Hi guys,

Just wondering if someone could give me their thoughts on if our ASHP is the right size.

Our house is ~129.5 square meters, I’ve seen a few posts above talking about the wattage of the ASHP, I asked our installer and they have said ours is 7kW. I imagine its much more complex than “yes, that sounds about right” or “no, that sounds too small” but just wondering if anyone has any insight?

The back end of Feb was a bit warmer so we came in at about £125 for Feb. Bringing our total electricity bill for Dec, Jan and Feb to £415, and a total of 1,742kWh.

Would if incorrectly sized the wattage of the ASHP impact the bills? i.e. would it have to work harder than a more powerful ASHP?

@dant Under sizing certainly would have an effect on your bills as well as putting the machine under fair pressure. I am dealing with one at the moment. The house is a well insulated 'Eco' house and has a 9kW installed when it should realistically be around 12 to be comfortable.

As a result, it is a new build, they have a noisy outdoor unit and no heating in the last snowy conditions.

Other problems are apparent in the installation.

@dant I am currently looking at my Air Source heat pump sizing and I really will depend on several factors such as how many people are in the property, how well insulated, is it doing the DHW as well as heating, is it underfloor heating etc. The size of house is going to play a major part of the overall requirement, my house is just over 270 m2 and I plan on fully renovating to include lots of new insulation in the entire house and use underfloor heating both ground and first floor. I have been advised that I will need a 14kwh air source pump for heating and the DHW.  I think as Bobbt9866 states under sizing will ensure that it overworks at its full capacity all the time and would be better to have something slightly over to meet the requirements. let us know how you get on but in the first place I would ask your installer WHY do they suggest that size and what is it based on ? you want to ensure a proper heating plan has been done to ensure that it can meet your U values within your home for heat loss etc. Good luck

We have just had a quote for an ASHP for our reasonably well-insulated 165 sq metre bungalow.  We're getting a 14kWh Mitsubishi Ecodan that will run 12 (new) radiators.  It's slightly oversized but the next size Ecodan down (11.2kWh) was borderline.  Your usage sounds OK; I dream of bills like that!  Our electricity usage Dec, Jan & Feb was 8608kW @ £707.  Although most of that is relatively cheap E7, it's a lot and we're hoping for savings from the ASHP system. 

If you're getting RHI you should have a detailed heat loss calculation and specification. 

We have just revisited the issue of sizing and performance with our ASHP manufacturers. There are several components to this, but it boils down two major things. The first is the heat-loss calculation which is based on the fabric (U values) and volumetric space you're trying to heat. This will go some way to determining the size of pump you need.

The second component are the emitters (rads and/or UFH). These two things are connected. The pump needs to be large enough to heat sufficient water to heat the house, but the rads need to be big enough to emit sufficient heat to warm rooms up and keep them warm.

The safest way to determining the right size pump is to get 2-3 installers to put forward a proposal. This will go some way to giving you a ballpark figure of what you need. Compare that to your current boiler's output. In our case, we had 5-6 installers, and most came in around the 14-15kW mark. One company tried to push a 12kW and another 2 x 16kW (32kW) Ecodans. Crazy. The model we went for was a 18kW, and it produces more than enough heat.  

Just started looking into ASHP and some of the observations I’ve read elsewhere are scarey! We have just moved into a 1950’s ish detached home in rural Derbyshire. We were well aware changes would be needed. The house has recently been fully double glazed but that’s it so far as insulation is concerned. The house has also recently been rewired and a solar connection, unused at present, was installed.
So obvious jobs initially is loft insulation, cavity wall insulation and under floor insulation beneath the original parquet. A radon survey is being carried out now and won’t finish until end of May so no building work of any sort until then. The insulation work will then be carried out regardless of the heating solution. At present an oil fired Aga is providing hot water (sort of) and two wet radiators upstairs. All other heating is dealt with (poorly) by wall mounted electric heaters and a lovely log burner stove in the living room.
No mains gas (or sewage) so it’s a toss up between oil, electric or alternative. We both got excited when first hearing of ASHP as an option and fully intend inviting firms to advise and quote. However the risk of paying a goodly sum of money to possibly end up with a noisy, inefficient system and suffering the travails experienced by some is a worry. I don’t know of anywhere that I can even see and listen to an ASHP in action to at least allay the noise factor fear. I will need some convincing during my research because one way or another this house will have a central heating system to radiators installed before next winter. I don’t want it to be by an oil fired boiler but if needs must then……………
Fingers crossed it’s a quiet, efficient ASHP system but the guys that come here to survey, evaluate and quote better know their stuff or else it’s oil. As much as I want to be more green I cannot be cold like this next winter. 🤞

We are in a similar situation and have just signed up for an ASHP system.  We currently have storage heaters, various electric heaters and solid fuel.  We had a night storage Aga but I took that out.  A horrible contraption IMO and cost a fortune to run.  Our house is quite well insulated and we're getting new windows and doors. Like you we will need a full system including radiators and tank so it should be a good oppurtunity to design and efficient system from scratch.  Also like you I'm concerned about some of the horror stories of huge bills and cold houses but as we already have both of these the bar for improvement is set quite low! 

We briefly considered oil but discounted for financial and environmental reasons; I don't think heating oil will be reducing in price long term and the RHI makes up a big chunk of the cost of the new system. 

It will be intresting to know what you decide. 

Hi Morgan,

You are starting at the correct place by improving insulation.

ASHP's do work, and can be beneficial not only for the environment, but also your bank balance, when coupled with Solar PV.

The important requirements are correct sizing, correct installation and correct commissioning and optimisation.

I would suggest that you watch this video and then come back with any specific questions relating to your particular situation.

removed link

I hope that you find this useful.

Regards,

Derek.

Hi Morgan,

For some reason the system has removed the link to the video I mentioned previously.

It is on youtube, so if you copy and paste this it should hopefully find it.

youtube.com/watch?v=vwaRlYbKCkI

Regards,

Derek.

@derek-m, we will add a feature on the weeks ahead to embed videos directly into posts - it's on my to-do list 😀 

hi

My property is 1930's semi detached with 115 sq m floor space. Due to our current refurb we are fitting new UFH downstairs, new electric UFH in the bathroom (apparently it is easier to fit) new double glazed windows and new double cavity walls. So fingers crossed we are well insulated! Our heat loss came to 9287 and our installer has planned for a 10K unit.

with his advice we went for a Vaillant unit. This was based on the fact that they are the newest one's on the market and with branded trust. (i've had Vaillant gas boilers for years, and love them.) Also they are very efficient and quiet. The Nibe came a close 2nd and the Mitsubishi was ruled out on supply problems basis. All 3 unit were very close in price only a few hundred £ between them. 

The company we went with were recommended by our architects, and the designer was very knowledgeable. Plus they were 'freelance' and had a range of manufacturers to choose from. 

Im hoping to be moving in, in middle of May and see how things work.

I feel I know more than I did when I started down this road from the knowledge gained from Mars videos, as I am a complete novice about ASHP's.

So any tips would be welcome and I'll let you know if any issues occur which you need to know about.