ASHP - Size and Type

[Conor]

^^ good point. From what I can see, my 9kW has a min output of 2kW.

[jack]

One small thing to consider, especially anyone fitting a relatively low power ASHP:

 

We've been very happy with a 5 kW ASHP doing our hot water and heating for the last few years. Recovery time isn't too bad, and no issues keeping the house warm (roughly PassivHaus levels of insulation and airtightness) and sufficient hot water in the tank. 

 

I've recently found the one shortcoming: we're trying to move over to doing as much as we can in the 00:30-04:30 cheap period we get on Octopus Go. Unfortunately, looking at the data I recently started collecting, I think I realistically need to allow up to two hours in very cold weather (and very cold incoming water) for the tank to fully heat up overnight. That leaves me with only two hours in which to heat the slab with the ASHP.

 

I plan to increase the flow temperature by a few degrees - it's currently 25 deg C (the lowest setting) for all but the coldest weather - in the hope I can get more into the slab during the heating period. However, two hours of heating is not going to be near enough to get us through the day in very cold weather.

 

Another option I'm thinking of is using the immersion heater during the cheap period for at least some of the DHW during the coldest weather. Even if the ASHP could manage a COP of 3 during the coldest part of the year, it's still cheaper to use the immersion at 7.5p/kWh than the ASHP is at the full rate of around 34p/kWh. I'd then have the ASHP for up to four hours to do the UFH.

More data and thinking still to come, but it's an interesting topic. 

[Chanmenie]

@jack how big is your house ? 
I'm aiming for .11u for floor .13u for roof and .15u for walls, not sure what passivehaus levels are.
I’ll be 190m2  so might be able to get away with a 5kw or 6kw unit 

I used Jeremy’s spreadsheet and got a heat loss of 3.1kw but not sure how much to allow for DHW

[JohnMo]

Your house is very similar to mine, my walls 1.4, floor 0.09, roof 1.3, our heat demand is 3.1kW @ -5 NE Scotland.  Our temps have dropped quite a bit more than yours.  So much so, that the heating has been on since the beginning of Oct.  Interesting is for the average temp in Oct, our heat demand should be 1.5kW, but so far this month we are averaging at about 20kWh a day including DHW.  So about 0.6kW heating input.

 

The only thing to watch for is look at the HP output at your lowest ambient, to make sure you have some room for DHW.  If you are batch charging UFH, this allows a bigger HP to be used, if doing WC you will be setting back at night a couple of degrees, so you will have a few hours where the heat pump isn't doing that much anyway.

[jack]

Just now, Chanmenie said:

@jack how big is your house ? 
I'm aiming for .11u for floor .13u for roof and .15u for walls, not sure what passivehaus levels are.
I’ll be 190m2  so might be able to get away with a 5kw or 6kw unit 

I used Jeremy’s spreadsheet and got a heat loss of 3.1kw but not sure how much to allow for DHW

 

289 m². 

 

Your U-values are good. It's possible you might meet PassivHaus standards (which set energy consumption targets rather than insulation values), depending on other factors such as solar gain, windows, form factor, etc.

 

5-6kW might be okay, but a little more overhead might be useful. It also depends where you are in the country - I imagine someone living in the north would have greater heating requirements than me in the south. 

 

One thing you might consider if it suits your layout is waste water heat recovery, which recovers some of the heat energy from showers while they're in use. Depending upon what proportion of the hot water water in your house is used for showering, the net effect can be significant. Some have argued that the payback is too long, but there are decent energy savings to be had, and they're about as set-and-forget as anything you can install when building a house. They do take some planning when it comes to drains.

[Chanmenie]

Thanks @jack and @JohnMo

I’m in Norfolk, I’ve not had my heating on yet, my current house is not as well insulated as the new one will be.

according the the spreadsheet my heat demand was same as yours JohnMo 

3.1K @ -5 

 

Good idea with the waste water heat recovery Jack 

[richo106]

Hi again

 

I have spoken to a local fitter recommended to me, it’s looking very likely I will be going with him. 
 
He says he fits the panosonic mono aquarea T cap 12kw R32, he did say it might be slightly oversized but would make hardly any difference and would mean less stress on the ASHP

 

Has anyone installed this ASHP, any reviews/information at all?

 

Many Thanks  

 

 Panasonic Mono Aquarea T cap 12kw 

Edited October 26, 2022 by richo106

[HughF]

The box in the garden isn’t really the important bit, it’s the controls and design that’s important. Ask him to tell you what controls package he’s going to fit and how he’s going to do the plumbing.

[Marvin]

On 23/10/2022 at 13:15, George said:

I thought air changes were only be a small component of heat demand. 

Hi @GeorgeAs buildings become better insulated the percentage of the heat demand required for the air changes increases. Ours is about 29% of the heat demand, about the same as the windows, even though we have a MVHR system.

[George]

Yeah I'd done the calculation wrong!

 

I don't have a decent measure on ach in my house but making assumptions it'd be about a third of my heat loss.