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UFH + Combi boiler, a happy match?


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Peer pressure has led me to rethink my plan not to have UFH which is now causing me to question my original plan to fit an LPG combi boiler. Overall build cost concerns mean I want to keep the heating system as low tech and simple as possible hence no DHW tank/store is planned.

 

In the past week I have read that a combi boiler is ideal for UFH because the low water temp of UFH fits the condensing temperature range of a modern boiler. An opposing view is that constant cycling to top of the UFH slab will work the boiler inefficiently. 

 

I remain a little circumspect about the advantages of UFH in my self build situation e.g. both of us will work for another 10 years and so heating a UFH slab all day in a less than passiv house feels wrong. In view of this I will regard the UFH as a winter/long-weekend heating source with a few radiators and a couple of wood burners for occasional quick heat autumn and spring.

 

There will be just two ground floor UFH zones with 30 m2 and 33 m2 slabs sitting on a typical 150mm of sheet insulation.

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@epsilonGreedy

Your proposed system sounds very similar to the one I installed in my own build. Like you I wanted to keep everything as simple as possible.

The main difference is that mine is a small 71sqM single storey holiday home. It’s built to minimum building regs insulation values and I have a bulk 1200 litre LPG tank feeding a combi boiler with u/floor heating. The combi also supplies all our hot water and cooking needs.

I work full time so we only use the house at weekends. The heating is set to a constant 24/7 ‘frost’ setting of 12 deg.C but I have a Hive controller and in the heating season before we go there I remotely boost the heating on the Friday night to a normal temperature. Looking at the Hive app after the weekend I can see that the temperature stays at a comfortable level for about half the week. 

We’ve now been through 2 full winters and I’ve only used 850 litres of LPG so far. The LPG cost 30 pence per litre so I reckon our annual cost will work out at about £130 per year of LPG + the rental of the tank.

Edited by Ian
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I've an old Edwardian house but we have 75% rads and 25% UFH - one loop covering the kitchen/dining area on a combi. Works lovely.

 

Due to the age of the house I run the flow temp at 60degC to keep enough heat coming out the radiators but I have a thermostatic mixer+pump on the UFH which brings the flow temp down enough.

 

Also linked to a Honeywell Evohome system which has a cycle timer setting, 5mins on mine I believe.

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33 minutes ago, Nickfromwales said:

Can we assume no PV, or would you like some solutions based on Combi - UFH - PV ? Just so I don't derail this :)

 

 

Best assume no PV. The architect showed an airing cupboard upstairs, I will retain that space for a later DHW thermal store upgrade in say 10 years time and ensure it would not be too problematic to connect up with spare pipes and high current wires.

 

Brick & block construction just meeting current thermal regs but hope to exceed air permeability by a large margin.

 

L-shaped 2-story house 10m x 6m + smaller 2 story block of 4.5m x 4m. All dimensions external.

 

There is in fact an even smaller 3m x 4m single story utility room/toilet block attached to the far end of the 4.5m x 4m block, however given the relatively large perimeter to floor area of that section of the house I feel that a couple of small rads will do there.

 

Current plan is that the upstairs will be heated by 1 small rad in both bathrooms plus 1 mid sized rad in the master bedroom. No science behind that thinking just a hunch.

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Shop around is your answer.  My Air source heat pump cost me £455, I have just bought all the downstairs under floor heating kit for £502, and the hot water tank was the expensive bit at £875

 

That's the heating and hot water system all bar the pipe and some valves for £1832.  How much is a combi and your radiators?

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15 hours ago, epsilonGreedy said:

Best assume no PV. The architect showed an airing cupboard upstairs, I will retain that space for a later DHW thermal store upgrade in say 10 years time and ensure it would not be too problematic to connect up with spare pipes and high current wires.

With a combi all your hot and cold pipework would be in 15mm. Upgrading later would require larger bore pipework, or tees cut into the 15mm supplies at the max load point and larger bore pipework taking to the A/C now ready / in anticipation. 

Are you saying PV may feature later on ? If no then why waste time and spend money future proofing that far ahead ? At the most I'd run the cold feed to the combi VIA the A/C so a Sunamp could be fitted for preheating the DHW cold feed to the combi. That's one pipe and one 13a fused spur.  

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