing temperatures. This I may create problems if one circuit is slower/cooler than the other and you could end up with thermal back pressure on the slower pump or indeed the array.
    • CommentAuthorgyrogear
    • CommentTimeSep 10th 2018 edited
    Posted By: ModelmechanixThe winter gains won’t be as much as summer clearly

    really ?

    Per link, & depending where you live, winter insolation should be around 40 % more than summer...
    Depending on your climate, cold will remove more useful heat, fair enough, but you should still get more winter gain with a vertical set-up

    • CommentTimeSep 10th 2018 edited
    Posted By: gyrogearPer link, & depending where you live, winter insolation should be around 40 % more than summer...

    Would you care to explain that claim a little more, especially what 'per link' means?
    • CommentAuthorgyrogear
    • CommentTimeSep 10th 2018
    OOPS, apologies to all, I failed to post the link...


    Per the tables (I selected Bradford, UK), theoretical insolation on a vertical collector is 42% greater for the eight months of winter, than for the 4 months of summer. Viz. 14.43 kWh/day vs 10.12 kWh/day/ per square meter.

    Since the OP talks of "gain", I defer by agreeing that colder winter temps will naturally hit some of this "surplus" energy. Just how much depends on the efficiency of his collector, of course.

    Hope this clears things up a bit.

    • CommentTimeSep 10th 2018
    That link helps a bit. But I don't see where your 14.43 kWh/day vs 10.12 kWh/day/ per square meter numbers come from. I see

    Measured in kWh/m2/day onto a vertical surface

    Jan 1.11
    Feb 1.83
    Mar 2.14
    Apr 2.50
    May 2.70
    Jun 2.54

    Jul 2.62
    Aug 2.61
    Sep 2.35
    Oct 1.96
    Nov 1.31
    Dec 0.98

    No numbers anywhere near as big as yours, and the peak in the summer as expected, though it is showing the double-peak as expected for a vertical surface.
    • CommentAuthorgyrogear
    • CommentTimeSep 10th 2018
    never mind !

    Thanks Nick and Owlman, some good open questions there.
    I see combining systems as one large array really but totally accept there could be an averaging of temperatures across both and/or back pressure issues or worse.
    I guess I’ll just have to connect it and see how it goes. It won’t cost much as 90% of the work is already in place.
    I’ll do it and circle back, though it could be a couple of weeks.

    Thanks for the comments Gyrogear and Djh.
    I should say that I am in the north of Ireland so have quite low kWh per m2.
    For my latitude, I always understood that vertical solar instalation performed better in winter as the sun was lower on the horizon so the array was closer to 90degrees to the sun. Less that optimal angle in summer but that was generally compensated by the overall higher insolation.
    I’ve never seen figures as high as you mentioned gyrogear, were you quoting the total kWh for my particular 6.6m2 array?
    I checked the link and it’s quoting figures closer to what djh mentioned and that I’ve seen from other sources when researching previously but admit that it’s not an area that I’ve spent a massive amount of time on. Where are based gyrogear, are you closer to the equator and are those figures typical for you?
    I’m happy to be educated more on the subject and greatful for everyone’s input!
    • CommentAuthorgyrogear
    • CommentTimeSep 10th 2018 edited
    Posted By: ModelmechanixI’ve never seen figures as high as you mentioned gyrogear, were you quoting the total kWh for my particular 6.6m2 array?


    No, I was aggregating for summer and winter, like I said: thus 8 months and 4 months respectively.
    So by such a definition, winter gain is greater than summer gain, because winter is twice longer !

    Like you said, a steep angle optimizes for winter collection OK.

    BTW, just to repeat oneself, those figures are per square meter per day, so if you have 6.6 m2, more power to your elbow .

    Also don't forget that that table is for insolation (received energy...) so since your collector will (unlikely) be 100% efficient, you will need to knock a bit off ! Like 50 %, rule of thumb...


    good luck with the install...:devil:
    gg, I am self-confessedly rubbish at mathematical manipulation, but while I see the point re winter being twice as long as summer, I cannot then see how you can come up with a potential yield figure *per day*. Or are you saying that the insolation per *2 days* of winter is greater than that for *one* average day of summer?

    Confusedly yours,

    • CommentAuthorgyrogear
    • CommentTimeSep 11th 2018 edited
    Hello, Nick,

    Me no maths neither !

    Here I go for another try !

    From the table, aggregating the daily insolation for the 8 months of winter (Oct; Nov; Dec; Jan; Feb; Mar, Apr; May = 8 months), gives 14 kWh per square meter per day.

    Doing the same for Summer (the other 4 months) gives 10 kWh/m2/day.

    It is not necessary to actually CALCULATE the monthly figures as the number of days ("30") is a common denominator...

    This was to respond to the OP's statement that "winter gains won't be as great as summer"

    Effectively, I cannot prove the opposite: I was just trying to show that with a vertical collector, winter *insolation* is 40% greater than summer insolation - a fact which (I seem to think...) often passes unheeded.

    (declaration of interest: I have ten square meters of vertical collector...)

    • CommentTimeSep 11th 2018 edited
    Posted By: gyrogearFrom the table, aggregating the daily insolation for the 8 months of winter (Oct; Nov; Dec; Jan; Feb; Mar, Apr; May = 8 months), gives 14 kWh per square meter per day.

    No! When you add numbers like that the units don't somehow stay the same. They would become, in this case kWh per square meter per 8 days. Similarly the summer figures would be in units of kWh per square meter per 4 days. We then need to divide by 8 and 4 respectively to get them back to the same units (per day) so we can compare them. That gives:

    winter = 14/8 = 1.75 kWh per metre² per day
    summer = 10/4 = 2.5 kWh per metre² per day

    and then it's quite clear that as you would expect, it's sunnier in the summer than the winter. :bigsmile:

    edit: you can of course compare the total generation over the whole season but then you'd do better to move to Sweden where it is said they have only two seasons; winter, and July. So they get even more energy in winter, since it's 11 months long.
    • CommentAuthorgyrogear
    • CommentTimeSep 11th 2018
    ... and eight times 1.75 = 14,

    and four times 2.5 = 10,

    thus forty percent more in winter, like I said !

    Well I’ll be, you really do learn something every day. Well I do at least, especially here!

    So in summary, my understanding is that, if you look at a single days insolation in one of the 4 summer months the kWh per m2 per day is higher than a single day in one of the 8 winter months.
    However 😀
    If you consider that winter is 8 months long, then..... the aggregate insolation (for want of a better term) over the 8 months is higher than the 4 months of summer.

    Did I get that right?

    So my off the cuff statement re winter gains not being as good as summer was clearly wrong.
    I did mean the gain per individual day but I hadn’t factored the “shoulder months” making winter 8 months long.
    I wasn’t specific in that so I’m glad to have been educated, thanks everyone.

    I will still circle back once I have something to share on the sharing of the coil in the thermal store.

    Now, off to research more on the vertical mounting of the 50 EV tubes, I’ve heard mixed advice saying that they “need” to be at a slight angle for the small amount of liquid inside to condense at the top and rollback down to be heated again.
    • CommentTimeSep 12th 2018 edited
    Except that it's only gg that thinks winter is eight months long. The meteorologists think it's three months long, like summer, and spring and autumn. Specifically, they say it is December, January and February. And normal people think it's three months too, just starting from a different date.


    The whole 'more energy in winter' thing is pure bunkum.
    But by angling the collectors with a winter bias you can get better collection in the winter (when you would need to make best use of what there is) at the cost of less than max collection in the summer (when you probably don't need all you can collect)
    • CommentAuthorbhommels
    • CommentTimeSep 12th 2018
    What Peter says!
    Do you have freedom in the vertical angle of the array? If so, playing with PVGIS
    allows you to maximise for winter collection by setting the vertical angle.
    If your array is facing South exactly, the angles would be:
    Summer yield optimum: latitude + 23.5 degrees
    Winter yield optimum: latitude - 23.5 degrees
    I would start off by looking at mounting angles of the winter yield optimum + 10 degrees or so.
    • CommentTimeSep 12th 2018 edited
    Posted By: Peter_in_HungaryBut by angling the collectors with a winter bias

    The numbers we're discussing ARE with a winter bias. Have been all along.
    • CommentAuthorowlman
    • CommentTimeSep 12th 2018
    For Karl;
    If you proceed with your "Tee in" experiment It'd be wise to look at pipe sizes e.g. first the coil bore then secondly the respective sizes of the two ST pipes presumably 22mm. If you end up "throttling down" the flow in the coil when both arrays are pumping it could cause one array ( the weaker), to go into stagnation and therefore be ineffective, defeating the whole object.
    Less of a problem if the coil is oversized.
    <blockquote><cite>Posted By: owlman</cite>For Karl;
    If you proceed with your "Tee in" experiment It'd be wise to look at pipe sizes e.g. first the coil bore then secondly the respective sizes of the two ST pipes presumably 22mm. If you end up "throttling down" the flow in the coil when both arrays are pumping it could cause one array ( the weaker), to go into stagnation and therefore be ineffective, defeating the whole object.
    Less of a problem if the coil is oversized.</blockquote>

    Thanks Owlman, I have the same concern but thought that temperature rather than pressure might cause that effect so I intended on setting/playing with the delta T on the FP to only trigger the pump at a similar point to the EV array. Thus “pumping” the system at similar temperatures, reducing the risk of the EV array sending higher temps to the FP array and wasting energy.

    Here are the details of the systems, I’d be very interested in opinions;

    The coil is 3m2 with inputs sized at 3/4 inch copper.
    The FP solar is plumbed in 3/4 inch copper completely.
    The EV tubes will be plumbed in 3/4 copper externally, will join into 1 inch copper once they come through the south facing wall (reusing spare wood burner flow and return pipe work as already runs 95% of the same route) and then reduces down to 3/4 inch copper at the “Tee” joint with FP solar entering the solar coil.

    It’s worth mentioning that the FP pipe run is c. 12 meters of 3/4 inch copper total and the planned EV pipe run is c. 20 meters copper total (with c.6m being 3/4 inch copper & 16m being 1 inch copper).
    I have A rated willo pico modulating pumps that I can use if that would balance pressure?

    Perhaps it’s just going to be a case of plumb it and see....
    • CommentAuthorowlman
    • CommentTimeSep 13th 2018
    Perhaps "teeing in" the two 3/4" ( FP & EV) flows and returns into the 1" defunct pipework may give the extra capacity for both flows and maybe act a little bit as a Neutraliser ?

    I still think using your 450l twin coil cylinder is a better idea, with the main flow and return on a simple pumped,
    ( thermostat controlled ), circuit to your main thermal store. That way you could utilise the extra storage capacity to even out demand.
    If you've got the cylinder already it's only a pump and a stat you need. If you wanted to further sophisticate that new set up and avoid any ST stagnation you could put in a heat sink rad.
    Sharing the 1 inch pipe is not an option with my particular set up as the FP pipe work comes from the roof and enters the plant room directly from above whereas the pipe work intended for the EV runs along the ceiling of the basement and enters horizontally. Im not explaining very well but trust me, it’s not practical.
    Good idea though Owlman and I’m sure it would help.

    I’ll keep the 450ltr option as a plan b, that could be quite effective with both solar arrays heatings a smaller volume of water, then when it hits the desired temp, the pump transfers it to the thermal store. The idea is growing on me.
    I’ll see how the coil share goes first as it’s an easy connection and almost ready.
    I promised to update and close this discussion out once the install was commissioned, happy to be able to circle back and do that now.

    I went with the Teeing option and both arrays are working independently, actually quite similar to an east-west system. I’m delighted with the results and whilst there is a little warming of the return to the FP by the EV, I am minimising this by setting differs delta t for both systems.
    So far, it seems the FP collector kicks in first and does the heavy lifting with a delta t of 10c on / 8c off.
    I have a delta t of 30c on / 20c off on the EV array. This works well because of the volume of fluid in the 1inch pipe given its c.20m run from the south wall to the TS. Currently, it lets the FP pretty much run continuously during the morning and mid afternoon with periodic bursts of high temp from the EV array. Then when the FP falls off, the EV continues for another 2 hours or so late afternoon. I lifted the 1000ltr. TS from 8c (after empty and refill to change some immersion heater port arrangements in preparation for PV install) to 69c yesterday just on solar.
    Yesterday was a very cold but clear and sunny day in Ireland so that energy harvest won’t happen often but it shows the system is working.

    I’m pretty happy, given the low expenditures and I’ll continue to tweak the settings to optimise as far as practical.

    Thank you for everyone’s input!

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