Science and Art of specifying ASHP

[severnside]

Hi All,

 

As mentioned in my previous posts, we are renovating a 70s house and at some point would like to add ASHP. I have read lot of posts on ASHP here and most of the response fall in these camps

- House need to be super insulated for ASHP

- A correctly specified ASHP will work in any building

- ASHP calculations need special considerations and lifestyle factors and it's easy to get wrong system (This is what worries me and most of people I speak to)

I want to start this thread to list the key points needed to specifying ASHP for any building. I strongly believe it should be couple for formulas which can be added in a successful repeatable process

I would like to hear and compile everyone's opinion on this

 

Science:

1. Do proper heat calculation of building e.g. using MCS calculator, Jeremy's sheet, PHPP. Specify heatpump for worst case temperature for the area (e.g. -3C) with 10-20% margin for DHW

2. Specify emitters based on low Delta T of Heat pump

3. Specify pipe size for low temperature (15mm to radiator seems to be common for most cases)

4. Bigger Hot water tank than heated by fossil fuel

5. Location of ASHP and size/insulation of flow/return pipe

6. Noise vs proximity of neighbours, bedrooms etc

 

Art

1. Noise, placement, concealing of ASHP unit

2. Mono vs split?

 

Thanks 

Edited March 22, 2022 by severnside

[Spreadsheetman]

Planning? Some of the recent monobloc systems are over the 0.6m3 allowed under permitted development, so add extra hurdles to the process.

[SteamyTea]

9 minutes ago, Spreadsheetman said:

Some of the recent monobloc systems are over the 0.6m3

Is that the external dimensions of the unit?  Not heard of this rule.

 

31 minutes ago, severnside said:

Art

1. Noise, placement, concealing of ASHP unit

2. Mono vs split?

Only the concealment could be considered art.

 

31 minutes ago, severnside said:

Do proper heat calculation of building

You must already have a lot of data, get your old bills out and see what you have been using.

[Spreadsheetman]

5 minutes ago, SteamyTea said:

Is that the external dimensions of the unit?  Not heard of this rule.

 

Only the concealment could be considered art.

 

You must already have a lot of data, get your old bills out and see what you have been using.

See https://www.planningportal.co.uk/permission/common-projects/heat-pumps/planning-permission-air-source-heat-pump

[Marvin]

35 minutes ago, severnside said:

Do proper heat calculation of building e.g. using MCS calculator, Jeremy's sheet, PHPP. Specify heatpump for worst case temperature for the area (e.g. -3C) with 10-20% margin for DHW

Hi @severnside Bit concerned about the low temperature figure. I checked the records here over the last 50 years and it said -8C was the lowest and I am on the isle of Wight. IMHO I would want the ASHP sized on the lowest temperature (plus hot water) in the area over many years, so when you have very cold days its not struggling which is what we did.

[SteamyTea]

1 minute ago, Spreadsheetman said:

See https://www.planningportal.co.uk/permission/common-projects/heat-pumps/planning-permission-air-source-heat-pump

Thanks.

[severnside]

35 minutes ago, SteamyTea said:

You must already have a lot of data, get your old bills out and see what you have been using.

Yes, I am using that but we are also doing some extention and improvements so need to do some paper analysis as well.

 

29 minutes ago, Marvin said:

Hi @severnside Bit concerned about the low temperature figure. I checked the records here over the last 50 years and it said -8C was the lowest and I am on the isle of Wight. IMHO I would want the ASHP sized on the lowest temperature (plus hot water) in the area over many years, so when you have very cold days its not struggling which is what we did.

I looked at Met office data near Bristol and minimum is -4C. Also I assume minimum is just for few hours in day not -4C for whole day so average for day should be higher.

[Marvin]

10 minutes ago, severnside said:

I looked at Met office data near Bristol and minimum is -4C. Also I assume minimum is just for few hours in day not -4C for whole day so average for day should be higher.

 

Hi @severnside I understand it is -7C at one point during the day. I think the whole day averaged -4C however it has definitely got colder than -4C please see info:

 

https://www.weatherbase.com/weather/weather-summary.php3?s=62730&cityname=Bristol,+England,+United+Kingdom

 

https://www.bristolpost.co.uk/news/bristol-news/bristol-weather-met-office-shows-6526858

 

https://www.martynhicks.uk/weather/data.php?page=extremes

 

I would want my ASHP to handle the -7C event without immersion heating backup.

 

Good luck

 

Marvin

 

[severnside]

Thanks Marvin, Makes sense to plan for lower than my original -3C.

 

Another questions/input on Science bit of design

- At what size does it make sense to have multiple units instead of one large. e.g. is it better to get 2x 8kW for a single phase house than 1x 16kW

 

Edited March 22, 2022 by severnside

[Marvin]

16 minutes ago, severnside said:

Thanks Marvin, Makes sense to plan for lower than my original -3C.

 

Another questions/input on Science bit of design

- At what size does it make sense to have multiple units instead of one large. e.g. is it better to get 2x 8kW for a single phase house than 1x 16kW

 

Now you need input from someone else from further-a-field capable of slaying dragons.....🤣

 

I have read discussions about using different ones for different functions.. for example one for the hot water and limited heating, and one for full heating, however I'm not knowledgeable about the outcomes. 

Edited March 22, 2022 by Marvin
further thought

[SteamyTea]

Weather Underground had plenty of contributors in most area, find one local and then look at the daily data, collected every few minutes.

I have often wondered if Bristol gets warmer in the winter when the tide comes in.

 

Regarding the size if an ASHP, generally the fan size is similar  with just more fans added to a larger radiator area. So doubt it makes that much difference.

16 kW is a large unit, can your current electrical supply satisfy it?

Remember that a combi gas boiler is usually sized to the DHW supply, not the space heating.

[Wil]

2v1:

 

Benefits:

Some level of resilience depending on config

Lower start-up currents - less impact on grid if sequenced properly

Ability to heat and HW at the same time

Greater capacity on single phase supply

Efficiency during summer months when second unit not needed

 

Drawbacks:

Higher installation cost and maintenance

increased complication in controls

Increased pipework

 

 

[severnside]

35 minutes ago, SteamyTea said:

16 kW is a large unit, can your current electrical supply satisfy it?

Our last years Oil consumption for heating was approx 2600litres. DHW is heated with overnight electricity.  The house will be extended further but we will also improve on insulation, so need to do final calculations.

Back of envelope calculations suggest 10-12kW ASHP. I was thinking 16kW with margin.

16kW ASHP should use 5kW or 22Amps which should be doable on Single Phase

[SteamyTea]

22 minutes ago, severnside said:

last years Oil consumption for heating was approx 2600litres

So about 26 MWh. Do you know that mean outside air temperature you turn your heating on at? Mine comes on when it is less than 10⁰C for a week.

DHW on E7 is going to be quite costly now, but you can add extra insulation to the cylinder, heat to a lower temperature, and use less, though a bath with a showers worth is never very appealing.

[Marvin]

1 hour ago, severnside said:

Our last years Oil consumption for heating was approx 2600litres. DHW is heated with overnight electricity.  The house will be extended further but we will also improve on insulation, so need to do final calculations.

Back of envelope calculations suggest 10-12kW ASHP. I was thinking 16kW with margin.

16kW ASHP should use 5kW or 22Amps which should be doable on Single Phase

Hi @severnside

Hmm. Not sure a 5kW ASHP will supply 16kW in the dead of winter.

The principle is the closer to the outside temperature the ASHP produces the better the COP.  Think mine is a COP of 1 to 3 depending.

 

Edited March 22, 2022 by Marvin
Add info

[ProDave]

My ASHP was sized to give enough heat to heat the house in the worst case here of +20 inside and -10 outside.  That was calculated with Jeremy's spreadsheet that turned out to give a much more accurate result than the SAP calculations.

 

I am in the "An ASHP works well when properly specified" camp.  But at the same time, I also recognise if you have mains gas available then an ASHP is very unlikely to be cheaper to run than a gas boiler.

[Kevm]

1 hour ago, severnside said:

Our last years Oil consumption for heating was approx 2600litres. DHW is heated with overnight electricity.  The house will be extended further but we will also improve on insulation, so need to do final calculations.

Back of envelope calculations suggest 10-12kW ASHP. I was thinking 16kW with margin.

16kW ASHP should use 5kW or 22Amps which should be doable on Single Phase

 

Just be careful what you think is a 16kWh ASHP.  Some manufacturers can only achieve their rated output under very benign conditions  I know Midea is one, there may be others.  Mitsubishi can achieve theirs (and more) in most conditions; there will be others like this too.   

[ReedRichards]

The MCS norm is to specify a heat pump for a temperature that is exceeded 99.6% of the time (if I remember correctly).  If you specify it for the lowest temperature achieved in the last 50 years then it will be bigger.  Any boiler or heat pump will be oversized to meet your heating requirements most of the time.  Does being more oversized cause a heat pump to lose efficiency?  I have seen data for a Mitsubishi heat pump suggesting that it does, although I don't know if this is the norm.  

[SteamyTea]

49 minutes ago, ReedRichards said:

Does being more oversized cause a heat pump to lose efficiency

Generally no.

There has to be a distinction between oversizing for the worst case, and over sizing the the most likely case.  This is why knowing your local weather profile is important.

An example, based on the Central England Temperatures for January and July shows several things.  January is, on average, colder than July (3.5°C, 16°C.  January has a larger spread of temperatures than July (19.5°C, 16.5°C).  Both months are pretty evenly distributed around the mean (0.91, 0.89), but the kurtosis is visually quite different, with January being flatter (platykurtic) than July (leptokurtic), though the number, which really measure the tails, not the peaks, are not so different (0.3, 0.2), showing that the distribution is normal.

So to get 99% heating via an ASHP, you need to find a model that does not drop below a CoP of 1 down to -6°C for your given flow temperature i.e. 35°C.  DHW heating will have a different (higher) flow temperature i.e. 55°C.  That is where you may struggle.

Edited March 23, 2022 by SteamyTea

[severnside]

Just checked Valiant 15kW spec. 

- MAx current 25A

- At -5C output is 12.65kW ScoP 3.28

 

They have a temperature table at Page 28 of brochure which has min temperature to achieve heating in 99% cases. Lowest figure is -3.9 for Glasgow

https://www.vaillant.co.uk/downloads/product-brochures/arotherm-brochure-2006193.pdf

 

So if my house can manage with 12.65kW constant input then it should be enough

 

Another question to everyone. Most of our calculations use temperature delta as Indoor Temp - Min Outdoor temp. But the heat degree days concept generally calculates heating below (indoor temp - approx 3.5C). Which is correct method of delta calculations 

Edited March 23, 2022 by severnside

[ReedRichards]

I'm never sure about the correct way to interpret such temperature distribution curves @SteamyTea.  It's typically coldest in the middle of the night and I use a night time set-back temperature of 2.5 C so my heat pump can cope with a colder outside temperature overnight because I'm not asking it to work so hard.  And that is if it comes on at all, almost always the retained heat is sufficient that it does not need to come on until it starts warming up the house in time for me to get up.  Given that my heat pump is rated to keep the house at 21 C down to -3.8 C outside, I would only be in real difficulty if the temperature did not get as high as -3.8 C for 24 hours or more.     

Edited March 23, 2022 by ReedRichards

[SteamyTea]

30 minutes ago, ReedRichards said:

I'm never sure about the correct way to interpret such temperature distribution curves

We all struggle.

The y-axis is the probability (or time, occasions, count), the x-axis is the classes (usually called bins) that demarcate the data (temperature, speed, wind direction).

 

Taken at their most basic, all they show is the probability of something happening, and how often.

This does not mean that something will happen, and if it does, it will last the same amount of time as it did previously.

If you want to do that, then the usual way is to divide the time series into blocks i.e. 4 hours, pick the mean temperatures of the two previous blocks and see what those mean temperatures where.  Then see if there is a correlation i.e. if it is 10°C now, what were the temperatures 4 and 8 hours ago.

You can, if you want to get really predictive, look at historical data and see what the temperature are during the following two blocks.

 

Edited March 23, 2022 by SteamyTea