[Closed] ASHP what to look for

Have a look at "technology connections" on youtube. He's done a few videos about ASHPs. US based, but certainly gives a good basic understanding of the modern tech and options etc.

You don't say how much you already understand about ASHPs so sorry if this is stuff you already know.

Each brand and appliance might have a COP labelled on it but the COP depends a lot on how the whole thing is set up. In simple terms, the COP is higher when the ASHP doesn't have to work as hard. So for example if you have been away for 1 month and you come home to your cold house then switch the ASHP back on to heat the house the COP will be really low as it works hard bring the tempurature up. Once it reaches the desired temp and it just has to work to maintain that temp then the COP is much higher.

It is mostly all the other elements that you consider in your heating system that will dictate how good your COP is. Such as insulation, window spec, radiator spec, piping spec and general layout of the thing.

When designing a heating system it is always best to work backwards so that the actual heating unit you spec is almost the last thing. First you need to look at how well insulated your house is. If you've got money to spend insulate insulate insulate your house as much as possible. The more insulation you have, the less you'll have to warm your house as the less heat you'll be loosing. Same with your windows and doors, these are a big heat leak, seal up all your draughts. This will mean your ASHP doesn't have to work as hard and you'll get a better COP if it has to work less to maintain a certain temp.

Next is to consider the piping and radiators in your house. If you have small or not enough radiators then then those radiators have to work harder to maintain a certain temp in a room/house and they'l typically have to be hotter (as they are with gas boilers). Typically ASHPs heat the water in the system to a lower temp but you will have larger radiators or more of them. The best way to do this is to have a continuous wet underfloor heating system that is heated by the ASHP, one giant radiator in the floor that runs at a low temp. Same principal applies to your pipes, if you have older smaller diameter pipes the ASHP has to work harder to push the low temp water around the system more often. So larger diameter pipes allow low temp water to circulate easier allowing the ASHP to work less hard again.

These are the sort of things that will get your COP low. I would guess most ASHPs will have a pretty good COP claimed on their spec sheet.

@anyexcusetoride - I know very little. I thought the CoP was driven by the ambient temperature, so the higher the temp the more energy available to use for heating.

I hadn’t really considered that whether the unit is maintaining or increasing temperature might come in to it. What’s the cause of this driver- the required temperature of the output from the ASHP?

I have tripe glazed windows, cavity wall and around 300mm loft insulation. In hindsight I should have put additional insulation on the exterior walls in each room when refurbing, but didn’t. I think my rads are oversized, according to the online calculators anyway.

I thought the CoP was driven by the ambient temperature, so the higher the temp the more energy available to use for heating.

It is. How long it's running makes some difference but not much unless it gets to the point of iceing up the exchanger - which is why some have anti-iceing systems beyond just shutting down. Humidity has a role too. If you want numbers I'll go with COP 3.5 if the outside temperature is at the highest you will need to heat your reasonably well insulated house to 1.2 on the coldest night you'll ever experience in the UK.

When designing a heating system it is always best to work backwards so that the actual heating unit you spec is almost the last thing. First you need to look at how well insulated your house is etc etc

For me, massively this. But also that your installer needs to be a genuine specialist and take an interest in all this wider context to make sure both that the heating system properly takes all this into account but also that where there are opportunities for other aspects to reduce the system size or impact its effectiveness, you make that happen. I have been involved in 3 as a client. The people involved have made a vast difference between the best, worst and just good. Annoyingly, my home is the worst of the three. Despite good hardware (the heat pumps themselves work fine in all three cases) and good insulation etc, and being the only new build, I've got some bad basic plumbing and a control issue. Sometimes the simple stuff turns out to be very important.

Air to air heat pumps are the most economical, we're all electric and use an electric shower so an air con unit makes a lot of sense.

This guy has a few ASHP videos and he's seems to know what he's talking about. This one is an installation in an older property. Don't be put off my the title he just gives some reasons why bad installations don't work and shows how to do it properly. The size of some of the rads needed is surprising!

Air to air heat pumps are the most economical

In the short term maybe, but long term the 4.5 COP ground/underfloor units will pay back with reduced running costs. In a new build on a generous plot ground/underfloor is the most econmoical long term, as energy costs rise the pay back period will be reduced.

I did my Part L course last week, and, obviously, ASHPs came up. Apparently, the current Valliant ASHP is the best in the market, it has taken the level of efficiency a step higher. We didnt go into any more details, but the Lecturer, who seems to know his stuff reckons its great, it doesnt use CFCs/refrigerants, it's propane iirc used to transfer the heat.

I'm running a simulation on a UK install of a solar boosted HP system right now!

Basically 2,000 liters of low temperature water buried in the garden and heated by solar electric immersion (to maximise scavanging during the less sunny days). Because the heating really only needs to run for a few hours each day mostly in the evenings when it is dark and colder, having a 4.2MJ/K storage buffer is looking to make a huge diffence to installed CoP 😉

@maxtorque How do you link to water? I’ve got rather a lot in the garden which realistically can’t afford to heat in the winter but maybe could use as you suggest.

heated by solar electric immersion (to maximise scavanging during the less sunny days).

I would have thought that solar thermal would have been better suited for that purpose.

Edukator, in a new build on a generous plot, i think you may have rigged the deck there.😂

I hadn’t really considered it but I could use solar thermal to heat the pool. Is that going to be better than an ASHP and solar PV (for the same surface area of panels?) Although the Solar PV could be used for other stuff (albeit if the house could then efficiently utilise the heat via some sort of heat pump then it wouldn’t be wasted)