Which heating system to use on our new build?

[Johnny Jekyll]

Hi everyone. Our new build is finally water tight. Traditional small 3 bed detached over two levels with cold loft for storage. Based in South London.

 

So now to decide on which heating system to use? Any advice much appreciated.

 

Insulation to be as follows:
300mm floor insulation (EPS and PIR) with screed (probably anhydrite) on top. 
100mm full fill PIR with medium dense blocks both skins, will be adding 62.5mm PIR insulated plasterboard on the inside.
400mm loft insulation.
Excellent airtightness utilizing parged walls, airtight paint and membrane where needed.
MVHR.

 

We are not going down the air source heat pump route. We are going for a more traditional gas boiler system.

 

With the above in mind, what would you go with and reasons:
Vented / Unvented / Combi?
Radiators / Underfloor heating inside screed / Underfloor heating on top of screed e.g. Wunda?

 

Thanks very much. With prices going through the roof, heating bills would I guess be a key consideration.

 

[tonyshouse]

I prefer insulation to heating 🙂 no ongoing costs 

 

what are your U-values?

[joth]

How many showers? If 1 I'd probably do a combi, if more than one shower then an UVC.

I'd put the wet UFH pipes in the screed because no reason not to. With 300mm insulation under it it will work fabulously.

Only the wall insulation letting U values down a bit (and windows?). I'm guessing in South London the plot is a bit space constrained against larger walls?

 

Will you do an airtest? Best do it before the plasterboard is in.

 

Well done. 

 

[Conor]

56 minutes ago, Johnny Jekyll said:

 

 

Thanks very much. With prices going through the roof, heating bills would I guess be a key consideration.

 

 

I think the cheapest is an ASHP running on economy 7 or other night rate tarrif. Works out at about 2-3p per kWh of heat output Vs 6p or more for gas. Who knows what it'll be in the future, bit I can't see gas coming down in price ever again.

 

If definitely gas, go for an UVC as gives you the ability to switch to other heating source in the future, use solar PV to heat hot water ( a bigger energy component in a well imsualted house than people think) and run multiple hot water outlets at the same time.

 

A combi is a far cheaper install (£1k easily) and takes up less space.

 

Have you designed in the required plant space, service voids, floor build up etc for all these options?

[JohnMo]

With the floor installation you have, I would go UFH in screed.

 

Small system boiler, not sure if you get them but one where you can run the heating circuit at a lower temp than the DHW.  Unvented cylinder.

 

Your UFH will come on once per day for a few hours and shut off until the next day, irrespective of what you want it to via thermostats.  Run the whole ground floor as a single zone from a single timer thermostat.  No need for anything fancy, as they will not work as required in a high performance house.  I took my thermostat out except one in the living space and one in each bedroom, as we are in a bungalow.

 

Put electric towel rails in bathrooms.

 

Ask yourself do I need heating in the bedrooms as the heat will move upwards and they will be heated anyway.

 

I went slightly different route.

 

Combi with thermal store upstream, to collect PV immersion heat; to offset gas costs in the summer, the boiler takes the preheated water up to 55 degs (via mixing valve) it adds heat if it needs to only. 

 

Will also be adding solar thermal in a couple of months also, to help displace heating costs in the shoulder months.

 

Boiler works with no short cycling as the UFH manifold (temp set at 30) is fed from the thermal store.  Boiler heating circuit is used to heat thermal store to 34 deg (no real losses from the cylinder) with a 4 deg hysteresis.  Boiler heating flow set to 40.  When running the boiler runs for about 15 to 20 mins and slowly creeps up to a max allowable temp of 55.  Then spends about 10 to 15 mins resting.  (Still making changes to setting to get it to run most efficiently).

[Marvin]

Hi @Johnny Jekyll

 

I think the sizing of your heat source will be related to your hot water demand as my guess is that you will need little heating. Do a heat loss calculation assuming your proposed Airtightness, Insulation and MVHR. (AIM)

This will give you an understanding of how much heat you require, however you have to take into account the low temperatures of winter. My understanding is that the low recorded in London was -16C in 1962, and the lowest 24 hour temp was in 1987 at about -8.3C. 

 

Good luck

 

Marvin 

Edited March 24, 2022 by Marvin
Can't spell

[Johnny Jekyll]

Thanks guys.

 

Forgot to say have 2KW PV solar panels on south roof (45 degrees and no shading), so they should work well for what they are.

 

@tonyshouse windows 1.3, front door 1.5, bi folds 1.6, I think these are the values - all secured by design, laminated, toughened. They will be fine but I understand not near passive levels. Walls will be pretty good once I fit the internal insulation to them.

 

@joth two showers,  yes constrained by plot size, yes will definitely do an air test before internal fit out.

 

@Conor Thanks will look at unvented cylinder route in more detail, plant space is restricted which is why combi is not off the table, quite like the look of a powerful Viessmann if combi. 

 

@JohnMo good info thanks, UFH downstairs on single circuit, then towel rails in bathrooms and small radiator in bedrooms using smart heating thermostats. I don't know anything about thermal store upstream, looks like you have a good system there.

 

@Marvin thanks is there an easy way to do a heat loss calculation?

 

Thanks very much, all info is much appreciated.

[Johnny Jekyll]

I'm currently looking at unvented systems.

Any recommendations? Typical like Worcester Bosch boiler or another make?

Would there be systems which are best to work with UFH downstairs and radiators upstairs?

Sorry for the simple questions. Get to a new part of the build and start learning from scratch. Only ever had a combi in previous house.

 

Also, best finish floor to use. I learnt that porcelain tiles are great for allowing heat to pass through, but would my preferred luxury vinyl tile perform much worse?

Edited March 24, 2022 by Johnny Jekyll

[Marvin]

58 minutes ago, Johnny Jekyll said:

@Marvin thanks is there an easy way to do a heat loss calculation?

I used my own Excel spread sheet which is complicated, but others here should be able to guide you to a simpler one.

 

If you don't get help finding it by tomorrow, start a new topic asking for it.

Edited March 24, 2022 by Marvin

[tonyshouse]

Veissmann boiler if you must have one 

[Iceverge]

4 hours ago, Johnny Jekyll said:

100mm full fill PIR with medium dense blocks both skins,

Edited, sorry I read back and didn't realise your walls were done.

 

 

 

 

 

Edited March 24, 2022 by Iceverge

[ruggers]

5 hours ago, Johnny Jekyll said:

400mm loft insulation.

 

Valliant got some of the best system boiler reviews in my research. Combi's I've found baxi platinums to be excellent, now with 10 year warranty. Insulation in the liquid screed will work the best and 50mm will give a good reaction time. You won't get better than the porcelain tiles for heat conductivity, nice and cool in summer too, LVT will still be great though due to it being so thin. 

 

I've just been told to also use 400mm loft insulation, is this the new standard, it was 270-300mm?

4 hours ago, JohnMo said:

Small system boiler, not sure if you get them but one where you can run the heating circuit at a lower temp than the DHW.  Unvented cylinder.

 

 

 

@JohnMo What's the reason for this, in an unvented cylinder would doing this not mean that the CH is constantly drawing temperature away from the stored DHW, using gas only, how do you heat the DHW tank if the CH is lower?

[Johnny Jekyll]

@ruggersSimply going to put 200+mm insulation between 9by2s and then another 200+mm over with loft legs up to loft floor, so will be minimum 400mm criss-crossing, plus wrapping well over into the eaves which are already at 200mm, plus high quality passive standard loft hatch to finish off. No reason other than doing my very best to insulate.

[Johnny Jekyll]

1 hour ago, Iceverge said:

Edited, sorry I read back and didn't realise your walls were done.

 

 

 

 

 

@Iceverge I saw your original message in my email notification. Yes fully agree with everything you say. If only I could do it all again = It's a very long story (won't bore you). Airtightness should be excellent at the boundary. Am adding insulation inside to lessen cold bridge effect. Again it's a very long story...!

 

[ruggers]

24 minutes ago, Johnny Jekyll said:

@ruggersSimply going to put 200+mm insulation between 9by2s and then another 200+mm over with loft legs up to loft floor, so will be minimum 400mm criss-crossing, plus wrapping well over into the eaves which are already at 200mm, plus high quality passive standard loft hatch to finish off. No reason other than doing my very best to insulate.

Make sure adding the insulation into the eaves doesn't prevent airflow from soffit or facia vent flow, it can sometimes cause mould issues on breather membrane but having your ceiling vapour control and air airtight loft hatch will at least prevent anything from leaving your living space. 

[JohnMo]

7 hours ago, ruggers said:

 

@JohnMo What's the reason for this, in an unvented cylinder would doing this not mean that the CH is constantly drawing temperature away from the stored DHW, using gas only, how do you heat the DHW tank if the CH is lower?

UVC is for DHW only. 

 

I know a combi can deliver different temperatures to CH and DHW.  Would have thought there would be system boilers that can tell the difference and do the same?

[SteamyTea]

13 hours ago, Johnny Jekyll said:

Our new build is finally water tight

Have you got this far without thermally modelling your build?

[Johnny Jekyll]

11 hours ago, ruggers said:

Make sure adding the insulation into the eaves doesn't prevent airflow from soffit or facia vent flow, it can sometimes cause mould issues on breather membrane but having your ceiling vapour control and air airtight loft hatch will at least prevent anything from leaving your living space. 

Indeed, fully on top of that. 👍

[Johnny Jekyll]

4 hours ago, SteamyTea said:

Have you got this far without thermally modelling your build?

 

@SteamyTea thanks for this question, we have all the SAP Assessment Reports, but haven't done any thermal modelling. Building Control haven't asked for anything like that. I'm doing my best to build a really warm airtight good spec house to live in. Sorry can you provide a little more info on thermal modelling? where I can look into it? why I would need it? is it expensive? Thanks.

 

[SteamyTea]

2 minutes ago, Johnny Jekyll said:

Sorry can you provide a little more info on thermal modelling? where I can look into it? why I would need it? is it expensive? Thanks.

Have a hunt around on here for Jeremy Harris's spreadsheet. That is about as good as you need.

As for why you should do it, it is because it allows you to change thermal performance of the fabric elements. That can save you thousands.

 

[Johnny Jekyll]

17 minutes ago, SteamyTea said:

Have a hunt around on here for Jeremy Harris's spreadsheet. That is about as good as you need.

As for why you should do it, it is because it allows you to change thermal performance of the fabric elements. That can save you thousands.

 

 

@SteamyTea thanks for the reply. We are already up and water tight so may be a bit too far down the line for us. But will have a look in case it can help at all. Thanks again. 

 

[JohnMo]

But it certainty helps to know for yourself what each element (floor, roof, walls etc) of houses' U value is and worst day heat loss, even if it's to verify what size boiler/ heat pump you need. Or do a sense check on heating quotes etc. Takes about 20 mins to fill in.

[MikeSharp01]

You can find the JS Harris spreadsheet here:

 

 

 

[Johnny Jekyll]

Thanks guys