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Green Building Bible, Fourth Edition
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    • CommentAuthorRobWeir
    • CommentTimeJun 11th 2022
    Hi everyone,

    First post - I'm a long time lurker, it's a great site but it's taken me 15 years to get around to finalising and commisioning my heating system..

    I’d like some help please, selecting the right TMV and pump for mixing down the Central heating supply temperature - being taken from a 350litre tank (working partly as a buffer tank, partly thermal store). This is probably quite a simple problem for professionals and folk that have done it before, but I’m doing all this for the first time (I’ve split an old pub into two, so I’ve got a second more complex living unit to do after this one).

    Should I use a TMV and Grundfoss pump (like a lot of UFH systems) – or a motorised valve (which must need a controller and add complexity and cost).

    I’m struggling to ID a good value Pump/TMV array that will output 70-80 deg C water and accept 95 deg input. The standard UFH and DHW TMVs regulate to lower temperatures than I think I want…

    This is for a Wood stove backboiler based heating system. I’m using a single vented 350 litre tank/thermal store for DHW and CH, fed directly from a woodstove/boiler, and indirectly by Solar (and possible in future by Heat pump – but I’ve no dedicated coil for that).

    I’ve only got a single 15mm flow and return circuit feeding 3 rooms (plus a towel rail). 2 bedrooms with TRV and a living room which has thermostatic controller built into the fan convector.

    I’d like the freedom to bring the tank up to its maximum safe working temperature (90-95 deg C say) so as to maximise the thermal heat storage capacity of the tank.

    The Overtemp safety valve on the Woodburner is set to 95 degC. The cylinder manufacturer recommends a 90 deg max temp (‘to avoid boiling’), and my main open plan living room radiator (a DIMPLEX SmartRad fan convector) states max CH Feed temp should be 85 degC…

    Please check out the attached diagram (cobbled together but quite representative).

    Can you suggest a Thermostatic Mixer Valve that will provide adjustment between say 65 and 80 degC? Can you suggest a CH pump that is right sized for this size of job.
    If I am sourcing separate Pump and TMV does the pump always go downstream of the TMV?

    Controls: I wasn’t thinking of switching the pump on and off with a room thermostat, I was hoping I could use a basic timer with a simple on/off manual override. In this case I’d want the flow rate to throttle right back (to nearly zero?) when the TRVs were all closed off? Is there a smart pump that does that? If I need a bypass VV I’ve seen a suggestion (here) to bypass in a loop back into the supply line (wouldn't it be better if the bypass was fitted from the output of the pump back to it's input? That way it wouldn't stir the thermal store?).

    There seem to be three good reasons for a Thermostatic Mixer Valve (or equivalent motorised VV) in the CH supply. 85 deg C is what Dimplex have specified as a max supply temp. Mixing down would improve or rather maintain stratification, and it should reduce any burn hazard for occupants. Note that all 3 of the room radiators in the house are Low Surface Temperature designs.

    There is a good reason to allow quite a high CH supply temperature…I am using a fan convector in the main open plan kitchen/living area, as part of a ‘low thermal mass’ heating approach. This convector is oversized (maybe 5 times) by conventional standards. The intention is to be able to heat up the room so fast that the heating can be off when the room is not being used, or when the occupant is out of the house. The higher the feed temperature, the easier it is to make the occupant feel they have access to ‘instant heat’, and so can allow parts of their house to be cool when they are not in use.

    I’m pretty sure 70 degC CH Flow temp would perform adequately as a maximum temp but I am truly lost looking to source the right pump and TMV on the web - there is so much choice and variation in cost.

    I see lots of mass market DHW and UFH mixer valves that seem good value, but when I look for a TMV or a load unit spec’d to 65-80 deg C I cannot find them or the prices seem to be 3 times higher. Am I looking in the wrong places or at the wrong things? Should I be buying separate Valve and Pump components for best value/compactness?

    Pump wise - I was considering a reconditioned Grundfoss pump (off of a well known auction site). The house has such a small system with TRVs on all the radiators that a small modulating pump is not needed to be working very hard so a pressure sensitive pump that modulates down to nearly nothing would be good.

    I’ve rebuilt a small listed pub in a Scottish village, splitting it into two housing units.

    I’m commissioning the smallest 2 bed unit first. It is about 70 m2 with MVHR and a design heat load of under 3 kW. It has a single downstairs open plan living space, with two bedrooms and a bathroom upstairs. There are only 4 radiators on 15mm pipework in the house, all with TRVs.

    There is a single vented thermal store (350 litres) for DHW and CH, which is directly heated from the Wood burner (through a load unit). The stove is supposed to output 2.3 kW to main living room and 6.1 kW to the water (but who knows). The stove has a thermostatically controlled damper on an external air intake.

    There is a solar coil which I’ll probably have to hook into a wet solar thermal panel system for summer DHW. I’m really tight for space locating panels, otherwise I’d put in PV with an immersion heater for simplicity. I can’t use the roof because the building is listed, and I’ve very little unshaded yard space for PV panels so it has to be Solar Thermal, or maybe just use the Solar coil for a Heat pump option instead.

    The bedrooms have small (Low Surface Temp) radiators and the main room has a fan convector unit (DIMPLEX ‘SmartRad’ SRX180EM). I am using a Fan convector so that heating can be switched off when house/rooms are unoccupied (because the convector can heat up air quickly when occupied).

    Of course, the Convector gives me the option of using low grade heat when the store is nearly exhausted or to maintain a good COP if an air/water heat pump is added in future (I’d like to think in the long run we’ll most of us have access to a district heating scheme).

    My initial building warrant was approved >15 years ago, utilising a small oil boiler. So I need to redo the SAP calcs and submit B. Warrant changes adapted to whatever is now allowed!

    In winter, I’m expecting to light the fire (in the downstairs living space) each night. The intention is for there to be sufficient energy left in the store tank to have a shower and heat the living space for breakfast without relighting it, then go out to work leaving the heating off. Ideally there would be a smidgeon of heat left in the evening to take the edge off whilst the stove is lit!

    My plumber is good, but hard to get hold so I’m making all the design decisions, buying the gear (possibly reconditioned Grundfos pumps) and mocking up the pipework …so he can make a neat compact job as fast as possible, when he gets a moment. We’ve got Solar, Wood Boiler and possibly a future Heat Pump CCT as well as DHW and CH… in quite a small airing cupboard space!!

    When I’m happy with the small unit I’m doing similar but more interesting stuff in the adjacent 4 bed unit which is heated only with oversized convectors and uses a 1000m3 Thermal store.

    Sorry to say so much – any comments on any aspects welcome 😊.
    • CommentTimeJun 11th 2022
    Not sure how much I can help but there've been no other takers so far ...

    Just a few thoughts.

    1/ Running the radiators as hot as you intend seems like a bad idea to me, since it restricts the choice of heat sources considerably and you said that was fairly important. A gas boiler should run below 57°C IIRC in order to ensure it condenses properly, and heat pumps are more efficient (i.e. have higher COP) at even lower temperatures. So designing a system to suit only a flow temperature of 75°C or so is pretty rstrictive, I think.

    2/ Most TMVs etc for DHW either truly domestic or even in hospitals and factories etc will be limited to 60°C because that is the legal limit. So you might be looking for industrial units or something custom.

    3/ PV panels aren't prohibited on listed buildings, you just need listed building permission. Not my area of expertise so I can't say more.
    • CommentAuthorRobWeir
    • CommentTimeJun 12th 2022
    Thanks so much djh,

    That's really interesting/food for thought about PV, I'll follow up with planning and my architectual agent (pity I rebuilt the roofs without integrated PV, I could have saved a lot of slates!)...

    I do feel a bit out on a limb, not having anyone to discuss this stuff with (I think WillInAberdeen must live nearby but I can't see his email)

    I haven't been very clear about the CH mixer temperature setting. I'll try and clarify...You could be thinking of a steady state heating system, and I'm trying to accomodate the changing performance as the CH section of the store falls from 90+ to 35 degC. This is primarily a Wood Boiler stove based system, which could run on the immersion in an emergency, with a third option to add a HP that I'll fit later if I have to.

    When the store is at 90+ deg, and heat is demanded, I want it to mix down to say 75 degC on the CH flow. The oversized Fan convector will be able to belt out approx 5 or 6 kW - which would heat the air in the living room up (from 14 deg say) in less than 5 mins. In this mode there is no need to preheat the living space, it can warm up when the occupant enters (which saves energy). At 40 degC flow I wnat the mixer to alow through teh 40 degC 'hot water' from cylinder and not mix in any cold return water. The convector can still output 1-2 kW which will warm up the living room (air) in about 10 mins. 10 mins is a bit of a wait, but I'm OK with that. The convector fan modulates down after reaching its set point.

    I'd recommend my occupier to light the fire if the CH section of the store is at or below 40degC and it's cold outside.

    If the store was running from a Heat Pump (a sort of plan B/C mode), then the target charge temp for the HP controller would be much lower (it's not a great use for a heat pump to load a large buffer but this is a backup option which would avoid using the immersion). The 'Best' use of the oversized Fan convector would be to allow the HP to run at low temperatures and so achieve a higher COP. In this mode the user would not have the option to 'Boost' the heating to the same extent and might be tempted to leave the heating on when not present (something I'm trying to make unecessary most of the time). You would save energy by getting a better COP and lose some by leaving the heating on longer than you would otherwise..

    This and especially the connected main house heat systems are being designed for 'fast response' 'low thermal mass' spaces. A key measure of success (for all heating systems) is how low the average temperature of the living spaces can be whilst maintaining user comfort. Low average temperature is achieved by zoning as many rooms as possible and only heating them when occupied and if the user is intending to be there for a while (the heating sort of follows the person around the house). Comfort with this sort of system depends on being able to heat the spaces up fast most of the time (and knowing that you are saving resources).

    For houses and lifestyles that involve low room occupancy, this sort of approach can save energy. I don't see much about it and wanted to provide an example to see whether it would help. In the small house (this one I'm doing now) it's all a bit of a compromise, because it's open plan. In the larger house I'm doing next there are 6 main rooms all independently heated and ventilated so the fast response approach will make more of a difference.

    Again - sorry for the length - I struggle with 'concise'.

    Posted By: RobWeirAt 40 degC flow I wnat the mixer to alow through teh 40 degC 'hot water' from cylinder and not mix in any cold return water. The convector can still output 1-2 kW which will warm up the living room (air) in about 10 mins. 10 mins is a bit of a wait, but I'm OK with that.

    My thermal mixing valves for the CH on my thermal store are the 35 -60 deg. type turned up full. If your convector can still output 1-2 kW at 40 deg. and you are OK with that whats wrong with a 60 deg TMV, cheap and easily available ?

    Posted By: RobWeirI'd recommend my occupier to light the fire if the CH section of the store is at or below 40degC and it's cold outside.

    By occupier do you mean tenant? My experience with tenants is that anything more than turning a dial to increase the temp. (which never gets turned back) or flipping a switch to turn heating on will create problems after week 2. Expecting a tenant to manage a TS, to know when to light the stove and when to stop putting wood on the stove because the TS is up to 96 deg top to bottom and you don't want it to boil just doesn't happen in my experience. E.g a current tenant can't be bothered to (or can't cope with) setting a programmable CH thermostat so they set it on 23 deg 24/7. (gas combi system) When I pointed out the extra energy and cost of doing this they shrugged their shoulders and just pay the bill. The point being that any heating system for use by a tenant needs to be bullet proof and simple to operate.

    Edit to say From my experience I would go for a Grundfos Alpha 2 CH pump.
    • CommentAuthorSimonD
    • CommentTimeJun 12th 2022
    Something like the Reliance Heatguard Tempering Valve can deal 95 degree with the supply of water up to 70 degrees C. Obviously not quite what you're looking for but close. This is designed for DHW rather than CH so you'd have to fashion a method of feeding the CH return through the tempering valve. How well this would work I'm not sure so recommend a call to the Reliance technical team to find out.

    For me, when I read what you're trying to do I can't help but wonder whether it's getting overcomplicated with too many potential areas of problems, particularly around the desire for manual control input. But then the desire you have is similar to the newer systems using 'smart' controls that modulate heating output depending on the behaviour of the occupant - this involves quite a lot of monitoring and input into the system.

    I would agree with djh in that designing your system to run at 75 degrees is restrictive, but also that when using a thermal store, it might not be the most effective way to resolves the problem you're looking to solve. If you design the ch for a lower flow temperature - what temperature drop are you looking to achieve btw ? - you'll put less demand on the thermal store and provide yourself with better output over a wider range of store temperatures.

    You've clearly been doing your research but having fast heat-up time for fast response/low thermal mass, is just one way of skinning the cat as sometimes, reducing the extent of micro-zoning/intermittent heating can actually improve the overall efficiency of the system.

    With respect to pump selection, you really need to look at the pressure drop across your system together with flow rate and select the pump from there.

    I would go back to the basics and ask what your heat loss calcs tell you about your heating needs, dhw demand, target temperatures together with adjustment for intermittent heating (or no intermittent heating for comparison). Then look at typical user behaviour and patterns. This will all feed into the sizing of your heating emitters, flow temperatures etc. and then you can finalise the design of the heat input needed from your thermal store/back boiler.

    Sorry if this is just teaching to suck eggs but I can't see this info above - maybe I missed it?
    • CommentAuthorRobWeir
    • CommentTimeJun 14th 2022
    Thanks for the replies Peter and Simon,

    Bear with me - I'm not sure how to get the nice formatted quotes from previous messages.

    Peter Q1: I'm not really satisfied with 60 deg C. When more is available (in the store) I'd like the option to pump it around the radiator CCT. I know ESBE has all sorts of mixer VV on their load units (I've actually got a couple of these). So I will approach ESBE.

    Peter Q2: I am going to be the occupier for a year or two, since I've got another house at about the same stage next to this one and a small tumble down stable at the back to rebuild also. I had thought of renting it out in the end, so I think you're right 'on the money' regards 'Keep It Simple...'.

    Peter: Thanks for the pump recommendation 'Grundfos Alpha 2' it is then!


    I've sent off a query to Reliance, thanks for the suggestion.

    Despite my complex description, I am aiming for simplicity - and low energy costs. Anyone who lives in the small house after me and wants heat, will either have to light the fire if the store is discharged, or switch on the heating. The main room convector unit will have a controlling thermostat so they'll have to be able to cope with that, and TRVs in the bedrooms. I'm not planning to have a 7 day timer, so thats one thing that is simpler.

    You are right though - I do quite fancy spoken voice control, and systems which can adapt to the occupants. At the moment, I'm not prepared to use eTRVs if the batteries have to be replaced frequently. I could provide low voltage DC power to the radiators though...

    My folks are getting on now and voice control would make a big difference to their lives.

    If you think my CH is getting overcomplex, don't ask me about the MHRV mods I'd like to make! Or the (self opening) air tight, thermal shutters I want to make! Both would need to sense room occupancy ..That's probably a lifetime's work for me, but it would be fun to try out some ideas.

    Simon - you ask "what temperature drop are you looking to achieve btw". AFAIK deltaT is about 10->15 degC. The fan convector modulates between 3 fan speed settings (and 'off), and has a constant flow rate. So the CH fluid temp drop must vary quite a bit.

    I was trying to explain (in my second post) that the CH flow does not really have a 'design temperature'. The system will work (with the Convector modulating its fan rate) with 85 degC CH flow or with 45 deg.

    I'm trying 'Fast response' as a sort of experiment (can't you tell!) - it's 'a road less travelled'. I actuallly can't wait for winter to come round again and we haven't had a summer yet!

    Simon, it's a long time ago that I estimated SAP like design calcs/got Heat pump quotes etc. Roughly though (back of fag packet style)...Its a small two bed house with design heat load of 3 kW (is that at -5 degC?) so for sizing the heat source we need (3 * 24) + 4 (DHW) kWh/day for combined CH + DHW. Lets round up to 80 kWh per day, to run the house at say 19 degC for 24 hrs with 2.5 showering occupants (there is no bath).

    The wood stove (already installed) is rated just over 8kW at full blast (2 to room 6 to water). So would need to be on at full blast approx 12 hours in mid winter if you were warming house fully for 24 hours/day. So worst case/taking no intermittent heating allowance.

    Behaviours/use cases for a couple staying in. I'd expect some users to throttle back the wood burner so it can stay in nearly all day without boiling the store. I'd expect other users to do an early morning burn and evening burn or else burn during mealtimes and go out between times. Total full heat burn time about 12 hours, but the living space might be a bit too warm if they did that, so the'd probably throttle back and burn for longer.

    The 350 litre store might store say 20 kWh of usable heat between approx 35 and 95 deg. Assuming I provide an option to tap off the top 1/3 for CH (normally reserved for DHW).

    The radiators are already in but the main open plan living room can manage 1.5kW at medium fan setting and average flow temp of 40 deg (flow rate per fan convector data sheet says 0.15 m3/h). Since the top floor has same area as the open plan ground floor I'd say total flow rate for the whole house would need to be double that. So approx 0.30 m3/h

    The radiators upstairs are only normal LST rads good for 0.75 kW each at (probably) 60 degC, so they would be undersized as the thermal store drops in temperature. I think there would be quite a lot of incidental gains up there - from the living room below, so I'm not to worried atm. I'll change whatever doesn't work after testing.

    The bathroom has a towel radiator and TRV and also underfloor electric tile warming.

    Anyway, many many thanks for the help and advice, I'm nearly there now with the pump and TMV decisions.

    Wishing you all well

    • CommentAuthorborpin
    • CommentTimeJun 14th 2022
    I have an and a Thermomatic EC Home controller with a 4 way bivalient valve fitted to an Akvaterm Solar Plus tank to control the output temperature to my UFH.

    Mixes the return, and 2 x flows (from different parts of the tank) for a fixed output temperature.

    It is cleverer than that with the possibility of wether compensation, but I don't need the advance features.

    The UFH has a pump and that controls the heat flow, the mixing valve does the rest.

    Works perfectly :)
    • CommentTimeJun 17th 2022
    Posted By: RobWeirpart of a ‘low thermal mass’ heating approach
    I had one belated thought about this, prompted by the current weather down here in the soft South (well, East). One reason people tend to incorporate some thermal mass is because it helps to keep buildings cool inside when it's hotter outside. There are precious few other ways to keep a house cool when the temperature outside is high, apart from active cooling. There's shading, and external window blinds and suchlike but they just slow down the overheating. Perhaps not as much of a problem in sunny Scotland but it might be worth a thought.
    • CommentAuthorRobWeir
    • CommentTimeJun 19th 2022
    DJH -

    Yes, overheating does seem to eba risk...it's already evident on warm sunny days on the top floor, that it is possibly going to be a struggle in the summer to keep cool.

    I had intended to take the MVHR unit intake via an underground pipe system (potentially 10 degC all year round). Which would have helped a lot. So I may still need to dig into the yard and provide cool fresh air source for the summer (when the MVHR runs in bypass mode). Of course I'd rather turn it off and open window.

    I'll write in again after I've bought the kit and update when I've got some performance figures

    Thanks again


    PS - I spent a weekend (at least) reading all the "Heating and Cooling" topics before posting but only found this topic last week. Its from ten years ago but is highly relevant and points me again in direction of motorised VV
    • CommentAuthorRobWeir
    • CommentTimeJun 20th 2022

    I'm getting some quotes for TMVs with water flow temps up to 65 degC.

    I've also found a german online shop within ebay.


    They (and other german shops) offer integrated CH pump/valve assemblies with many of the high temp mixing capabilities discussed above, using motorised valves. Price over there 300+ euros for a unit including, pump and mixer unit with isolation VV I think. I'm not sure if they will export here or what that would cost. If the Germans are doing them then they can probably be bought from eastern european EEC countries as well.

    Anyway nice to see it's a commodity style product over in Europe.
    • CommentAuthorJonti
    • CommentTimeJun 21st 2022
    Posted By: RobWeirFolks,

    I'm not sure if they will export here or what that would cost.

    According to the website it would cost 21.99 Euro to have it sent to a UK address with a 14 day delivery date.
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