Air source Heat Pumps any good?

[HamsterJam]

Aside from the lower grade heat they produce (they don’t get as hot), the main issue I have with ASHPs is the comparitive running cost.
Unless you have an alternate electricity source like solar, the price capped cost per unit of electricity of the grid is about 4x that of gas. Unless I missed something, that straight away negates any advantage of the 300% efficiency.

 

[XTiffy]

PDW125, very welcome comments.
It is quite common to use heat pumps in Scandinavian countries that regularly have these low temperatures, so what is the problem? Our own installation ten years ago would cycle on and off quite often in the winter but not constantly and no they did not disturb us. Yes they are best teamed up with underfloor heating and serious insulation but there are specially designed low temp. radiators that as I recall have more fins, but are no larger. I do not understand how anybody can quote what they see as "a problem" without a bit of research, which is so much easier these days. The only problem is "sorting the wheat from the chaff". More power to PDW125's pen!
Does it concern me that having all energy supplied by electricity, well yes, but I don't recall any gas boiler I have had in the past 50 years working with a power cut.

 

[Phil Pascoe]

There will be power cuts (you need rose coloured glasses to think otherwise) - there are no major generating capacity increases, sunshine and wind are intermittent, and demand will keep rising.
We have no gas, so the prospect of electricity cuts in mid winter was yet another reason to have a multi fuel stove installed. Three of my immediate neighbours obviously thought the same in the last few years.

 

[Jaco]

The unit is fitted in our courtyard next to the kitchen window.
The water pipes go up the wall into the ceiling to the geyser.
It has a small PC control panel where you set the on/off times and the temperature maximum in the geyser (it has a line up to the geyser) I have set it at 55c
The geyser thermostat is set at 60c
You also set the low temp where the pump kicks in automatically. Set at 40c
(Yes I admit to cocking up the settings and in the middle of winter a cold shower is not appreciated )

This is what our pump looks like.

It is a 3 blade fan 400mm diameter.


These are the fins inside in a half circle behind the fan.
Fan pulls air in from the back of the fins.

The circulating pump is in the bottom of the cabinet.

It has just started running

 

[Terry - Somerset]

Gas vs electric seems to depend on the relative cost which per KW from October will be ~27p for electricity and ~7p for gas - a multiple of 3.9.

The co-efficient of performance of a heat pump is (AIUI) typically in the range of 3-4 depending on manufacturer, ambient temperatures etc. Thus gas is probably a little cheaper right now.

My current system is time served (albeit still functioning) and likely to need replacing in the next year or two. It seems sensible to plan what I would do if/when it fails rather than go into panic mode when it does. So the issue is the future decade or two, not what is now:

• I can install PVs to reduce electricity charges, I can't extract gas from my garden
• what will happen to relative prices over the next decade
• what are the comparative installation costs including any disruptive work required of each

Hence my interest in the subject!

 

[PDW125]

Unless you have an alternate electricity source like solar, the price capped cost per unit of electricity of the grid is about 4x that of gas. Unless I missed something, that straight away negates any advantage of the 300% efficiency.

Some of the benefit with heat pumps when you combine them with underfloor heating and a concrete slab is load shifting - that is you use the slab as a large storage heater and then you can run the heat pump overnight, and the slab then releases its heat during the day. Doesn’t work with rads and you need to do a lot of forward planning on design but no reason why new builds shouldn’t do this.

It is quite common to use heat pumps in Scandinavian countries that regularly have these low temperatures, so what is the problem?

Relative humidity !!! The UK is very damp and the dew point means that the evaporator fins ice up quicker at the lower air temperatures (ie 1-5°C) which requires more defrost cycles. Only way to defrost is reverse the process - that then means the heat pump either uses a resistance heater or draws hot water from the house which reduces efficiency. Another factor here is water flow temperature - the hotter you work them, the more icing up and therefore the more inefficient. Most heat pumps are rated at “A5W35” which means air at 5°C and outlet water at 35°C - on some you’ve got a CoP of over 5 which is quite impressive until you realise how cool 35°C water is ..!!

There are new refrigerant gases in some units now that will output up to 75°C at CoP 2, these are ideal for retrofit but tend to be more costly, but the tech is going the right direction

but I don't recall any gas boiler I have had in the past 50 years working with a power cut.

No modern boiler will run without electricity - all the controls require 230v as does the pump. Wood stoves are the answer there !

 

[Dave Moore]

Apparently Octopus are offering heat pumps for £500 at the moment.

 

[yngndrw]

Aside from the lower grade heat they produce (they don’t get as hot), the main issue I have with ASHPs is the comparitive running cost.
Unless you have an alternate electricity source like solar, the price capped cost per unit of electricity of the grid is about 4x that of gas. Unless I missed something, that straight away negates any advantage of the 300% efficiency.

I'd argue that running colder is a higher grade of heat. Both heat pumps and even gas boilers should really be run continuously, modulated by the outdoor temperature. This means that neither will cycle as much (Only when the required output drops below the minimum modulation.) - This is important because, aside from the wear of heat cycling a boiler every time it heats up and cools down, for a heat pump there's a large inefficiency during startup. There are a few other benefits such as reduced peak heat load which for a heat pump means less icing, continuous condensate production which means that the condensate pipe is less likely to freeze, continuous hot water flow which means that for an outdoor unit the pipes won't freeze. For gas boilers, the lower temperature also keeps it in the condensing region for longer, improving efficiency.

The other benefit is that your radiators remain a constant just-over-ambient temperature and your house remains a consistent temperature. I'd take that over piping hot radiators personally.

The issue for gas with doing the above is that fuel boilers are traditionally oversized, which means that they don't modulate down low enough, so they'll always be cycling.

For running cost, see below.

Gas vs electric seems to depend on the relative cost which per KW from October will be ~27p for electricity and ~7p for gas - a multiple of 3.9.

The co-efficient of performance of a heat pump is (AIUI) typically in the range of 3-4 depending on manufacturer, ambient temperatures etc. Thus gas is probably a little cheaper right now.

My current system is time served (albeit still functioning) and likely to need replacing in the next year or two. It seems sensible to plan what I would do if/when it fails rather than go into panic mode when it does. So the issue is the future decade or two, not what is now:

• I can install PVs to reduce electricity charges, I can't extract gas from my garden
• what will happen to relative prices over the next decade
• what are the comparative installation costs including any disruptive work required of each

Hence my interest in the subject!

I don't know what the current gas price is as we're on oil, but I think you might have missed off the boiler efficiency. If we assume a boiler efficiency of 80%. (Probably generous considering most boiler installs.) the gas cost would raise to 8.75p/kW which brings the break-even COP to 3.1.

Gas is actually where heat pumps make sense, a well-designed ASHP should be able to achieve a SCOP of over 3.5 which will easily break even and that's not considering the PV benefits you've mentioned. For oil things are a little harder as oil is very cheap right now. (Although that depends on where you are in the country) The break even cost for us right now on oil is 4.2 which is GSHP territory, but who knows how fuel and electricity prices will change over the next year.

Legionella has been mentioned a couple of times, it's worth pointing out that even when an ASHP is outputting a higher temperature, it should still be achieving a COP higher than a resistive electric heater would. Please see the following chart for a 65C flow temperature. (Which you'd only use once per week in order to kill the Legionella bacteria - In practice you'd see a substantially higher COP than this on average as you'd use a lower temperature such as 45C) The left chart is the COP (Efficiency) with outdoor temperature on the Y axis, compressor speed (Modulation) on the X axis.

COP is affected by the design of the installation more than it is by the heat pump manufacturer. Consider this when looking at the very low installation costs of some companies.

 

[yngndrw]

I did also want to add a bit about insulation as there's a common misconception about older houses and their suitability for heat pumps. The misconception isn't entirely untrue in a roundabout way, but I'd like to explain it a bit as there are some misunderstandings. Sorry if this has already been explained, I did read through the thread but I may have missed something.

The misconception is that old houses (Drafty with poor insulation) are not suitable - This is untrue, in fact the chart I included in my previous post demonstrates this. (That chart is specifically for a flow temperature of 65C, I've included all of the charts for 35C, 45C, 55C and 65C in this response.) You can see in these charts that the efficiency (COP, left chart) depends entirely on the outside temperature, the flow (output) temperature and the compressor speed. (load %) Furthermore for heat pumps we'll typically deal with SCOP (Seasonal COP) which is the COP averaged across an entire year and therefore we can ignore the outside temperature component as long as this doesn't vary wildly from year to year. (The average will work just fine for this.)

In other words, the only real factor (Which we can adjust) which affects efficiency is the flow temperature - Exactly the same as with a conventional boiler.It doesn't matter how much energy you require, the efficiency (and therefore relative cost to a boiler) remains the same. In short, the amount of insulation you have does not affect the relative cost difference between a heat pump and a boiler.

Which brings me back to my caveat earlier, why the misconception is not entirely untrue and what it really means - A boiler is far more forgiving to higher flow temperatures than a heat pump.

A building has a heat loss - That is the amount of energy it loses through it's insulation and this depends on factors such as the outside temperature, the surface area of the building and how the building is constructed. That is to say that an older property will generally require more thermal energy to be put in, in order to maintain a constant temperature. There are a couple of ways to approach this, we could dump in large amounts of energy every few hours or we could put in a smaller amount of energy continuously. The former can be done reactively based on the inside temperature, the latter can be done proactively based on the outside temperature.

Heat transfer depends on a few factors, but for radiators and underfloor the main thing is surface area and the difference between the average water temperature within the radiator and the air temperature. The larger the temperature difference, the quicker the energy is transferred. This means that in order to maintain the same temperature within the property, there's a balance between the water temperature (Flow temperature) and the size of the radiators or surface area of the underfloor heating.

That's the real trade-off with older properties. Older properties have poorer insulation, which means a higher heat loss and more energy is required to maintain a temperature. (The same for both a conventional boiler and a heat pump.) The difference is that the surface area for your radiators needs to be larger as heat pumps are less forgiving. I.e. Older or poorly insulated properties can be just as efficient if the radiators are large enough or you have underfloor heating. Conversely a well insulated property with can have much smaller radiators.

Radiators are often oversized sized for on/off heating so they may well be big enough as is, or they may require an upgrade but this may be cheaper (Or more practical) than upgrading the insulation of a solid wall building. A heat loss calculation is required per property, rather than a general rule of thumb saying that they don't work in older properties.


Insulation is still often the cheapest way of reducing heating costs, but this affects both conventional boilers and heat pumps alike. Misconception that I wanted to tackle with this response is that less insulation = inefficient heat pump, where in reality less insulation = larger radiator surface area.

 

[NormanB]

I'd argue that running colder is a higher grade of heat. Both heat pumps and even gas boilers should really be run continuously, modulated by the outdoor temperature. This means that neither will cycle as much (Only when the required output drops below the minimum modulation.) - This is important because, aside from the wear of heat cycling a boiler every time it heats up and cools down, for a heat pump there's a large inefficiency during startup. There are a few other benefits such as reduced peak heat load which for a heat pump means less icing, continuous condensate production which means that the condensate pipe is less likely to freeze, continuous hot water flow which means that for an outdoor unit the pipes won't freeze. For gas boilers, the lower temperature also keeps it in the condensing region for longer, improving efficiency.

The other benefit is that your radiators remain a constant just-over-ambient temperature and your house remains a consistent temperature. I'd take that over piping hot radiators personally.

The issue for gas with doing the above is that fuel boilers are traditionally oversized, which means that they don't modulate down low enough, so they'll always be cycling.

For running cost, see below.



I don't know what the current gas price is as we're on oil, but I think you might have missed off the boiler efficiency. If we assume a boiler efficiency of 80%. (Probably generous considering most boiler installs.) the gas cost would raise to 8.75p/kW which brings the break-even COP to 3.1.

Gas is actually where heat pumps make sense, a well-designed ASHP should be able to achieve a SCOP of over 3.5 which will easily break even and that's not considering the PV benefits you've mentioned. For oil things are a little harder as oil is very cheap right now. (Although that depends on where you are in the country) The break even cost for us right now on oil is 4.2 which is GSHP territory, but who knows how fuel and electricity prices will change over the next year.
View attachment 166921


Legionella has been mentioned a couple of times, it's worth pointing out that even when an ASHP is outputting a higher temperature, it should still be achieving a COP higher than a resistive electric heater would. Please see the following chart for a 65C flow temperature. (Which you'd only use once per week in order to kill the Legionella bacteria - In practice you'd see a substantially higher COP than this on average as you'd use a lower temperature such as 45C) The left chart is the COP (Efficiency) with outdoor temperature on the Y axis, compressor speed (Modulation) on the X axis.
View attachment 166920

COP is affected by the design of the installation more than it is by the heat pump manufacturer. Consider this when looking at the very low installation costs of some companies.

Congratulations, one of the few posts in this thread written by someone who understands the issues and nuances of ‘the debate’.

Just one addition on the insulation required for a heat pump aspect. As you say in most every case insulation is a good thing no matter the heat source. Where it can present difficulties for a heat pump application is where the property is large and uninsulated resulting in a high heat loss which may put you out of range on the common size heat pumps. This then puts you into the territory of ‘larger’ heat pumps which have a commensurately high power draw (and starting current) which may have implications for your electrical supply system. These units will not be single fan and will require full planning permission. That said, likely a very small percentage of UK housing stock.

 

[Agent_zed]

Something I find interesting is how we can be convinced of things by the 'media'. There seems to be an active push against heat pumps for some reason. The funny thing is though we as humans seem to be able to ignore the faults in the system we already have.

If we were trying to bring in gas boilers there would no doubt be endless articles about how they aren't as efficient as they claim, how it's a con that only registered people can work on the system, the condensate drain pipes freeze up and cause the boiler to stop working, Gas could escape and blow your house up (31 domestic gas explosions/year on average), carbon monoxide could kill your family in your sleep so you need to have a special monitor/alarm that may or may not work even if you buy a reputable one (Carbon monoxide poisoning sends 4,000 people to A&E each year), you or your family could be one of the 863 burn victims from domestic radiators and pipes (1 April 2021 to 31 March 2022 stats). I'm sure there are other things I've missed but you prob get the point. Nothing is ever perfect and there is always someone with a vested interest in making you do something a certain way that benefits them.

 

[glenfield2]

A very interesting thread (albeit pretty much on the limit of my understanding in places).
The problem for me and i imagine many others is that the whole area is dominated by vested interests or, more kindly, specialist knowledge groups.
Our oil boiler is of an age where the next breakdown likely won’t be worth fixing. Our local and excellent heating engineer can quote for and fit a new one but that’s all he does.
If I want an ASHP there are any number of company sales people who can come in and price one up.
But where are the knowledgeable and independent people who can do heating calculations, recommend options and so on across the board?

 

[XTiffy]

When looking at installing heat pumps in a new build 12 years ago (pair of semidetached cottages each with a heat pump), I bought a book on the subject (heat pumps for the home, John Cantor). I used this to be able to understand the subject, when dealing with the firm(s) for installation. I actually used NuHeat, who are in Devon and I paid for one of my employees to attend a one week installation course. We went for underfloor heating and that does match well with Heat Pumps. Though there are radiators available that are designed for a lower flow temperature and are not just oversized "normal" rads.
I am afraid Heat Pumps have become the new "Double glazing installation" scam and avoiding some of these people is important.
There are plenty of heat loss calculation programmes out there. I know I came across several when resarching the subject.

 

[yngndrw]

I personally used Heat Geek to find an installer. (Roger Bisby linked to a video with the owner at the start of this thread) They are a training company (Not just for heat pumps, although that is their main thing now) have a lot of educational videos on YouTube and they do challenge some of the "traditional" wisdoms in the industry with well reasoned arguments. I'm not saying that they are always correct, but their engineers to seem to get good results for both heat pumps and traditional boilers.
https://www.youtube.com/@HeatGeek/videos
They have a map for their engineers:
https://www.heatgeek.com/find-a-heat-geek/
They push a couple of general concepts, mainly for heat pumps as they are more sensitive but these also apply to gas boilers:
- Steady state heating with weather compensation
- Smaller boilers / heat pump (I.e. Sized to the heat loss) to prevent cycling
- No zoning
- Hot water prioritisation

I think the best thing you can do is to learn about it yourself. You don't have to learn everything, just enough that you can have a proper two-way conversation with the engineer and I think that will stop you from being taken for a ride.

 

[XTiffy]

I agree 100% with yngndrw's last post, do your research so that you can make a rational decision.

 

[dickm]

The general feeling that the cowpersons are jumping into this one was confirmed yesterday when I got a letter "to the householder" offering to get me grant aid for all sorts of wonderful heating and insulation stuff, because "your house has been identified as having very low enerty efficiency". We have six inches of fibreglass in the timber walls and ceiling, triple glazing and complete air sealing with heat recovery forced ventilation. I wonder what else a high energy efficiency house would have? They also offered to install a gas boiler. I thought these were due to be phased out as soon as it's not electorally difficult??? Plus the nearest gas main is half a mile away and anyway it's the main high pressure line from St Fergus North Sea terminal, so not really any use domestically!