How to decide between ASHP or Gas boiler for New Build

[Meabh]

Hi - 

 

We are demolishing an existing bungalow (with mains gas) and building a new ICF home on the East coast of Scotland 500m from the sea (just outside Edinburgh). We will have UFH on the GF, electric towel rads in the bathrooms and MVHR. We are trying to decide how to heat it!

 

The house is made up of 3 "zones" -  2 single storey pods (1 is kitchen/L/D & 1 is the kids pod which can be shut down/rented out when they are older -  3bed, 2 bath) and a 2 story pod with master/office/living/bathroom/plant. They are all one house but you can shut doors to "close them off"

 

We ran the PHPP software out of interest and got 

 - Specific space heating demand 29 kWh/m2
 - Maximum heating load 15 W/m2

 

The SAP passes with

 - ASHP

 - Gas boiler with 4.5kWp of PV (we have a south facing roof that I've always planned to have PV on anyway)

 

How do we decide?  I always thought we'd just go the green route with the ASHP but now we aren't so sure?

 

Thanks

 

 

[ProDave]

The PHPP are annual heating requirements per square metre, so a good measure of overall performance but not something you can size the heating system on easily.

 

What does the SAP say about maximum heating demand?  And what is the overall SAP rating with the gas boiler and solar PV?  Does it only pass SAP with the PV?

[jamieled]

Best get it built before 2024 if planning gas!

Scotland proposes 2024 new build ban on natural gas boilers - Heating and Ventilation News (hvnplus.co.uk)

 

[Carrerahill]

14 minutes ago, Meabh said:

Hi - 

 

We are demolishing an existing bungalow (with mains gas) and building a new ICF home on the East coast of Scotland 500m from the sea (just outside Edinburgh). We will have UFH on the GF, electric towel rads in the bathrooms and MVHR. We are trying to decide how to heat it!

 

The house is made up of 3 "zones" -  2 single storey pods (1 is kitchen/L/D & 1 is the kids pod which can be shut down/rented out when they are older -  3bed, 2 bath) and a 2 story pod with master/office/living/bathroom/plant. They are all one house but you can shut doors to "close them off"

 

We ran the PHPP software out of interest and got 

 - Specific space heating demand 29 kWh/m2
 - Maximum heating load 15 W/m2

 

The SAP passes with

 - ASHP

 - Gas boiler with 4.5kWp of PV (we have a south facing roof that I've always planned to have PV on anyway)

 

How do we decide?  I always thought we'd just go the green route with the ASHP but now we aren't so sure?

 

Thanks

 

 

Go Gas Boiler with PV - but with a caveat.

 

I'll give you my thoughts based on my personal home situation and my professional observations (we have the gas/ASHP/PV/Sunamp debate for new buildings weekly).

 

I have PV and a gas boiler, the boiler is relatively cheap to run, its not even a year old (I made the call to replace with gas and not go ASHP last Feb). On a crisp bright winters days I can also heat some space I am using with some small electric heaters, for free if I want. For example I am looking at electric UFH for the bathroom and adding an electric option to my office and maybe loft room so I can make use of day time sun for heat in the room I occupy most, I can whittle away at my energy import and balance things out a bit through the year. I then have the gas boiler for the simple hot water production and on demand space heating when needed.

 

I am currently working on building services designs for a 4 residential developments ranging from 7 storey flatted development in London to smaller blocks of flats in Glasgow and a house, some will be gas boilers with a PV allowance per house, some will be ASHP some will be Sunamp's charged straight off the grid. When looking at heat loss for these flats or houses and the heating method we cringe at the cost some of them will cost to heat. The fact is electricity is expensive, things like ASHP's are expensive the whole lot will just be painful until electricity becomes cheap to produce nationally/globally.

 

If it was me building from new, I would stick a gas boiler in, a hot water cylinder with a water coil and element fed from PV, more PV than I need, and electric heating options in various spaces, I would spend a bit more on insulation and try and create a building which needs little heat. I would leave in allowance in space and technical spec for more PV and battery storage when prices come down and my long term plan would be to lose the gas boiler, well, if that is the way things go, maybe we will see an alternative gas supply and boilers to suit, in which case I will be pleased I have the gas infrastructure. My thinking is that this will give me green credentials and cheap electricity to use however I wish, but the low capital cost of a has CH system but with options further down the line. 

 

If I go ASHP now, then to my mind, I have just signed away a lot of money on technology the industry is frankly still having trouble with, I also diversify my energy sources, I do think we are going to see more energy issues in the coming years, it could be gas or electricity shortages or sever price hikes. If you had told Germans 2-3 years ago, look boys you might run out of gas and struggle to keep the heat on most would have laughed, it is very real now. Which is why I am keeping my options open, it is why I have a diesel and a petrol car, at present my petrol is cheaper to run as Petrol is 20p a litre cheaper round here and my Petrol & Diesel get almost the same MPG. 

 

Spend some of the ASHP money on future proofing, make allowances for things you might install so you are sitting pretty and can, if you want, when technology and prices improve and maybe electric costs stabilise, go ASHP in the future. High temp versions are coming through now that will work as a direct replacement to a gas boiler without rad changes, maybe even increase panel size so that come slightly lower temps you still have good heating from them. 

 

[ProDave]

I will counter the above with my positive results with a modern well insulated house and an ASHP.  The annual electricity used by the ASHP to heat the house is £400 at todays high electricity price, but it would not be a lot cheaper if we had mains gas available (we don't)  Factor in the gas standing charge and they would be about the same.

 

Fuel costs of all types is rising quickly, so the best thing you can do whatever heat source you choose is insulate the building to death and make it air tight with an MVHR ventilation system.  And install wet under floor heating so if you do choose a gas boiler now, you can easily swap to an ASHP at a later date.

 

AS much PV as your roof will take or as much as your DNO will allow.  Then having an ASHP makes it easier to self use more of the PV you generate.

 

If you do choose gas, choose a system boiler with an unvented hot water tank. That will allow you to dump surplus PV not being used to hot water.  The very worst thing you could choose is a combi boiler. 

[SteamyTea]

17 minutes ago, Carrerahill said:

technology the industry is frankly still having trouble with

No, under qualified fitters, not the industry as a whole.

Entrenched ideas and philosophies are so hard to get rid of, I would terminate any conversation with a plumber that was negative about them, even if I was asking to have a combi boiler fixed.

[Carrerahill]

54 minutes ago, SteamyTea said:

No, under qualified fitters, not the industry as a whole.

Entrenched ideas and philosophies are so hard to get rid of, I would terminate any conversation with a plumber that was negative about them, even if I was asking to have a combi boiler fixed.

I would say this technology has many hurdles, one of them is certainly installers who don't know enough about them therefore, so it is 'technology the industry is frankly still having issues with', however there are many more issues with them from size, space to install them (block of flats, where do you put them?), some can be noisy, low temp issues on some models (creeping up now), not a good direct replacement for gas systems etc etc. 

 

In a new build with a properly spec'ed system they will be fine, but this is not the majority of UK housing stock, the industry needs to solve the retro-fit issue and they need to become more mainstream before the industry will come to grips with ASHP's.

 

We still have issues with gas boiler installs! Nearly every plumber you talk to always says, "We will just put in the big one and that will cover everything" which has lead to many issues over the years, including reduced efficiently, short cycling, boilers not condensing because they are running too hot etc. etc. 

 

 

Edited August 5, 2022 by Carrerahill

[JohnMo]

Couple of issues I see first off.  The 3 zones, will play havoc with either a gas boiler or heat pump.  As your heat demand on any one zone will be really low, a big buffer will be required for either system.  Our heat demand is similar to yours (slightly higher), the min turndown on our boiler is 6kW, with a max demand of around 3kW (no sun, -5 degC). We installed a 180l buffer.

 

We went with gas, as at the time I couldn't make the numbers work for an ASHP, with gas on the doorstep.  Still can't really.

 

We have a combi boiler, that can accept pre heated water.  In the summer excess PV is diverted to the immersion in buffer, to heat it up, a DHW coil in the buffer heats water up on its way to the combi.  A solar diverter valve is used, if above 45 degrees, water goes direct to the tap, if below 45 the water goes to the combi for further heating. Works well, our gas bill for the last couple of months has been mostly the standing charge.

 

Couple of different ways you can run your heating, dump heat into the floor for about 6 hours, the heat will then trickle out over the next 18 hours, then repeat.  Or very low flow temp constantly.

 

The call on gas or electric is your to make, any system can work well, as long as thought goes into the design of a low temperature heating system.  Then the system is pretty much future proof for any boiler design.

 

[Marvin]

Hi @Meabh

 

Lots of good stuff above...

 

As you are building new you could choose either the gas or the ASHP route.

 

As @JohnMo says the zoning could be tricky. But not only for heating but also for cooling (if your using an ASHP) and the MVHR system.   

 

@ProDave hits the nail on the head about peak heating demand. I designed our home to heat to 23C when the outside temp is -8C - four degrees below the lowest temp here in the last 10 years. Also I put as much PV on the roof as possible. - great advice if your going for PV in my book, especially if have a Solic 200,an ASHP and/or EV (Electric Vehicle) We have AIM (Airtightness Insulation, MVHR) and APE (ASHP, PV, EV). AIM works well as a group, and I think that's where your at.

 

I have a different opinion than some about using PV.

APE also works well as a group.  We use the PV to heat the hot water 205 storage tank using a Solic 200. When the ASHP is not needed for heating or cooling we turn it off as on standby it uses about 93Watts an hour, which is more per day than we would use to heat the hot water using the Solic 200. We have had hot water by this method for weeks now. Also it stops the need for the cooling ASHP to suddenly change tack and heat the hot water when cooling the home on a hot day.

 

As @Carrerahill says most of the UK housing stock fails to achieve a suitable standard for ASHP ( poor AIM levels).

 

Good luck

 

M

 

 

There have been many discussion on Build hub about ASHP installations and I am of the opinion that from an installation point of view they are more technical than a gas boiler with regard to positioning the various items and they are more complicated to programme and fine tune for each individual property than a gas boiler. That being said we are happy.  

 

Our EV is at home most of the time and we have charged it up fully buying the power about 5 times this year. Most of the time we use a little mains or no mains at all when charging. When home and not full we plug in the car and through the use of CT clamps and relays we slow charge the car when the PV is producing over 3kW. We estimated that our PV power costs about 10 pence per kWh assuming it all lasts 7 years. We achieve about 4 miles per kilowatt.

 

 

 

 

[Ferdinand]

Some very good comments above 🙂 .

Am I correct that a Solic 200 is essentially the same as an Immersun?

[SteamyTea]

33 minutes ago, Marvin said:

about 93Watts an hour

 

33 minutes ago, Marvin said:

We achieve about 4 miles per kilowatt.

You mean kWh, for both 

So the ASHP is using about 100 W, 2.4 kWh a day.

The car is using 0.25 kWh per mile.

[Marvin]

2 minutes ago, SteamyTea said:

 

You mean kWh, for both 

So the ASHP is using about 100 W, 2.4 kWh a day.

The car is using 0.25 kWh per mile.

Yes.

[Marvin]

2 minutes ago, Ferdinand said:

Some very good comments above 🙂 .

Am I correct that a Solic 200 is essentially the same as an Immersun?

Yes. Any brand will do in my book as long as it works. I think some can be connected to more complicated setups but ours is simple.

[SteamyTea]

3 minutes ago, Marvin said:

Yes.

Edit on your blog as well 

[Marvin]

Just now, SteamyTea said:

Edit on your blog as well 

Goodness your busy! will do

[Ferdinand]

1 minute ago, Marvin said:

Yes. Any brand will do in my book as long as it works. I think some can be connected to more complicated setups but ours is simple.

 

I see that in your blog you noted that it could deliver up to 3kW.

 

Is that the limitation of the Solic 200, or the solar setup - thanks?

 

(I have 9.98 kWp of solar panels, and am working through options.) 

[Ferdinand]

1 minute ago, Marvin said:

Goodness your busy! will do

 

I thought ST worked in a cafe, so intermittent.

[Marvin]

2 minutes ago, Ferdinand said:

 

I see that in your blog you noted that it could deliver up to 3kW.

 

Is that the limitation of the Solic 200, or the solar setup - thanks?

 

(I have 9.98 kWp of solar panels, and am working through options.) 

The Solic 200 that I have will only allow up to a 3kW immersion. We have 5.12kW PV installed. Wish I had room for 9.98kW PV ...... We could do it but we would have no garden left.

 

M

[JohnMo]

I only have 3.1 kW installed, but have a garage begging for some to be installed on it.  Prices are too high at the moment due to a high demand and demand v supply issue.

[Marvin]

7 minutes ago, JohnMo said:

I only have 3.1 kW installed, but have a garage begging for some to be installed on it.  Prices are too high at the moment due to a high demand and demand v supply issue.

Took us £360.00 and 3 months to get DNO permission to go over the 3.68kW installation....

[JohnMo]

Was looking to go fast track G99 route, with zero export inverter.

[SimonD]

52 minutes ago, Marvin said:

I am of the opinion that from an installation point of view they are more technical than a gas boiler with regard to positioning the various items

 

I can't speak of the technical programming of the heat pumps themselves, but I think there's a big difference between what should be done and what is done in the market. To design a heating system using a heatpump or a gas boiler is exactly the same process if you want a properly designed heating system. As follows:

 

• calculate heat loss
• calculate hot water demand
• calculate total heat demand
• size heat emitters based on design temperature drop of the system and flow/return temperatures
• select your heat emitters based on heat factor and pipework emissions (this also varies depending on radiator connections)
• calculate system flow rate including pressure drop and flow velocity
• size pipework and pump accordingly (with heatpumps this might include the buffer)

The differences between your fossil fuel boiler and heatpump is largely the flow/return temps and temperature drop across the system (which can be managed using a buffer in heatpumps) and thus flow velocity and rate.

 

Using this method it's a fairly simple process to future proof the heating system for upgrade to heatpump in the future.

 

The problem as I see it is that most installers doing gas/oil, don't bother to do these calculations, using rules of thumb instead which can be totally random. But with these systems you can get away with it because you just chuck in a big engine. Not so with a heatpump. I reckon if we rejigged all the existing fossil fuel systems we'd manage to save well in excess of 15% of gas, if not a lot more and have decently heated houses.

 

Talk in the industry appears to be edging towards hybrid heatpumps using small heatpumps added to existing fossil fuel boiler or new hybrid boilers as a transitional phase.

[JohnMo]

The design for gas boiler is usually for a high temperature system.  You can design it for a low temperature system (as ours is), but this totally confuses the normal plumper.

 

But a low temp system is the same design broadly, the delta T can be the same for both a gas boiler or heat pump.  Our boiler at 25 to 30 degree flow temps does so with a 4 degree delta T.  Our heat load is low so the pipe sizes can be normal sizes.

 

Most gas boilers in a low energy house will need a buffer to operate without short cycling, not just heat pumps.

 

The trouble is to rejig the existing fleet of gas or oil boilers to run low temperatures; to get condensing mode all the time, a lot of homes would have plumbing of too small a diameter to get the mass flow through the system, at the required temperature.  Plus they would need new radiators, new controller and a good possibility those with system boilers couldn't do low temperature for heating and high temps for DHW heating.  So isn't a easy fix.

 

Hybrid systems do work, but reducing heat demand is the real fix all.

[Marvin]

1 hour ago, SimonD said:

 

I can't speak of the technical programming of the heat pumps themselves, but I think there's a big difference between what should be done and what is done in the market. To design a heating system using a heatpump or a gas boiler is exactly the same process if you want a properly designed heating system. As follows:

 

• calculate heat loss
• calculate hot water demand
• calculate total heat demand
• size heat emitters based on design temperature drop of the system and flow/return temperatures
• select your heat emitters based on heat factor and pipework emissions (this also varies depending on radiator connections)
• calculate system flow rate including pressure drop and flow velocity
• size pipework and pump accordingly (with heatpumps this might include the buffer)

 

Hi @SimonD

Think your right with all you list but there's a few more I would add:

 

The distance between the external ASHP and the buffer tank.

 

The location and insulation of the external pipework.

 

The location of the buffer tank ie in or out of the thermal envelope.

 

Also, and different to a gas boiler, the lowest temperature strategy which would give the ASHP the highest COP. This may be for instance heating the UFH up during the afternoon to last all night. 

 

Also again, an ASHP will produce a better COP when the difference between the demanded water temperature and the external air is the least, so choosing when to use the ASHP is also an important aspect. Generally, heating your hot water during the afternoon will use less electricity than at 4 in the morning because the temperature difference is usually less.

 

 

 

 

[PhilT]

On 05/08/2022 at 10:42, Meabh said:

The SAP passes with

 - ASHP

 - Gas boiler with 4.5kWp of PV (we have a south facing roof that I've always planned to have PV on anyway)

 

How do we decide?  I always thought we'd just go the green route with the ASHP but now we aren't so sure?

 

If cost is your main priority then gas (no PV), if energy/CO2 reduction then go for ASHP. I was faced with a similar choice 18 months ago but went with ASHP as the RHI grant made the net cost comparable to gas. Now the whole picture has changed due to inflation and grant reduction so my installation net cost has doubled (c. £6k to £12k). If that had been the case 18 months ago I may have gone for gas. Personally I would not consider PV as I don't have an electric car and my hot water usage is so tiny in the summer, they just don't make any financial sense for my situation, yours may be different.