Under-sized vs over-sized ASHP conundrum

[Kevm]

On 13/12/2021 at 15:13, muhrix said:

Hello,

 

To keep it short, I am in the process of having a Mitsubishi Ecodan ASHP installed at my property.

 

I had one installer quote for a 8.5kW model, while another installer quoted for a 14kW model.

 

I don't have any information regarding the calculations from the first installer (who did a survey taking measurements, looking at insulation in the attic, etc.).

The other installer provided a few design scenarios (as I requested).

 

For regular design temperatures, the heat demand is 11.7kW, while for 21C throughout it would be 12.3kW and 22C throughout 13.4kW.

 

I was told that a 10% factor was added on for intermittent heating, and then I was wondering (I do not plan to heat the house intermittently)...

 

Removing 10% from those calculations, I'd get 10.53kW, 11.07kW and 12.06kW respectively.

That means in two design temperature scenarios I could get a smaller unit (but not the warmest scenario).

However, it's still way above 8.5kW.

 

I have been reading a lot, trying to educate myself on the details regarding controls and heat demand.

As you all know better than me, those heat loss values are for the coldest days in the winter (~5% of the year in Eastern England?).

So most of the time I'm likely to need even less than the calculated heat demand.

 

A smaller ASHP (11.2kW) would be:

• cheaper to buy and to run
• quieter to run
• potentially cycle less
• able to modulate to produce very little heat (for "drip feeding" into the house) compared to a bigger one (14kW)

Am I right in my thinking here?
 

Any improvements to insulation would reduce heat demand. The house is already fairly insulated, but there are some drafty doors that I plan to fix (not sure the fact they are drafty was taken into consideration). Other than that, perhaps changing all windows to triple glazing (but getting rid of LPG is higher priority).

 

Are the SAP calculations normally on the conservative side w.r.t. heat loss?
While it seems that 8.5kW is under-sized (and hence unsuitable), and 14kW slightly over-sized (depending on the design temperatures mentioned above), is 11.2kW just right?
Or will the 11.2kW turn out to be a "false economy" and turn into a "nightmare"?

 

Oh, for DHW, there will be a 250-300 litre cylinder heated by the ASHP. The house has 17 radiators which will be sized appropriately according to the design temperature of choice.

 

Any help/advice would be greatly appreciated!

Thank you.

 

Note: these are the two installers I had constructive and positive discussions with. Quite a few just ignored me, others gave me quotes via email just by using my floor plan, another was extremely rude to us in their visit... overall, finding it very hard to navigate this "mine field". 

 

I'm a bit late in responding but I thought I would because I have a 14kW Ecodan.  Mine is definitely oversized because my house will need nothing like 14kW most of the time. My ASHP can also deliver quite a bit more than 14kW if it has to.  My house is a 160m2 bungalow with 13 radiators.  I keep it at about 21C, running the ASHP 24/7.  I don't use thermostats but control through weather compensation/flow temps only.

 

From the published data and my own observations my ASHP's minimum output is about 4kW.  An 11.2kW model looks to be about 3.7kW or so minimum, so not that different. 

 

In practice, this means that my ASHP cycles when it's warmer than about 7C ambient.  The cycling is about three times per hour.  Any colder out and it runs steadily.   At its minimum steady running it's consuming about 1.2kW.  I think I would have been OK with an 11.2kW model but I went with my supplier's advice and got the bigger one.  As you said, the smaller model will be a bit cheaper, smaller and quieter. 

 

Looking at the published data, it's clear that these ASHPs (and I assume others) are less efficient when working hard. 

 

https://library.mitsubishielectric.co.uk/pdf/book/ATW_Databook_R32_2020#page-72-73

 

It's a bit fiddly but if you take a winter's day of 0C and assume yourvhouse will need say 9kW to heat it at 45C flow then you can compare the COP with the different models.  I think what I'm seeing is that the bigger ASHP is providing its heat slightly more efficiently.  I think also that the smaller model may have the edge at the lower end.   If I had known what I know now I may have gone smaller  but for me I don't think there is a lot in it.  But if I were in any real doubt I would  gone large. 

 

Hope this helps.