Under-sized vs over-sized ASHP conundrum

[muhrix]

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". 

[muhrix]

Just now, muhrix said:

To keep it short

Oh that didn't work...

[J1mbo]

You do need some headroom to allow for DHW heating, unpredictable thermal losses, and unusual cold.
 

If you have some time, record your daily space heating energy consumption over the next month then plot that against degree days at your location for the same period and you will be able to determine both the heating load and the required weather compensation curve for the system when it’s installed.

[ReedRichards]

Whatever size heat pump you choose it will be oversized for most of the time because most of the time the outside temperature will be greater than the design temperature.  And probably undersized for those rare days when temperatures get to seriously sub zero.  On that basis I don't see that an oversized heat pump can be too terrible because they all spend most of their days being oversized.

 

If you plan to run your radiators 24/7 then that's unusual for radiators but your choice.  If they will be off some of the time (e.g. at night) then the bigger the output the faster you can increase the temperature so it might be worth boosting your radiator output to match the heat pump.      

[TW9]

Heat pumps don't perform at their best when you make them work hard. They like producing a low level of heat for hours. If yours is undersized it will either fail to get the house warm enough on cold days or will get less efficient as you try and get it to produce hotter water. It will end up being disappointing or very expensive (or both).

 

Good system design will avoid most of the things you're worried about (short cycling, modulation etc). As for noise, a 14 kW Ecodan will be a little louder than an 11.2kW one but mainly because they don't make the ultra quiet version in that size. But they aren't very noisy so I wouldn't worry about it.

 

Obviously your house will be insulated differently to ours but for comparison, we have a 1990s 4 bed detached with 12 radiators being fed from an 11.2kW heat pump. The only additional insulation we have, over what was fitted when it was built, is an extra 100mm of lift insulation.

[muhrix]

4 hours ago, J1mbo said:

You do need some headroom to allow for DHW heating

Good point, thanks!

 

4 hours ago, J1mbo said:

record your daily space heating energy consumption

I have an LPG tank with a very old gauge, so I think that’d be difficult, I think.

 

I like your suggestion, though - thanks. I’ll see how I can estimate our usage.

 

4 hours ago, ReedRichards said:

If you plan to run your radiators 24/7 then that's unusual for radiators but your choice.

I did not know this was unusual - I thought that is how heat pumps work best. Keeping them running when needed, allowing radiators to be like warm emitting a steady and low amount of heat. Doesn’t that apply to radiators? I was not planning on having them off over night. I assume the pump will switch off once return and flow temperatures are too close, then switch back on once the temperature has gone down and the thermostat calls for heat.

 

2 hours ago, TW9 said:

we have a 1990s 4 bed detached with 12 radiators being fed from an 11.2kW heat pump.

Would you consider your heat pump under- or over-sized? Or maybe just right?

 

Our house is about 200 sq. metres, detach, fairly well insulated.

 

Thank you all for sharing your views so far, much appreciated.

[john0wingnut]

9 minutes ago, muhrix said:

Good point, thanks!

 

I have an LPG tank with a very old gauge, so I think that’d be difficult, I think.

 

I like your suggestion, though - thanks. I’ll see how I can estimate our usage.

 

I did not know this was unusual - I thought that is how heat pumps work best. Keeping them running when needed, allowing radiators to be like warm emitting a steady and low amount of heat. Doesn’t that apply to radiators? I was not planning on having them off over night. I assume the pump will switch off once return and flow temperatures are too close, then switch back on once the temperature has gone down and the thermostat calls for heat.

 

Would you consider your heat pump under- or over-sized? Or maybe just right?

 

Our house is about 200 sq. metres, detach, fairly well insulated.

 

Thank you all for sharing your views so far, much appreciated.

 

My Bungalow is approx 180sqm, UFH with a 12Kw Heatpump. Insulation is 100mm walls and floors, 300 in the roof.

[TW9]

I think our heat pump is sized about right. It certainly doesn't have much extra capacity when it's very cold. Our house is smaller than yours (about 130sqm).

[Marvin]

8 hours ago, muhrix said:

The house has 17 radiators which will be sized appropriately according to the design temperature of choice.

Will you have Thermostatic Radiator Valves?

 

Do you expect to use a Buffer tank?

[ReedRichards]

2 hours ago, muhrix said:

  7 hours ago, ReedRichards said:

If you plan to run your radiators 24/7 then that's unusual for radiators but your choice.

I did not know this was unusual - I thought that is how heat pumps work best. Keeping them running when needed, allowing radiators to be like warm emitting a steady and low amount of heat. Doesn’t that apply to radiators? I was not planning on having them off over night. I assume the pump will switch off once return and flow temperatures are too close, then switch back on once the temperature has gone down and the thermostat calls for heat.

 

Most people with radiators either switch their heating off at night or use a set-back temperature.  I'm not referring specifically to ASHPs, just the generality of radiator-based heating.  I have radiators with my ASHP and I set back the temperature to 17.5 C at night; it varies between 18.5 and 20.5 during the day.  There are two reasons to do this:

1. It saves money by not heating the house when you don't need the heat.
2. It saves money by not running the heat pump (much) when the outside temperatures are lowest, which is typically overnight.

If you do this, as I do, then you want to be sure you are warm enough in the morning which means that your heat pump and radiators need to have enough capacity to raise the temperature at a reasonable rate.

 

If you have underfloor heating it responds very slowly and is best left on all the time.  Some of the myths about heat pumps have arisen because they are frequently used with UFH and the requirement of the UFH is mistakenly thought to be a heat pump requirement.  Also, it's unlikely your heat pump will know what room temperature you are trying to achieve and if it does know it's unlikely that it can do anything about it.  This would be a control feature called "load compensation" and it is rarely found with heat pumps.        

[J1mbo]

6 hours ago, ReedRichards said:

 

Most people with radiators either switch their heating off at night or use a set-back temperature.  I'm not referring specifically to ASHPs, just the generality of radiator-based heating.  I have radiators with my ASHP and I set back the temperature to 17.5 C at night; it varies between 18.5 and 20.5 during the day.  There are two reasons to do this:

1. It saves money by not heating the house when you don't need the heat.
2. It saves money by not running the heat pump (much) when the outside temperatures are lowest, which is typically overnight.      

With direct combustion heating it certainly saves money. The average temperature of the fabric will be lower and therefore the heat loss and heat demand are also lower.

 

However, the case with ASHP is not so clear.

 

Assume the night set back is 8 hours in duration and the low threshold rarely results in heat demand, this reduces the compressor run time from 24 to 16 hours (obviously).

 

Since the fabric loss is still high at 18 degrees when freezing outside (90%), the heat pump is being asked to provide nearly the same energy in 24 hours via less runtime. This obviously means higher radiator surface temperature, as much as 10 degrees higher, with the resulting lower COP only offset by a marginal energy demand saving.

 

I did some maths in another thread that showing a small saving when fully optimised in both scenarios to run 24x7. Of course it depends on the daily temperature profile and the length of the set back period and the thermal performance of the building.

 

But I don’t think it can be assumed that a night set back is cheaper with an ASHP when fully optimised.

[muhrix]

8 hours ago, Marvin said:

Will you have Thermostatic Radiator Valves?

 

Do you expect to use a Buffer tank?

Yes, I will have TRVs. I intend to have most of downstairs on “always open” since it’s all fairly open plan. The bedrooms will have smart TRVs, mainly to call for heat and maintain an even temperature (I won’t use them to switch off zones).

 

Our installer told us we need a buffer tank, but we don’t know where to put it (they haven’t told us the size yet). They suggested the attic, but it sounds wasteful).

 

Does it make sense to you? What are your thoughts?

[ReedRichards]

1 hour ago, J1mbo said:

But I don’t think it can be assumed that a night set back is cheaper with an ASHP when fully optimised.

 

I don't think my heat pump is capable of being fully optimised; is yours @J1mbo?   It will either target a fixed output water temperature or it will target an output water temperature that varies according to the outside temperature ("Weather Compensation").   So my radiator surface temperatures vary as the heat pump cycles but the peak surface temperature only goes down if the outside temperature warms up.

 

This was my entire house power consumption yesterday (relatively mild for December).

 

[joth]

3 hours ago, muhrix said:

Our installer told us we need a buffer tank, but we don’t know where to put it (they haven’t told us the size yet). They suggested the attic, but it sounds wasteful).

 

Does it make sense to you? What are your thoughts?

Yes - if you have smart TRVs and a possibly over-sized heat pump, a buffer tank makes a lot of sense, as it allows individual rooms to call for different times satisfied from the buffer, and the buffer is then recharged as needed from the HP, without any risk of short-cycling.

 

Buffer in the attic only makes sense if it's a warm loft. If so, and if the rest of the plant can go there too (circulation pumps, valve, and the HW cylinder) it's not a bad call, assuming it doesn't make the circulation loops gigantic. 

 

Otherwise, you can can UVC with an integral buffer tank, which might save space.

[J1mbo]

41 minutes ago, ReedRichards said:

 

I don't think my heat pump is capable of being fully optimised; is yours @J1mbo?   It will either target a fixed output water temperature or it will target an output water temperature that varies according to the outside temperature ("Weather Compensation").   So my radiator surface temperatures vary as the heat pump cycles but the peak surface temperature only goes down if the outside temperature warms up.

 

This was my entire house power consumption yesterday (relatively mild for December).

 

Observe the time where the system has zero output. But the structure doesn’t magically have zero thermal loss in that same window, and that cumulative energy deficit has to be replenished by the ASHP during the next heating period, and to do that the radiators must produce more output than they would otherwise be needed to, meaning the surface temperature of them will need to be higher than if the structure were heated continuously.

 

There is clearly a trade off in running cost between running hours, average delivered flow temperature, average outside temperature, and likely electricity tariff rates that make this all a bit of a judgement.

[muhrix]

4 hours ago, J1mbo said:

I did some maths in another thread

Would you mind pointing me to that thread, I'd like to read that (I can try and search for it too, just in case you have the link or remember to title of the thread).

[muhrix]

1 hour ago, joth said:

Buffer in the attic only makes sense if it's a warm loft

I don't think it would be, since the insulation is currently exposed on the attic floor and the roof is just lined.
The DHW cylinder is located just below where the solar thermal plant is (in the attic) and where I believe the ASHP plant will go. We don't have a lot of space for that.

I'll discuss what is feasible and practical with my installer today. Let's see what we end up with.

 

1 hour ago, joth said:

Otherwise, you can can UVC with an integral buffer tank, which might save space.

Yes, I saw those and I liked the idea. I'm just not sure where the expansion vessel would end up given the extra height due to the buffer tank.

[Marvin]

16 hours ago, muhrix said:

Our installer told us we need a buffer tank, but we don’t know where to put it (they haven’t told us the size yet). They suggested the attic, but it sounds wasteful).

 

Does it make sense to you? What are your thoughts?

Well, I think you should read pages 62 63 and 68 of this report. It will give you some understanding. 

 

https://forum.buildhub.org.uk/applications/core/interface/file/attachment.php?id=50188 

 

Good luck 

 

Marvin

[SteamyTea]

8 hours ago, Marvin said:

pages 62 63 and 68 of this report.

The report that keeps giving.

 

[Marvin]

Yes. I think that anyone who's thinking of ASHP should read it. 

 

Fortunately our installation follows the recommendations, and now we're fine tuning the set up.  

 

 

[ReedRichards]

In my case the buffer tank is under the hot water cylinder (as a single integrated unit).  In makes the cylinder very tall (and heavy) but doesn't waste usable space and means the buffer is within the heated part of the building.

[Marvin]

12 hours ago, Marvin said:

Well, I think you should read pages 62 63 and 68 of this report. It will give you some understanding. 

 

https://forum.buildhub.org.uk/applications/core/interface/file/attachment.php?id=50188 2.58 MB · 49 downloads  

 

Good luck 

 

Marvin

You have @SteamyTea to thank for sharing the report.

[muhrix]

3 hours ago, Marvin said:

You have @SteamyTea to thank for sharing the report.

Thanks @SteamyTea!

 

Did you write that report?

[SteamyTea]

3 minutes ago, muhrix said:

Thanks @SteamyTea!

 

Did you write that report?

No.  I had forgotten all about it from when it was published, was the only time I read it.

Usually find it easier to just work from first principles when it comes to thermodynamics.

[Christiano]

On 13/12/2021 at 20:55, muhrix said:

I have an LPG tank with a very old gauge

 

We had LPG until the heat pump this year. (We still have about 1200l which we can’t use and it looks like we will have to pay for it to be taken away!)

Anyway Google says there are 6.9kWh in a litre of LPG. We used about 3000 litres a year which is just under 21000kWh. EPC says our heat demand is about 24000 but there’s only the 2 of us in a family house so I reckon it’s about right. Our gauge was quite difficult to read because the plastic cover was perished, but we also took readings when we did elec/FIT readings and I made a reasonable demand curve. (Tank is 2000l so 1% is 20l or 138kWh)

 

On 14/12/2021 at 07:23, muhrix said:

Yes, I will have TRVs. I intend to have most of downstairs on “always open” since it’s all fairly open plan. The bedrooms will have smart TRVs, mainly to call for heat and maintain an even temperature (I won’t use them to switch off zones).

 

With LPG we tried TRVs calling for heat and it was hopeless, took them all off. With the heat pump we only have 1 in the bedroom to stop it getting too warm.

 

Finally, WRT oversize or not. Ours is oversized “in case we build the extension (that would be nice but we don’t really need)”. A 16kW Samsung comes on at about 5kW and rarely is on long enough to drop down much lower. I’m trying to fine tune it but mostly to make it run for longer to get the lower kWh figures. I wish we had known this as we would probably have gone for a smaller HP and dealt with it if we did the extension.