Building extension, are we missing a trick to future proofing energy

thebizz

Hi,
We are just starting a renovation and a large extension to our 2-bed bungalow.  It will be about 120m2 and 4 bed.   We do not have a lot of spare cash for the build (quote is literally double the spec from 2 years ago but nothing we can do about that).  We do want to make sure we are not missing something though with the future-proofing of the house for the next 20 or so years.  
The plan for heating is very basic.  We are replacing the current gravity-fed gas heating system with a combi boiler and adding radiators to all the additional rooms.  The radiators will be a higher btr rating than needed just in case we end up with a low flow solution in the future. The current gravity-fed system has a pretty new boiler but it's in the garage (which will lead to some lost efficiency) and we are having a large dormer conversion so losing the need for a water tank frees up a square meter of living space.
There is also just the 2 of us so the instant hot water would be more efficient than having a tank of hot water that probably will not be used.
We asked about heat pumps and got a price of 10k back and a lot more than that for the ground source.  The initial costs and being a new technology put us off and the only other alternative is electric which unfortunately seems is still too expensive to be anh option.
We are going with radiators in the house.  We did consider underfloor heating but did not want to go retro dig out all the existing concrete floors and doing just the extension seemed a waste for all the extra plumbing that would need to go in.  We are also fans of thick carpets in most rooms.
The existing house already has cavity wall insulation and we will replace all windows with the current spec.   The extensions all have a fair bit of glass in them (4 m doors and 3 m lantern).  We are going with the most energy-efficient glass we can afford to cut down on heat loss.  The rest of the extension will meet current regs or insulation but we are limited a bit by having 70m2 of flat roof (no option with planning).  We will however get at least 150mm of pir with insulated plasterboard all around also.
The rear garden gets sun all day long (SW facing and elevated) but we are having flat roofs across the whole of the back so I don't think we can use solar panels otherwise we would have definitely had them even though the math for payback wasn't that great.  We have a big garden and a large detached flat roof garage so maybe solar panels could be an option if they are on a tilted frame somewhere but not sure if we need planning for that.
We are both working and will not be entitled to any means-tested schemes.  
The builder/plumber are good guys but don't really have any thoughts on anything out of the ordinary.
Thanks for any comments or advice.  

QrizB

Sounds like you've got it all thought out. One comment from me:

thebizz said:

The rear garden gets sun all day long (SW facing and elevated) but we are having flat roofs across the whole of the back so I don't think we can use solar panels otherwise we would have definitely had them even though the math for payback wasn't that great.

You can definitely put solar panels on a flat roof. Hers'a an example from an industrial building (photo from Ace Solar):

Section62

thebizz said:

We do want to make sure we are not missing something though with the future-proofing of the house for the next 20 or so years.  

If the aim is to future proof with a 20 year timescale then I think it is likely to be a mistake to get rid of the hot water cylinder and go combi.

Heat pumps aren't cheap at the moment, but there is a high probability your next boiler won't be a gas combi.

Moreover, if the existing boiler is relatively new and works fine, then dumping it is wasting money which will eat into your energy savings (if any).

Personally I'd keep the boiler and HWC and review the situation with heat pumps when the boiler eventually needs replacing.

If you need to gain 1sqm by getting rid of the HWC then I'd query whether the design of your "large extension" is as good as it could be.  Adding 1sqm to an extension is likely to be cheaper than the cost of modifying the space where the HWC currently is.

FreeBear

Section62 said:

thebizz said:

We do want to make sure we are not missing something though with the future-proofing of the house for the next 20 or so years.  

Heat pumps aren't cheap at the moment, but there is a high probability your next boiler won't be a gas combi.

They are getting close to the price of regular gas boilers. Prices starting at around £1600* for 7KW ASHP. But with a limited number of skilled/qualified installers, it is the installation costs that bump up the overall price.

As far as future proofing the property, I would say insulate everywhere - Walls, floors, and ceilings. Go more where you can. e.g. If you have cavity wall insulation, you could get away with 50mm of Celotex on the walls. Go to 75mm and don't forget to wrap at least 25mm round the door & window reveals (insulating internally gets round a number of headaches compared to external). If you fit uPVC windows with a chunky 80mm profile, that 25mm of Celotex plus 12mm of plywood gives you the impression of a much slimmer frame.

Have a look at installing a mechanical heat recovery ventilation system or a PIV one before having the windows replaced - When they tell you you have to have trickle vents, you can point to the ventilation system and say "nope. Already have background ventilation to comply with Building Regulations".

* https://www.plumbnation.co.uk/heat-pumps/air-source-heat-pumps/

thebizz

Thanks for the comments. Fair enough may not be that large an extension but is in our minds.  The bungalow is about 70m2 at the moment and we will be adding 40m2 loft conversion and a 45m2 rear ground floor extension.  I have just realised that makes it as big as our current 5-bed house!  So much for downsizing
Currently costs 2-3k a m2 square but it's as much the placement of the current gravity-fed system which would make it an expensive use of space.  As it can't practically be vented without being in the middle of a room.  We can go for an unvented tank but then we still have the issue of the current boiler being in the detached garage which seems inefficient. The boiler whilst over specced for the current house would also be at its limit with the new amount of radiators according to Worcester charts. The unvented boiler actually costs as much as a new combi boiler. In the current house, we are still in we pay for a full tank of water each day but do not use it all before it cools so that was the thinking behind the instant hot water although I appreciated it's more expensive per unit.
We have a third of an acre back garden so thought ground source heat pumps would be the way to go but I was shocked at how expensive it was.

It's a good point about the next boiler will be something different.  I'm not sure what that will be though, the heat pump guy actually said he wasn't sure what the future would be but did think that it would get better/cheaper in the future.  If they figure out a way of producing cheap hydrogen though it would all potentially change again.

I will relook at solar panels again.

Alnat1

Why do you heat a full tank of water if you don't need it? We heat ours at 7am for 40 minutes a day, does 2 showers, hubby's shave and a bit left to wash floors etc. We put it on manually for 15 mins if someone wants a bath.

Mstty

Alnat1 said:

Why do you heat a full tank of water if you don't need it? We heat ours at 7am for 40 minutes a day, does 2 showers, hubby's shave and a bit left to wash floors etc. We put it on manually for 15 mins if someone wants a bath.

Shhhh you will wake the heat the tank all the time brigade🤣🤣🤣

Joking aside I agree with this heat only what you need 👍

FreeBear

thebizz said:  If they figure out a way of producing cheap hydrogen though it would all potentially change again.

Pipes across the country will need changing, and 90% of the boilers/stoves/ovens will need new jets.

If interested, look up Hydrogen Induced Cracking, and then have a think about the miles & miles of steel pipes used in the distribution of gas. It is going to need a huge investment in infrastructure before we can use hydrogen in any quantity.

As for hot water, just fire up the boiler an hour or so before you need it (for a bath ?), and just boil a kettle for when you need small amounts. An electric shower will save quite a bit of energy over heating a full tank.

thebizz

Thanks freebear, not sure what the future of home heating will be personally.  I am not sure where the price difference comes in for heat pumps at the moment.  The heat pump was 8 times the price of a good combi boiler and that included locating to a new room.   They were also talking about retrofitting fanned radiators through the house and needed a separate water heating solution.
Maybe I gave up researching too soon.  I think you are right that there is a need for more qualified fitters.
To move to a heatpump in say 10 years, what do you think we could do to prepare for it?  
Good advice regards the insulation.  We will be working with the builders and window fitters to make sure no corners are cut.

thebizz

We may have to put some more thought into heating our water .  
There doesn't seem to be any love for combi boilers?  Is there a reason I am missing?
The tank in our current house has a big tank (designed for a large family) so we set it to lowest temperature and turn it on for 30mins at 6 so there hot water for morning showers and there is warm water for the rest of the day for washing hands etc.  Not having instant hot water from a tap is not an option.
If we got a tank in the bungalow then we would need to get an unvented one.  Fitting costs and purchase costs are actually the same if not more than a new combi boiler.  If we got one we could get one of an appropriate size for 2 of us.  
About 12 years ago in the last house we had a combi boiler and it worked well.  Gave a good shower and fuss free hot water when we turned the tap on.  I guess the higher unit cost for hot water is more of an issue with current gas prices but I was thinking this would be offset by the lack of waste.

FreeBear

thebizz said: To move to a heatpump in say 10 years, what do you think we could do to prepare for it?  
Good advice regards the insulation.  We will be working with the builders and window fitters to make sure no corners are cut.

As other posters have mentioned, oversize the radiators so that they can heat each room with a lower water temperature - Radiators are often sized based on a CH water temperature of 70°C and a room temperature of 20°C and quoted in the specs as either Δt=50 or deltaT=50. For a heat pump, you should be looking at radiators using a Δt=30 or lower. This generally means increasing the size of the radiator by 25% or even 50% (or fitting two instead of just one). The alternative is underfloor heating, but this really needs to be installed during construction and not as an afterthought.

Even if you don't fit a heat pump today, having oversized radiators means that you can run the existing (condensing ?) or new combi boiler at a lower temperature which should enable it to run at a higher efficiency.

Don't forget, a heat pump will (usually) require a hot water tank, so if you like hot showers, you should consider having an electric shower installed.