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.

[take2]

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.

[take2]

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.

[Lucky C]

We have an addition that had an awful attempt at being heated by ductwork (1 small vent with no return), and we are fine with that being more or less a three season room, so we put in a mini split to take care of both heating and cooling needs (in the northeast so heating is more important). So in the dead of winter when the heating will be inefficient, we won't bother running it unless needed. The room wouldn't get down below freezing except in extreme cold weather anyway, due to being attached to the main house, solar load, and ground temperature (sits on a slab).

That type of special situation where the main heat source doesn't cut it, and you don't need to run the unit during extremely cold temperatures, is where these units do well.

[Riggerjack]

You have a gas boiler, so your existing heat is piped, with water radiators.

Converting to a heat pump would either mean air to water, to use your existing radiators, or running new ducting, to distribute the heat. Heat pumps don't provide as hot a heated air as gas, so even air ducted furnaces usually need a duct upgrade for conversion to heat pumps.

Another option is a ground source heat pump, which is set up for water to water transfer. But that heat must come from somewhere, and I am thinking you have eliminated this option for lack of that source. Consider vertical wells, but remember that you would be robbing heat from the whole property, to heat the house. Maybe just a few degrees, but that can make a difference...

Remember, that all electric resistance heat is 100% efficient, so those energy sucking resistance coils people complain about, are as effective as baseboard heaters or wall units, or incandescent light bulbs, or LEDs, for that matter.

[Lucky C]

Fossil fuel power plant -> electric grid -> resistance heating is usually less efficient than a fossil fuel boiler in the house, which can be close to 90% efficient for oil and over 90% efficient for gas. The main inefficiency is at the power plant converting from fossil fuels -> steam -> turbine -> electricity (there would be large variation in inefficiency from plant to plant), but there are also some transmission losses in the electric grid. So with resistance heating you will likely be facing higher emissions (if the source is a fossil fuel power plant) as well as almost certainly higher costs to run the electric heat.

If the electric heat is from a renewable source then maybe your emissions would be lower than from a fossil fuel plant, but your costs would still be high. Unless we're talking about something like rigging up some used solar panels with some life left in them that you picked up for cheap, to a space heater for supplemental heat.

[Riggerjack]

Fossil fuel power plant -> electric grid -> resistance heating is usually less efficient than a fossil fuel boiler in the house, which can be close to 90% efficient for oil and over 90% efficient for gas.

Uh, ok. Still, resistance heat is 100% efficient. As in all of the waste is in the form of heat. Most people don't worry much about systemic losses beyond their control.

As to the efficiency of generating and transporting energy, that's a completely different conundrum. For instance, where I am, in the PNW, most of our electricity is hydro, but we still have transmission losses.

But fossil fuels aren't in most people's back yards. How efficient is a 90% fossil fuel furnace, when you factor in the energy to explore, exploit, refine, and transport it to the customer? Which has the higher costs for the environment? Honestly, I don't know.

I would expect anyone who was worried about it to just install a PV solar array, then round off the energy costs to produce and transport the PV solar array, because eventually, a decision needs to be made...

[Lucky C]

vexed87 stated the goal is to reduce emissions, so yes, losses beyond the power line must be taken into consideration. I fully understand the concept of 100% resistance heat efficiency within the home but it's not an appropriate argument in this context and I'm sure vexed87 is already way beyond that level of understanding anyway. Fossil fuel exploration, extraction, refining, and transportation losses/costs would apply to both fossil fuel power plants and residential customers, but would vary greatly across different plants and customers. Generally, an efficient combustion heat source inside the home should be a source of lower fossil fuel emissions than a sourcing electric resistance heat from a fossil fuel plant. However vexed87 is also on a renewable electricity plan so maybe a space heater supplement in certain strategic locations is appropriate.

[Riggerjack]

@ lucky c,

You seem to have an odd obsession with fossil fuel justification.

. However vexed87 is also on a renewable electricity plan so maybe a space heater supplement in certain strategic locations is appropriate.

Read up on heat pumps. They don't create heat (except as waste) they move heat. Wikipedia says:

Air source heat pumps can provide fairly low cost space heating. A high efficiency heat pump can provide up to four times as much heat as an electric resistance heater using the same amount of electricity. ...

So, fossil fuel to electricity involves loss, transmission causes loss, but at point if use, heat pumps add a multiplier. Up to 400% multiplier. Though he is going with an air source in the UK, (mild climate) so I wouldn't be surprised that the actual multiple is only 1.5 or 2.

I bring up the 100% efficient resistance heat, because it is very common for heat pump users to complain bitterly about how inefficient resistance heat is. And it is, comparatively. But rarely is a real comparison done.

How much transmission loss is necessary for a fossil fuel furnace to emit less carbon than a heat pump? Assuming that the power plant is only as efficient as the furnace (I think this is unlikely), and that literally half of the electricity generated is lost in transmission (again unlikely) and that vexed87's heat pump only has a multiple of 2, he still comes out ahead a furnace, in emissions. Costs will vary.

This math gets complicated, and sources are not clear, and all factors are local. But it takes a pretty harsh set of circumstances for a furnace to come out ahead of a heat pump, in emissions. But much easier for costs.

I think if we were talking about The Animal, in central Alaska, a furnace would come out ahead in emissions. Harsh climate and localish fuel.

[Lucky C]

@Riggerjack yes I understand. I have an oil furnace and ASHP for an addition and know all about how they work. I would have upgraded to a GSHP but the cost would have been twice as much as I had anticipated, so it sadly was not within our budget. I would always prefer heat pumps over fossil fuels.

The problem is I misunderstood when you were only talking about the resistance coils in a heat pump. Sorry! I guess I skimmed the post too quickly and thought you were talking about electric resistance coils in general, as if running your whole house on electric space heaters 100% of the time was a fine low GHG emissions solution. I figured you knew what you were talking about, but didn't want other readers to think that using purely electric heat (no heat pump) was a good efficient solution. Though I would hope most on this forum would be smarter than that.

[Riggerjack]

@ lucky C

Thanks, I think we both had the same concerns, approached from different angles.

FWIW, my math at the end of the last post is... sloppy. But I'm leaving it in place, to show the concept, regardless of accuracy. Or, because I am lazy...

[ZAFCorrection]

My, admittedly still fuzzy, thinking is that heat pump + PV is a robust individual solution assuming some future risk of gas not being available at a reasonable price. It's questionable whether throwing up a bunch of solar panels and the other gizmos for a renewable, all-electric system is good for reducing energy emissions, but it does the have the advantage of being maintainable in some fashion through an adult's lifespan down to the level of tinkerer/scavenger, assuming the panels themselves are not damaged.

[Lucky C]

To help figure out a rough estimate of how much the electric heater would kick in vs. the efficient heat pump, I used www.degreedays.net to see Heating Degree Days (HDD) for the past few years. If you use it, make sure you pick a highly rated weather station (3 star rating).

Say I want to keep a room at 65F all winter long. DegreeDays data in my area shows about 5500 HDD/year for a 65F base temperature. Now if I re-run it with base temperature set to 32F, it drops to only about 400 HDD/year. This is lower than I expected at only about 7% of the total heating load. I guess it makes sense since even in the winter months, a lot of days get above freezing. I guess I'll try setting a timer to only heat during the middle of the day in the winter and not care how much it drops at night. Then I won't be going into the inefficient range much at all.

My mini split COP is around 3 (efficiency multiplier 300%). This COP is based on its HSPF spec (Heating Seasonal Performance Factor) which is supposedly the effective efficiency over the entire season, but the industry doesn't make it very clear exactly how that seasonal adjustment works.
However the new models these days are very efficient and designed for below-freezing operation anyway, so I guess I shouldn't really be concerned with such a small proportion of my heating load in the < 32F range.