Caernarfon 18kW Air Source Heat Pump Review – Autumn

by Mars
air source heat pump in snow

Our Caernarfon air source heat pump has now been operational since the spring. It’s time to see how it performed over the course of the past three months as part of our autumn air source heat pump review.

To this point, our air source heat pump has had it easy and has been covered in our summer review.

To see how it performed over the autumn, including a very cold November, please check out this video.

Caernarfon air source heat pump review

For detailed information, please see the breakdown of our running costs below for September, October and November 2019.

Numbers for Sep, Oct, Nov 2019
Solar energy produced1.152MW
Total electricity used (after solar) 2,787mW
Electricity used by ASHP2,164mW
Coefficient of performance (COP)2.5
Electricity cost for ASHP£298
Oil cost in Sep, Oct and Nov 2018£750

Based on our electricity tariff, we spent £298 on heating over the past quarter, which equates to just under £100 per month.

We hope you enjoyed this review, and if you’d like to get informed when our winter update is out, please subscribe to our blog and YouTube channel.

UPDATE Feb 19, 2020: the winter review is available here.


If you’re interested in an air source heat pump from Global Energy Systems, you can use this code when you contact them and you’ll get £200 off your installation (and we’ll get some Amazon vouchers): GESRFAF000160

Notify of

Newest Most Voted
Inline Feedbacks
View all comments
Andris Meszaros
Andris Meszaros
3 years ago

Hi. After watching your last video I had an idea how to solve the problem with those rooms that stay cold.

Put your thermostat that controls the radiator zone into the coldest room. Turn the radiator into full position. Then have all the others rooms set to desired temperature with the thermoststic valve on the radiators. When those rooms warm up the valve shuts off and hot water will only flow into the rooms that are still cold, until the coldest room is up to temperature and then the room thermostat will shut it and the ASHP off.
I hope this helps.

3 years ago

Thanks for the suggestion Andris. I’ve tried what you said, to no avail. The amount of hot water to the coldest room is not sufficient. It’s the only room in the entire house that we can’t get to temperature.

I also removed our smart TRV and replaced it with a conventional one hoping that it would allow for more flow. It didn’t change anything. The temperature was exactly the same.

Reply to  Mars
3 years ago

In another perplexing twist, it’s been colder outside than on any other day this week and we’ve not tampered with any settings or valves. Incredibly, the water is flowing through the rad in the coldest room like a raging torrent and the room is up to temperature. It’ll be interesting to see if this maintains or whether it stops as abruptly as it starting working.

3 years ago

[…] £150 per month on heating versus oil at the same time last year (you can get more info in this post and also see a full autumn review of our heating system). This has meant that we’ve saved around […]

3 years ago

[…] level of acceptance and wanting to try new things is something to worry about. We moved to an air source heat pump to heat our property. It’s not perfect, but we’re learning how to use it and it’s getting […]

3 years ago

Insightful content on ASHP – thanks. I am trying to work out what would be best for my home, which is also a large-ish detached rural dwelling that is heated by oil.

It would be good to understand a bit more detail on your home – specifically the fabric, what work you did prior to going for a heat pump. The ‘fabric first’ mantra is great, but we don’t have the option to retro-fit all rooms in one go!

Reply to  nikcal
3 years ago

Hi Nick

Thank you for comment and you’ve raised some excellent questions that I never thought about addressing before.

The best place to start is with the EPC certificate. Our grade was D when we purchased the house with a potential of C. This is important because if you’re below D, I don’t think you’re eligible for RHI payments, so that should be a consideration.

Your property is likely to be like ours and we got comments from a few installers saying air source heat pumps will not work in our property because the footprint of the house was too large, there was insufficient insulation and that we had radiators that were intended to run off oil.

As it turns out, an air source can heat and keep this house warm, so that’s rubbish.

To answer your questions directly, our Victorian farmhouse is built from old brick – the previous owners put in interior stud walls throughout the property and insulated the wall cavity spaces. This helps keep cold air out and warm air in.

All our windows are double glazed, which is hugely important.

Our first floor loft space is very poorly insulated as we recently discovered, and this is a definite project for the summer that will again assist with heating efficiency. We’ve also had to place beading in front of skirting boards to prevent drafts in some areas in the house. Insulating rooms, one by one, is the way to go and it’s a definite case of doing everything you can to keep cold air out.

Air source heat pumps, as I’m certain you already know, don’t get rads extremely hot – it’s a steady warm heat that heats rooms, so you need to try and keep out the cold… that’s definitely the best advice I have because if you don’t the efficiency will be poor and rooms will never be as warm as you like.

In the coldest north facing rooms in our house, we struggle to get them any warmer than 21C, which I appreciate is sufficiently warm, but you could get those rooms roasting with oil or gas fired boilers.

I think you need to just ensure that you get the right sized pump for the size of your property.

I hope that provides some initial answers to your questions – if you have any more, I would be more than happy to provide you with further details.

3 years ago

[…] installed an air source heat pump, which was an expensive investment, but it means we’ve not been burning 500 liters of oil a month […]

3 years ago

[…] Caernarfon 18kW Air Source Heat Pump Review – Autumn […]

3 years ago

[…] spring and autumn, our ASHP operating costs were much cheaper than oil. The real test was always going to be the […]

You may also like

Please leave a comment – we'd love to hear what you thinkx