Electric or wet underfloor heating

[Anonymous]

I am currently involved in the design of a small 45sqm chalet style dwelling, Walls are timber frame insulated with wood fibre and external wood fibre sarking board to eliminate cold bridging. Roof is insulated between rafters and below with turf roof to north and 1.5Kw (might actually be 2Kw) solar array on the south roof face.

We are yet undecided whether to have a suspended timber floor or reinforced concrete slab this might relate to choice of underfloor heating system.

My question relates to the heating system. Secondary heating will be provided by a small wood burner about 3-4kw in the main open plan living space. The primary heating source will be underfloor heating. We can not decide whether it is better to use an electric underfloor heating system or a wet system running from the hot water cylinder.

We want to have a hot water cylinder for domestic hotwater powered by an emmersion heater so we can make use of the solar electric generated. (I should point out that this is a replacement dwelling and the PV array already exists and is being reused. There is not enough roof space to incorporate a solar thermal panel.)

Can anyone suggest whether it will be more efficient / cost effective to run to use a wet underfloor heating system running from the hotwater thermal store, or to use an electric underfloor heating system.?

Cheers Pete

[Anonymous]

P.S. Forgot to mention there is no mains gas.
Cheers Pete

[Anonymous]

I am currently involved in the design of a small 45sqm chalet style dwelling, Walls are timber frame insulated with wood fibre and external wood fibre sarking board to eliminate cold bridging. Roof is insulated between rafters and below with turf roof to north and 1.5Kw (might actually be 2Kw) solar array on the south roof face.

Nobody has answered yet, so I'll dive in …

How well is it insulated? What are the U-values? How much heat will it need to keep it warm in winter? Does a 'small chalet-style dwelling' mean that it is inhabited year-round or only part of the year?

We are yet undecided whether to have a suspended timber floor or reinforced concrete slab this might relate to choice of underfloor heating system.

My question relates to the heating system. Secondary heating will be provided by a small wood burner about 3-4kw in the main open plan living space. The primary heating source will be underfloor heating. We can not decide whether it is better to use an electric underfloor heating system or a wet system running from the hot water cylinder.

We want to have a hot water cylinder for domestic hotwater powered by an emmersion heater so we can make use of the solar electric generated. (I should point out that this is a replacement dwelling and the PV array already exists and is being reused. There is not enough roof space to incorporate a solar thermal panel.)

Can anyone suggest whether it will be more efficient / cost effective to run to use a wet underfloor heating system running from the hotwater thermal store, or to use an electric underfloor heating system.?

OK, so the secondary heating is the wood-burner. Is that just a room heater or does it heat water as well? And why is it secondary, what does that mean?

You have a hot water cylinder, which in the next paragraph changes to a thermal store. What heats the thermal store? You've said that the PV heats water via an immersion. But that isn't for the whole year is it? Specifically, it won't do much in the winter. So what heats the house in the winter?

Sorry to ask so many questions, but it isn't apparent how to answer your question without more information.

[Anonymous]

Thanks for the reply.
The uvalue for the walls is 0.26
The uvalue for the roof will be minimum 0.18 maybe lower 0.16 still working on details.
The floor will be minimum 0.2
The windows will be minimum 1.6

The chalet has permission for full residential use so can and most probably will be inhabited all year.

The proposed wood burner is a Stovax view 3, 3.75Kw and is just a heater with no back boiler. This will be position centrally in the dwelling within the open plan living / dining / kitchen. I used the term secondary as this is a the term used in SAP for an additional heating source.

I stated hot water cylinder / thermal store I guess this would be dependent on whether a wet underfloor heating system was used. We only have electricity on site as a power source and not sufficient space to store enough wood to use wood as a viable main heating source. My thoughts on the wet underfloor heating where that the low grade heat in a tank could be circulated to heat the house.
The only method we have to heat the tank is electric so it has to be immersion heaters and or air source heat pump. I thought it might be useful to store some of the solar energy produced by the PV as heat to use later.

With regard to the PV panels, the panels face south looking out to sea, and have no obstructions. The project is in cornwall so we have some of the best light in the country.

Hope this additional information might help someone comment and send me in a useful direction.

Cheers Pete

[Mark Siddall]

Hi Pete,
A few thoughts:

1) It sounds like the chalet MAY be used year round, but may not. Which is more likely? (This could influence the selection of appropriate heating system.)

2) I am not comfortable with using high grade energy (elec) for a low grade use (heating water/space). This is really quite inefficient/wasteful.

3) Heat losses from the electrically heated cylindar will be lost to the space (thus heating it). If the challete is not perminantly occupied (at normal household levels) then this heat will then be lost to outside with no functional use – so this becomes even more wasteful of the PV energy. If you want to manage this you will need some sort of control system – more complexity, or careful management by the owner.

4) It sounds like your carbon emissions, either by (grid) immersion or biomass, will be pretty high comapred to LPG (0.57 kg/kWh elec and about 0.45 kg/kWh biomass). Why not reinevest the capital from the wood stove into upgrading the insulation a condensing boiler and a timer/thermostat and reduce the CO2 emissions for no additional cost?

5) What airtightness target are you aiming for? Less then 3 ac/h@50pa?

6) What is the ventilation strategy?

Mark

[Anonymous]

Thanks for the comments,
1) The client intends to let the chalet for holiday lets, this could easily be all year but hence the vagueness of occupation.
2) I also am not especially comfortable with use of electricity for heating purposes, but with no mains gas it is the preferred option by my client. Hence the question on the underfloor heating.
3) Heat loss from the cylinder will add to space heating but this will not be significant enough to heat the dwelling.
4) Dont quite understand this. Are you saying for heating water that lpg is the least carbon intensive.
5 / 6) Airtightness will be around 6m3/(h.m2) @ 50Pa with trickle vents and mechanical extract ventilation.

The client is really keen for underfloor heating partly due to lack of space for rads and for the feel. They need a system that is simple and easy to use. Electric mat underfloor heating is obviously the simplest answer but I am aware it is not such a green solution. I should point out that they are on the origonal feed in tariff so get the paid the higher rate for there electric and they also seem to get paid for the units produced regardless of whether they are using it. So from a financial point of view it makes sense for them to be able to use and store that electric if possible, however for the bigger green debate I can see that is not necessarily the best use of the electric.

Going back to my initial question is there any benefit with using a wet underfloor heating system from a hwc / thermal store perhaps the hot water could be generated using an air source heat pump. I know this is contradictory to the comments earlier about electricity being high grade energy. But underfloor heating is low grade energy anyway and I can pretty much guarantee they will install a HWC with immersion even if I advise differently.

CHeers P

[Mark Siddall]

Hi Peter,
Yes, I'm saying that in practice the carbon emissions will be lower with LGP.
Here is a good primer by Nick Grant and Alan Clarke: https://aecb.net/publications/biomass-a-burning-issue/
(Also check out the thread somewhere on this forum.)

[Anonymous]

OK very interesting article regarding biomass and its carbon intensity. I had my suspicions about all this wood we have been burning.
With regard to this project the wood burner is only intended as a secondary heat source (and NOT to heat the water), so mainly for that occasional cosy evening feel because the place will be mainly used as holiday lets.

This brings me back to the initial question, I know the client doesnt really want a LPG boiler. So can any one advise whether it will be more efficient to run an air source heat pump in conjunction with a thermal store / hwc and a wet underfloor heating system or to use just an electric underfloor heating system.

Thanks Pete

[David Olivier]

I think objections to LPG are inconsistent if the alternative of an ASHP is likely to prove undersized and need resistance heat top up, with greater CO2 emissions. Even without resistance heat, its emissions look level with or higher than LPG and don't forget in the long term it's as possible to make a renewable version of gas as to make renewable electricity.

Also why should other electricity users who only run lights and appliances and heating controls off it have to pay to reinforce the cables of the rural national grid? At their present size some of these would probably melt if most buildings used heat pumps. Most of it was sized for a peak demand of around 1 kWe per dwelling not 2 or 3 or more kWe.

See THE GREEN ELECTRICITY ILLUSION written 8 years ago on this site somewhere and points still mostly valid.

[Anonymous]

Using an immersion to run wet underfloor heating is not going to be intuitive to occasional residents & is likely to lead to the thermal store being depleted when they arrive &/or they turn the heating on. Especially given the differences in temperature it makes sense to keep heating & DHW separate. So, if staying with electric, you have two choices:

1. Electrical underfloor heating mats;
2. ASHP with wet underfloor heating.

Which is the more cost effective depends upon how much you pay for the ASHP, the ASHP COP, how much you pay for electricity, whether you make effective use of Economy 7 & how many kWh are required per heating season.

David

[Anonymous]

There's the RHI and RHPP to factor in too if you're looking at an ASHP.

[Nigel Murray]

Hi Pete,
No-one has picked up your question about concrete or timber floor have they?
For me it depends on the site. If an Easy gradient then concrete can be fast, cheap and incorporate lots of insulation. Any other slope (like in Cornwall) a suspended Timber floor (like Edge'oCliff) is less carbon intensive.
If its an electric UFH… then the smallest heat sink in the floor would seem better for faster heat transfer to the space above.

As an aside, could they fit a Solar thermal system out of the building?

And when you have this fully designed I have a client who may buy the package !
Talk soon.
Nigel

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