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    • CommentAuthordelprado
    • CommentTimeMar 5th 2018
    Unless I am trying to be too clever (and in fact being stupid) wouldnt it be a good idea, especially with UFH, to have this feature? Otherwise, you are using your 70 degree hot water to be mixed with cold for your 45 degree UFH? Wouldnt it make more sense to just put out 45 degrees seperate to 70 degrees?

    • CommentAuthorDarylP
    • CommentTimeMar 5th 2018
    ... some can, they have two sep output settings, 1 for CH and 1 for DHW....:smile:
    Our Viessmann has a fully modulating output. When heating water it runs at max power.

    For UFH there are two zones. The higher temperature of the two (under wooden floors) runs directly from the boiler output. The lower temperature zone (concrete floors) mixes down from there.
    • CommentAuthorbhommels
    • CommentTimeMar 5th 2018
    Can OpenTherm controllers do this? I am looking to upgrade my back boiler to a modern condensing boiler and I am worried about the same thing. My CH system has a heat store, radiators (upstairs) and UFH (downstairs) attached, each controlled by a zone valve that woud have a specific temp requirement....
    Posted By: Simon StillOur Viessmann has a fully modulating output. When heating water it runs at max power.

    A modulating boiler is not outputting 2 temperature outputs at one time but rather varying the single output to the current needs.

    AFAIK boilers can't output 2 different temperatures simultaneously. This is usually done by mixer valves which are separate to the boiler
    • CommentAuthortony
    • CommentTimeMar 5th 2018
    The answer is yes and no

    Your heating system can be designed to have as many different flow temperatures as you wish. The boiler flow will only come out at one temperature but mixing this with cooler water from a heating circuit return can open up lots of options.
    • CommentTimeMar 5th 2018
    It seems to me much more likely that a combi boiler does produce water at two different temperatures, but only one at a time as PiH says. Doing this is relatively straightforward by switching the setpoint control as well as the flow diverter valve. Mixing additional water into the central heating circuit output would require extra plumbing that isn't present and perhaps an extra pump too. None of which plumbing is shown in the manufacturers drawings. And then what would happen if the user perversely set the DHW temperature lower? Would there be extra plumbing to allow additional mains cold to be mixed in as well? It all seems a lot simpler to make the switch electrically, which thus allows the boiler to produce two separate temperatures.
    • CommentAuthorTimSmall
    • CommentTimeMar 5th 2018 edited
    Posted By: tonyThe boiler flow will only come out at one temperature

    Strictly speaking, (with most boilers) it'll have a target temperature and an upper temperature limit which will be set at any given time (e.g. the amount that the actual flow temperature is allowed to exceed the set point temp before the output will cut out and start "cycling"), it may also support limiting the maximum gas input (modulation amount) too.

    Combis usually have two different set points (one for DHW and one for CH).

    A control system could change these to whatever is required at the time (on the fly), and could also vary which zones or stores etc. are active too - and so effectively run different zones at different temperatures (without resorting to mixing down, and losing efficiency).

    In practise it might be better to target running all your heating zones at the same temperature (by designing the emitters to match the worst-case heat requirement for each zone, and preferably targeting as low a temp as you think practical).

    In my case (using a 24kW Vaillant combi which came with the house) I chose to target a 35C avg emitter temp, with an indoor temp of 21C / 19C (depending on the room/zone), and an outdoor temp of -5C (IIRC). That should allow switching to a heat pump at some point in the future, whilst attaining a good CoP.

    I've also set the maximum CH modulation to be the minimum that the boiler is capable of (which is more than double the max steady-state heat loss of the house), so that it always runs at max efficiency whilst doing central heating (if you set it to anything else, then it'll unnecessarily "race" to get to a high emitter temperature and do so at lower efficiency).

    With the boiler running with a flow temp of 35C and limited to minimum modulation, the efficiency during CH (on the standard scale which is used in the UK/EU, so on a scale of 0% to "112%" or something) is between 97% and 100% I believe.

    DHW temp is set to 42C usually. When topping up tepid baths, it's temporarily changed to 55C or 60C or so. This is manual currently, but I have a plan to make it automatic...
    • CommentAuthorbhommels
    • CommentTimeMar 6th 2018
    Thanks for the replies. I should have formulated my question better.
    What I want is a controls system that sets the flow temp depending on who calls for heat.
    1- UHF zone calls: flow temp 30 C
    2- Radiators zone calls: flow temp 45 C
    3- DHW heat store calls: flow temp 50 C
    From what I have read, OpenTherm should be capable of doing this, however I have not seen any references to an implementation of the above. Anybody have any ideas/recommendations on this?
    • CommentAuthorowlman
    • CommentTimeMar 6th 2018 edited
    I know nothing about OpenTherm or how it works or for that matter about any super sophisticated modern modulating gas boilers.
    However a boiler is a boiler and as such it simply heats water. I would have thought that it/they,, can only regulate temperature by means of its own internal thermostat or by some single thermostatic external command, whereby it would cycle on and off or modulate.
    If you need differing delivery temperatures for different purposes as you suggest I recon ( I may be wrong), you'll only get that by having some sort of interface store, between the boiler and the end requirement. You can then have three or four different circuits each with its own controls drawing water as needed from that store.
    Those sorts of stores are called lots of different things but a generic term "buffer tank" explains. It's simply a storage facility between a heat source,-- boiler, wood stove, solar thermal, geothermal, you name it,- and the end requirement.
    The beauty of buffer tanks, accumulators etc., is that they are designed for and can do precisely what you seem to be after.
    Those of us with biomass boilers have such devices but I see no reason why you can't adapt a similar system for a gas or oil boiler, and you'd then maybe only need a simplified on/off boiler.
    Posted By: owlmanI know nothing about OpenTherm or how it works.

    Me neither
    However whilst a buffer tank would solve the problem mixer valves might be cheaper and take up less space.
    • CommentAuthorowlman
    • CommentTimeMar 6th 2018
    True Peter, but surely the mixer valves need something to draw from.
    What bhomels seems to be describing is a three zone hot water supply system where two zones go to space heating and one to DHW. That's simple enough with a buffer tank where each zone is a simple pumped flow and return, (3 pumps, 3 controls ). A three way infinitely variable motorised, thermo controlled, mixer valve links the flow and return of each zone/circuit. Each zone therefore operates independently but they all draw from one hot reservoir, ( buffer tank).
    Getting that independent zoned effect from a boiler alone that is either on or off, and pushing out water at its own internal thermostat setting may prove difficult. It may involve some very interesting plumbing; No?
    >>Getting that independent zoned effect from a boiler alone that is either on or off, and pushing out water at its >>own internal thermostat setting may prove difficult. It may involve some very interesting plumbing; No?

    That's pretty much what my Viessmann does, using their controllers. Heats the water as priority but once the (small) tank is full it puts out whatever the higher temp heating circuit needs, then theres a mixer that mixes down to the temp required by the lower temp circuit.

    All using standard Viessmann parts.
    • CommentTimeMar 7th 2018
    The point is that modern boilers don't operate fully on or off at a single temperature. They have one (or two in the case of combis) setpoint controls that determine the flow temperature from the boiler, and they modulate their output according to that setpoint and the flowrate through them and sometimes also external controls. Some modern boilers have an interface called Opentherm that allows monitoring and control of some aspects of the boiler through a digital interface. So a system like that Tim Small describes might be possible, using just diverter valves and no mixing. Though clearly some separation is needed between the potable water and the central heating and boiler flow water, so a thermal store or a PHE for example.

    Whilst I know such a thing is theoretically possible, I have no idea whether it is realistic. When I investigated OpenTherm in relation to my MVHR system, which also use the 'standard', I discovered that it is by no means 'Open' and it is by no means a 'standard'. All the manufacturers implement different variants of proprietary facilities. So the OP needs to find an expert in at least one of the systems he is considering. I know there are people on this forum who do understand the practicalities of OpenTherm somewhat more. Perhaps they will join in.
    • CommentAuthorbhommels
    • CommentTimeMar 8th 2018
    Thanks again for all the useful comments.
    For integrating the UFH, radiators and heat store I am thinking of a non-buffered, three zone system indeed, with each zone having its own valve & circuit, this would keep the plumbing relatively simple. What the control system should do is to use the boiler as efficiently as possible, so use the lowest output temperature depending on where the heat is going to be used. I don't see the point in a boiler heating water to 45 degrees only for the UFH to mix it down to 30 when it is a modulating boiler that delivers the heat.
    A buffer tank could provide a nice common point in the system, however it would have to sit at the highest temperature which is not very efficient. A buffer vessel with large T difference between top & bottom is not realistic for the system size unless the vessel is huge.

    From what I have read about it, there should be some kind of OpenTherm "master" that receives the calls from the 3 zones, and controls the boiler as an OpenTherm slave, receiving the setting for flow temperature from the master depending on which zone is calling for heat.
    BTW the heat store is providing DHW through a heat exchanger, so no danger of inhibited water coming out of the taps
    If the heat stores only function is to provide DHW why not use a conventional indirect DHW tank which does away with the heat exchnger
    • CommentAuthorbhommels
    • CommentTimeMar 8th 2018
    @Peter, there are a few (good) reasons for having the heat store
    1- current boiler is a back boiler, so needs a tank for DHW. It is very inefficient for DHW only so switched off in summer, with DHW relying on electricity instead.
    2- plenty of solar PV in summer to provide DHW through an immersion controller
    3- mains pressure hot water with an easy, flexible installation, capable of buffering heat across a very wide temperature range
    4- option to plumb in solar hot water should I get around to it
    5- it came for free, with DIY installation
    I am tempted not to rip the whole DHW system out to replace it by a non-vented steam bomb at high cost for very little gain. The boiler replacement & zoned system is going to be expensive enough as it is.
    I am struggling with this at the moment. Trying to connect a woodstove to UFH on the ground floor whilst still keeping high temperatures for conventional radiators on the first floor.
    Because my boiler is biomass I am using a laddomat to keep a high temperature into the heating circuit. The only solution I have found so far is an Omnie thermostat manifold for the UFH which will bleed in part of the heat for the UFH the remaining heat can be used for the radiators. Or am I barking up the wrong tree.
    Omnie thermostat manifold
    Yup - that will do the job.
    But then a standard mixer valve, pump and manifolds should also work. This would work the same as the Omnie just in separate components and possibly cheaper.
    You will need separate circuits for UFH and rads each with their own pump and non-return valves appropriately placed to prevent back flows.
    Are you running your boiler without a thermal store ?
    At present its a single pipe ring main central heating system with feed on the ground floor and return on the first floor ending at a coil in the top of the thermal store then onto the pump and the Dunsley Yorkshire biomass stove. Open system with header tank. Esse cooking stove has Laddomat to obtain high boiler temperature and goes upto coil in DHW tank with excess heat into thermal store than back to Esse Laddomat.
    Quite often get power cuts so any system must close down safely without electric. Esse is protected by gravity feed on Laddomat and Dunsley has Auto closure of air supply.
    Current thinking is for Dunsley to do UFH and Esse the DHW but that still leaves the upstair radiators.
    Given that the Dunsley Yorkshire has a fairly even split between heat to room and heat to water (about 7kW each) do you anticipate any problems in getting the heat to the UFH which will need a continuous supply of low grade heat without overheating the space (room) containing the Dunsley.

    You need to look at the heat loads vs. stove/Esse capacity. If you are looking to supply anything close to 7kW to water to maintain the CH then 7kW to the room will make the room unbearable quite quickly. Even if it is open plan you finish up with a no-go area close to the stove.

    From your description you have a DHW tank and a thermal store (TS) Spliting CH and DHW is the best, so that is OK. After that I would suggest running both the UFH and the rads off the TS - so neither connected to the Dunsley. In this way the Dunsley feeds the TS along with the excess heat from the Esse and the TS supplies the CH, both the UFH and the rads with each having their own circuit, pump and mixer valve etc. This also has the advantage of de-coupling the demands of the CH from the firing of the stove (up to the capacity of the TS)

    IMO the Dunsley should also have some sort of under temperature protection (= laddomat or alike) or a thermostat to shut of the pump below (say) 65deg. because it is possible to over cool the stove.

    Have you tested the Auto closure of air supply on the Dunsley at say mid burn on full throttle (worst case) to check that it actually shuts down the burn fast enough to stop a boil up?

    Is the location of the Dunsley and the TS relative to each other such that a gravity circulation could be installed. If this could happen then you have no problems with over cooling and no problems with power outages.

    Thanks for your thoughts which do make sense. I agree with your split of the DHW and Central Heating so the Esse will be dedicated to the DHW with the Laddomat creating the high temperature. Think the current valve is preset at 75 degree but can still get different temps if required. Now for the Dunsley at presentit never overheats the room as the thermal mass is massive ie 60 m2 of 600mm stone walls. Might change a bit when the floor insulation goes in but will live with that. At present the room has 2 additional thermostat radiators as well as the Dunsley. The TS is directly above the Dunsley so gravity circulation was part of the safety system. The Dunsley is specifically made for central heating and cannot be overcooled as it has a fully enclosed ceramic firebox with the boiler independent behind it. The Auto closure works extremely well and starts shutting off at 45C. as I say we have quite frequent power cuts so it has been well tested. Latest thought Dunsley output to Omnie then to TS down to pump and onto Dunsley input. Radiators fed from TS coil with own pump.
    Rads are better supplied directly from the TS rather than from a coil within the TS as it is difficult to get the heat output required for rads via a coil.
    I would still prefer to see both rads ad UFH coming from the TS and the stove heating the TS

    Only reason I am suggesting the UFH direct from the Dunsley is the time lag for heat transfer to the TS bearing in mind it holds 114 ltrs. Maybe the TS heated by the coil and then the rads from the top of the TS. This way the UFH would be working while the TS is still heating up. Already concerned about the predicted 4 hr delay between heat going into the UFH and getting through the stone flags but we have no choice as part of the LBC we have to reuse the original stone flags. (Dont mind they do look great so long as I can relay them level) Not to concerned with the rads as a warm downstairs gives a reasonably warm upstairs without any rads.
    Your stove will need about 1 hour to raise the temp of the TS by 50deg. However this does not tell the whole story because you will get useful heat from the TS as soon as the top of the TS reaches a usable temperature which could be within 15 mins depending upon the starting point of the TS. With a 4 hr lag in the UFH I don't think any delay in getting heat from the TS is going to be significant. And don't forget that the 4 hr. delay getting heat through the flags also works at the other end where heat will be emanating from the flags for considerable time after the heat input ends.

    I would still take both the UFH and rads from the top of the TS and I would prioritize the UFH (if that is what you want) by putting a thermostat on the TS to prevent the rads coming on if the TS is below say 60 or 65 deg. I would also have an over temp thermostat to switch on (regardless of other heating controls) either UFH or rads in the event the TS gets to 90deg. By taking both UFH and rads from the TS means that both can benefit from spare heat from the Esse.

    Have you accounted for any heat-sink output (typically a gravity rad) that may be required by the stove specs. or will the air shutoff system satisfy this safety requirement.

    What is the kW capacity of the coil(s?) in the TS and what are the kW loads of the UFH and rads.
    • CommentTimeMar 10th 2018
    Not read all the details of this thread, but has anyone mentioned the return temperature, which is quite important to keep a gas boiler condensing?

    Hopefully will be able to tell you the KW capacity and loads when the 3 quotes come back.
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