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).

MAIN QUESTION(s)...
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?).


GENERAL THINKING ALOUD…
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.

EVEN MORE BACKSTORY
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 😊.

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'.

Cheers

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?

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 :)

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

Rob

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
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9108

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.