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Weather compensated secondary circuits with Biomass

Wood Burners, Biomass, Small Scale, Biofuels & Other Heating Alternatives

Last Post by Sune 10 months ago

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AndyCoo

(@andycoo)

Active Member Member

Joined: 1 year ago

Posts: 7

Topic starter 28/06/2025 11:50 pm [#2369]

Hi all

Does anyone have experience or solutions for how to control mixing valves using weather compensation in the absence of heat pump or modern system boiler?

We have Windhager biomass at the moment, but installing UFH soon and would like to take the opportunity to prepare the system for upgrade to HP in future.

I've been looking at this system from Germany which looks like exactly the thing, but would be good to get any feedback about this system first, or any alternatives.

thanks in advance

Andy

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Mars

(@editor)

Illustrious Member Admin

Joined: 5 years ago

Posts: 4516

29/06/2025 7:11 am

If anyone’s going to know the answer to this it’s @sune.

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Sune

(@sune)

Trusted Member Contributor Professional Installer

Joined: 4 years ago

Posts: 27

30/06/2025 9:54 am

Hi @andycoo we do an LK 110 standalone weather compensating actuator with valve: https://www.firepowerheating.co.uk/lk-110-smartcomfort

LK are a top notch Swedish made. I haven't tried the German system, although it looks like it also does weather compensating mixing.

Tell the underfloor heating designer that the system is for a heat pump eventually, dT5, because they can then adapt it accordingly. And make sure that they've done a full room-by-room heat loss calc for your house to base it all on. Also get them to think about pipe spacings, and inform you about suitable floor coverings/materials so you can get those flow temps down, ready for your super efficient heat pump system. That can often mean that you avoid carpet and wood floors for rooms with a fair heat load. I'd also recommend dual fuel towel rails in bathrooms.

If you have a thermal store, and if you have the efficiencies for your Windhager at nominal and partial load to hand then - compare them. Sometimes the efficiency at partial load is better than at nominal and so turning the power down a bit can sometimes improve the setup even more, and give longer run times, with less cycling.

If you have a thermal store then you might also be able to use it when you do get a heat pump - I've been designing quite a few smartgrid systems recently where the heat pump ramps up the flow temp when leccy is cheap to store it in the thermal store, and you then have a weather compensating mixer off of the thermal store so the heat to the house is always consistent. Might be worth thinking about come the time.

If you think you might want the LK setup then drop me and email, sune@firepower.co.uk - and as you're a forum member I can make sure the team give you some discount on there too. And regardless of which solution you go for - good luck with it all.

🙂

Planning your dream house? Book a simple one-to-one consultation to get you on a good path - here.

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AndyCoo

(@andycoo)

Active Member Member

Joined: 1 year ago

Posts: 7

Topic starter 02/07/2025 8:26 pm

@sune thanks for the LK link, looks like a possible option for us.

Thanks for the advice on the UFH. It will be covered with limecrete and limestone flags so hopefully the transfer will be good, but I think we are going to need at least 100W/m2 in some areas so we might have a problem and require a plinth heater in the kitchen for example.

Good shout on the bathroom rads, the one we have in our en-suite already struggles to keep up with the heat loss with flow temp of 65. Were you referring to electric when you mentioned dual fuel?

We don’t have a thermal store at the moment because I think the original installer thought the Windhager modulation would be low enough. But even with its modulation down to 30%/ 7kW the best I can get from the Windhager is about 3 hours run time before cycling even at the coldest times. So I’ve been thinking at a buffer or thermal store will be needed, happy to be proved wrong! I didn’t really follow what you meant by ‘turning the power down a bit’ - please could you elaborate? I’ve also asked Windhager to share the control regime details used by the boiler as this is not generally available info.

Cheers

Andy

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Sune

(@sune)

Trusted Member Contributor Professional Installer

Joined: 4 years ago

Posts: 27

03/07/2025 11:53 am

@andycoo that flooring sounds ideal.

Extra volume is usually your friend when it comes to heating systems and I would certainly plan for potentially having to install a thermal store if and when you add the weather comp mixer. When we hook up thermal stores to pellet boilers either the boiler and its sensors handle it (top probe reads cold, return and/or bottom probe reads hot), or we use a latching relay to control the boiler - so that it waits till the store is empty (top probe reads cold) before then totally filling it up (bottom probe reads hot). I bet Windhager have a menu option for connection to a thermal store?

I'm used to Klover appliances as we're the importer for them in the UK - and with them you can easily turn down the power level in the homeowner's part of the controller - which limits the max output. So if you have one that is, say, 93% efficiency at lower output, 91% at higher, and it's connected to a thermal store, then it makes sense to have it at the slightly lower output, assuming that that output is still more than the heat load of the house. I am guessing that Windhager may have some equivalent setting? It might require going through your manual for a while.

Perhaps Thermaskirt could be used to provide a little supplementary heat in the kitchen? And yes electric dual fuel - mainly because then you can have the towel rail on if you choose to when the main heating system is off.

Planning your dream house? Book a simple one-to-one consultation to get you on a good path - here.

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