Viability of Air Source Heat Pumps?

[vexed87]

We are moving home to be closer to family to aid with child care for the next few years. If the sale of our house and the purchase of the new property goes through, we will be looking at a complete renovation project of a not very efficient 70s home. It could be our 'forever' home, so long as the energy descent is kind to us. The property as it stands has a grid fed natural gas water and central heating system with traditional radiators. The combination gas boiler however is past it's life expectancy and I expect it will need to be replaced imminently. Before we get into remodeling the house, we really need to think about insulating and future proofing the heating system as retrofitting it down the line will be disruptive, costly and messy. I doubt gas for household heating will be going anywhere soon, but noises are now being made about moving away from fitting new properties with gas heating, maybe as soon as 2030. That might mean extending to a ban on sales of replacement gas boilers in attempt for the UK to meet its emission reduction obligations.

Lets say DW isn't yet ready to move away from the space heating paradigm, so that leaves me with just two options. Replace the gas boiler with expected service life of 8-10 years or install an air source heat pump (ASHP) - 25 years. The former is fuss free, but locks us in to gas for another 10 years, the latter will still work with the existing plumbing, but that will mean rerouting some pipes from the current boiler's location to another external wall with more ambient airflow, hence the need to make the decisions before we insulate, replaster and fit new fixtures etc. It's fair to say that gas central heating is the tried and tested approach and I don't know much about the practicality of ASHPs but I hear that they are effective and efficient right down to -25C, so one would work fine in a temperate climate like ours.

My goals are to reduce overall CO2 emissions, whilst reducing heating costs and avoid being part of the CO2 emissions 'problem' as far as practicably possible. We are already on a renewable electricity plan, and don't have immediate plans to install PV solar panels. Electrifying our heating is the next logical progression in my quest to attain some form of sustainability. The heat pump system can be powered by PV panels should we move that way in the future. I know there's the possibility that grid powered ASHPs might inadvertently increase CO2 emissions over gas due to transmission losses and embedded emissions in renewable infrastructure, I'm still doing my fact checking on that. There are two other variables that we can't predict, which are the costs of gas and electricity going forward. Whilst gas becomes more scarce and EROEI falls, one might expect prices will continue to rise as they have been doing recently, however petroleum has shown us that isn't always the case, with demand destruction and price ceilings. Whilst the cost of renewables falls, I expect electricity demand to rocket as we slowly electrify the economy, which might make prices go up too as the generation capacity infrastructure to support the new demand needs to be expended/added. I worry there are too many variables, it feels like picking a horse at the races. What might be true now, may not be true 10-20 years down the line.

Has anyone else looked into installing air source heat pumps in their home to replace an older gas central heating system? Can you save me some leg work, or share some experiences? I would also appreciate any other thoughts on the matter of sustainable water and space heating, even if it's just to remind me that sustainable space heating is an oxymoron.

[anesde]

I renovated a two family home in northern NJ last summer that didnt have gas. I elected to replace the old baseboard heaters with ASHP instead of putting gas in the house.

In retrospect it may have been a poor decision as the cost to install was not cheap (~25K for three outdoor units and 6 indoor units) and I ended up replacing the driveway anyway so putting gas in the house would probably have been a wash. My key factors were ease of use by tenants and utility cost (though passed to tenants I wanted to keep them as low as possible). Gas would have been better on both counts, but as you say perhaps not as environmentally friendly.

A few key questions:

1) Where are you located
2) How well insulated is your house now / how willing are you to properly insulate
3) What system are you looking to install? Warranty is key - some do 10 years parts and some installers give ~2 years labor.

To note that the system working down to -25C isn’t really accurate. The systems start losing efficiency much earlier than that, so whilst they technically will work they won’t be able to keep the inside of the house as warm as it may need to be. This is less of an issue if your house is well insulated, and if you’re generally OK with a cold house in the winter (think 55F instead of 70F). More of an issue for me as it’s a rental. I ended up keeping a few baseboards in as supplemental heat if required. Cost to heat in the winter was higher than gas but lower than pure electric.

If you do elect for it make sure the installer stands behind the product and that it’s high quality. There are some cheap systems out there that will constantly break and leave you stranded if you don’t have a backup. Also hard to get experienced service as the technology isn’t as simple to fix as gas boilers. Mitsubishi, Fujitsu, Trane, etc. Not the Amazon brand.

[anesde]

Apologies, just re-read and saw you mentioned UK.

To be clear - are you looking for ASHPs that have water tanks (i.e external unit pumps air to heat up a water tank which then has radiators throughout the home)? Or air to air ASHPs which have indoor and outdoor units and allow both heating and cooling?

As you’re in the UK I imagine the former. I recently worked on a project to deliver social housing in Ireland where all homes were being set up with air to water ASHPs. We did an extensive analysis on this, and came to the same conclusions you mentioned. Gas was preferred by tenants because that’s what they’re used to but we implemented a lot of educational resources to help them adjust.

I find the UK/Ireland to be quite different from the states in that regard. Gas in the UK is more expensive and seen as worse whilst it’s the opposite in the US (at least NE US). My biggest pro for ASHPs on the rental home was the fact that it took care of heating and cooling, whilst gas could only heat. You don’t really have that problem in the UK, but you also don’t have extreme temperatures. ASHPs should work all year round without issue.

[vexed87]

To be clear - are you looking for ASHPs that have water tanks (i.e external unit pumps air to heat up a water tank which then has radiators throughout the home)? Or air to air ASHPs which have indoor and outdoor units and allow both heating and cooling?
...
You don’t really have that problem in the UK, but you also don’t have extreme temperatures. ASHPs should work all year round without issue.

Thanks, I appreciate your thoughts!

I have looked into Air to water systems, it would mean that it would work with existing plumbed in radiators, a tank would only be necessary to supply hot water to the kitchen and bathroom(s).
Cooling isn't necessary, and I don't think it's possible with air to water. As you say, this far north, our summers are not extremely warm, however having the option for cooling is something that might be needed in the future as heatwaves become more frequent.

As far as insulation goes, we are moving from a very well insulated property to a not very well insulated property, but are prepared to put that work into it. I haven't gotten as far as choosing a particular unit/model as it's still early days with the move, however I hear good things about the HITACHI YUTAKI-M. I think they carry a 10 year warranty.

[George the original one]

The ASHPs I've used were efficient down to freezing. At freezing temperatures, the auxiliary heating coils have to turn on (resistance heating coils) suck power like nobody's business.

[RealPerson]

We had an air source heat pump for many years. It cooled during the summer and heated until 40 degrees Fahrenheit. Below 40 the pump would shut off and the heating was done with a gas furnace.

It all worked well, but I had a few gripes: the pump wore out much sooner than a furnace and is expensive to replace. The other issue was that natural gas became much cheaper that electricity in my area. So it made no sense to use the heat pump in the winter. The whole system violated the KISS rule: early on we had a problem that was fixed under the warranty. It took days of technician time to find out what was wrong. The entire thing was just so complex.

[tonyedgecombe]

The new development behind us has air source heat pumps for each house. The installer told me they weren’t really viable unless you had underfloor heating and a really well insulated house.

[bostonimproper]

Our ASHP starts doing weird things right around 4 C so that's when we switch over to gas heating. Like RealPerson, gas is substantially cheaper in our area than electric per kWh, so we didn't bother with coils. For us the unit was substantially more expensive than just boiler replacement would have been ($18k vs $8k), but it means we get (super inefficient) central AC in the summer.