What heating source?

[Andehh]

We are half way through our new build, a large bungalow with smaller then average rooms (due to 4 kids & guest bedroom needs, but on a budget) but with significantly higher then average ceilings. 4.5m at high side of building, 3.2m at other side of the mono pitch. All brick internal walls.

 

Builder originally recommended we go with ASHP, and proactively included it in his tender price, same with MVHR. At the last site meeting though, he has recommended we drop the ceilings heights to something more normal, as he is uneasy about the warmth of the place with the warm air rising & leaving the bottom half of the rooms feeling chillier.

 

We declined reducing room heights, so he then suggested using a combination of a gas boiler & ASHP to ensure no risk of UFH struggling to maintain decent warmth during a cold winter. My view is this is too expensive & complicated, so its either ASHP or Gas. We are on mains gas... Builder has been 100% faultless to date, and I do trust him/have a good working relationship with him.

 

Going for a gas boiler in a modern home (SAP of around 87) feels 'wrong', but I always felt Gas would be the cheaper option installation & heating wise...just mains gas being so cheaper vs electric.

 

Just keen to understand anyone else's experience of high ceilings & UFH, and whether there is a risk ASHP could struggle on cold days? I am really 50/50, but starting to lean towards Gas just for the £ savings & reliability of turning up the output.

 

Welcome all views, thanks

[joe90]

Any chance of solar panels on the roof? Time you’re ASHP to daylight hours, win win. With a good COP electric costs about the same as gas to run currently. You need a proper heat analysis to work out your “needs”. In a well insulated house it’s DHW thats the major consumer most of the year.

[Andehh]

Thanks Joe, we have 3.99kw of PV, but cant install any more without risk of losing the FiT. Fitting a solar diverter + slightly over sized tank (300L) is on the cards to try and support the DHW.

[SteamyTea]

27 minutes ago, Andehh said:

Just keen to understand anyone else's experience of high ceilings & UFH

A few on here have UFH with an ASHP and do not bother with heating in the upstairs rooms at all, and they are 2.5+ metres higher.

The energy to heat an extra volume of air is pretty minimum, in itself, losses from walls, windows and air changes will dominate.

Not sure why he (the builder) thinks an UFH system will feel cold at floor level just because the air temperature at celling level may be 3°C higher.

 

Temperature is not energy.

May be worth reviewing the heat loose calculations just to be sure.

[JohnMo]

Let's start with basics.

 

Have you done a detailed heat loss calculations?  What are your U values for floor, walls, roof and windows?

 

We have a 6m high lounge and all other rooms go up to about 3.5m, we don't struggle to heat the house, with UFH.

 

A heat pump heating system is fine as long as you design for low temperature heating.

 

[Andehh]

Final U values of 0.13 floor, 0.15 ceiling, 0.22 for walls.

 

SAP scores are looking to be high 80s, following an 83 score before we increased the depth of floor and ceiling insulation by 25mm (PIR). 150mm pipe spacing. I am going to ask the architect to refresh the SAP calculations to help with this decision, as current one are out of date (due to insulation uplifts) - for both Gas & ASHP.

[Russdl]

40 minutes ago, Andehh said:

we have 3.99kw of PV, but cant install any more without risk of losing the FiT.

There is no FIT anymore.

 

If you have the space for more PV I’d say “go for it” regardless of what other decisions you make.  
 

 

[Andehh]

Original FiT...we have lifted old system, swift replacement of the bungalow below (!) then reinstalling original system like for like to maintain the 'original' FiT.

[Russdl]

Just now, Andehh said:

 

Original FiT

 

Ah. Got it. 👍

[joe90]

53 minutes ago, Andehh said:

but cant install any more without risk of losing the FiT.

Is that the case ? (would anybody notice, do they do spot checks 🤷‍♂️).

[ProDave]

4 minutes ago, joe90 said:

Is that the case ? (would anybody notice, do they do spot checks 🤷‍♂️).

Yes, if your quarterly generation figures are suddenly a lot higher they would notice.

[JohnMo]

Your wall insulation is pretty pants, can you do anything improve?

 

But as it is, your UFH will have no issues heating the place.

 

So to get a heat pump or boiler to work, do a heat loss calculation.

 

Good one here

 

Then you'll know what your working with.  Do the calculation and post your results.  From that you flow temp and flow rate can be calculated.

 

 

Edited August 6, 2022 by JohnMo
Spelling

[ProDave]

20 minutes ago, Andehh said:

Final U values of 0.13 floor, 0.15 ceiling, 0.22 for walls.

Wall values could be better.

 

We have bedrooms with high vaulted ceilings and don't notice much of a temperature variation between floor and high ceiling.  In a well insulated house it all pretty much attains the same temperature throughout.

[Kelvin]

Our previous house was a huge converted barn. Ceiling heights varied throughout the downstairs from 6m to 3.5m. The only source of heating was an ASHP and UFH downstairs.  We never had any trouble heating the place and the insulation could probably have been better in the walls. 

[Andehh]

Thanks all, yes the walls are 125mm Knauf dritherm 37, will always be a regret we couldn't upgrade them to 150 cavities but by the time I realised it was too late. Though one option I am debating it the savings going from ASHP to Gas used to request insulated PB for the external facing walls - though due to 3.5m - 4.5m ceilings everywhere it wouldn't be cheap. Floor & ceiling is 150mm Kingspan PIR.

 

I have done the Excel spread sheet as best I can, it is a real abnormal bungalow, in the sense it is ''H shaped'' with the middle bit being the flat roof entrance hallway and two 'wings' being large 10 degree monopitches coming down to the middle flat roof entrance. As a result we have the 'perfect storm' of big floor areas (230sqm ish + garage), big roof areas (250sqm flat roof) AND high walls everywhere (3.2 low side, 4.5m high side)...with over sized windows/glass everywhere....

 

 

 

 

3180_s4_100J_Floor Plan.pdf Heat loss calculator - Master (1).xlsx

Edited August 6, 2022 by Andehh

[Andehh]

Would very much welcome thoughts from all, thank you for your insights!

[TonyT]

Can’t you over sheet the wall.

 

I have a 140mm thick stud wall with celotex type insulation inside the stud bays and then over laid 50mm on top, service gap/battens/plasterboard.

[JohnMo]

As it stands your heat loss is just slightly over ours, with slightly bigger floor area.

 

5kW ASHP or small system boiler would too large on anything else but the coldest day for central heating - UFH throughout, balance the system so when your living space is 19/20, your bedrooms are 18 degrees.  Bathrooms at 20-21.  Run as a single zone on weather compensation with a 2 degree setback at night.

 

Your biggest demand will be keeping up with hot water, if your bedrooms are being utilised, which would drive a large cylinder and bigger boiler/HP, and a buffer on the central heating.

[Andehh]

Thanks very much John, out of curiosity what are your bills with that setup? Any PV?

[Andehh]

4 hours ago, TonyT said:

Can’t you over sheet the wall.

 

I have a 140mm thick stud wall with celotex type insulation inside the stud bays and then over laid 50mm on top, service gap/battens/plasterboard.

Thanks Tony, yes one thing I am considering is savings made going from ASHP to Gas be used for insulated plasterboard on inside walls.

[JohnMo]

1 hour ago, Andehh said:

Thanks very much John, out of curiosity what are your bills with that setup? Any PV?

I'm on gas combi boiler, and last heating season was not very good, as I had a lot of short cycling of the boiler, three designs later I have where I need it.  Basically a re-plumb of buffer and adding a big plate heat exchanger on the boiler side in addition to the buffer heating coil. 

 

First two months of heating boiler was in continuous short cycle trying run at very low temperatures with a buffer heating coil that was way to small for those temperatures. Used lots of gas while I got my head around the issue.

 

These are issues no one tells you about building low energy homes.  Learned loads in the process though.

 

End of the heating season I was batch charging the UFH for around 6 hours at a time, using 20 to 30kWh a day.  I was heating for half of April and the gas usage was 500kWh for heating and DHW.

 

The last three months with 3.1kW of PV, my DHW has mostly been taken care of by PV. Bills on the region £15 to £20.  Last month I used 60kWh of gas, 20 of those in two days, no solar production and doing a trial on the water heating system.

 

[Radian]

12 hours ago, JohnMo said:

Your wall insulation is pretty pants, can you do anything improve?

 

@Andehh gave the U value of walls as 0.22

An overlay of 50mm PUR might get that down to 0.155 but at what point do diminishing returns start to kick in?

[JohnMo]

Depends on the end user, how far they want to go.  As many say insulation is a one off cost, your heating bills are for ever and with the high prices of energy now, and with no end in sight, who really knows when diminished returns start or stops.

[SteamyTea]

1 hour ago, Radian said:

at what point do diminishing returns start to kick in

Really not the way to look at it.

It is not really just the insulation's k-value that is important, it is the whole design philosophy. 

[Andehh]

Thank you again all!

 

John - really interesting to hear about your challenges, this is something I am aware of & wanting to avoid...low energy house & sizing heating input accordingly. What kW is your boiler out of curiosity? We would go with a System boiler & large tank, but would be interesting to know all the same. Builder suggesting a 12Kw ASHP for the 240sqm of UFH - pre plumbed package deal from Joules, supply & fit. We havn't discussed gas boiler, and i'm not sure what the equivalence is for a 12kW ASHP unit.

 

Agreed on insulation being the priority - but for us its more of a desire to be top 5% not invest to be top 0.5%. Law of diminishing returns is something we are aware of, but the walls are fixed now (ie built & roof going on) and the wall/ceiling are already up to a good standard. Walls I do regret not upping (didn't know enough at the time) and will look at PIR Plasterboard, but one of those things...too late now!