ASHP - Investigations and calculations

[richbee]

When I was looking at figures for our refurb the degree days base figure always seemed quite arbitrary and yet can have a significant effect on the overall expected energy consumption. For example, a leaky building that is then made airtight will use less energy to heat - seems obvuous but nearly all the calculations you see assume a baseline of 15.5C in the UK. I found this article helpful: https://www.energylens.com/articles/deg ... re-problem - it is from a UK company I believe.

You do realise that the 15.5C is the outside temp and the starting point for calculating degree days. Basically they assume that at this temp no heating is required due to passive heating from occupants and sun.
I think you need to look more closely at the SAP 2012 calculations which underpin the EPC ratings. Most installers start their quotes off by just using the EPC fig.

The SAP calculations take account of the locality and openings eg chimney, and vents but perhaps not so much leaky doors etc as it assumes these are airtight.

The excel calculation sheet also calculates heat loss from chimneys, air change per day (more air changes for old rooms vs new rooms) etc - it gives a value of W per room based on walls, doors, windows, chimneys, air circulation etc, which is based on the temperature difference between inside and outside - eg. 21 degrees inside and -4 outside.
It also works out a total annual kWh usage for each room and each contributing part - but that is wholly based on the W/m2 and the degree days (based on 15.5 outside) it does not reflect the actual internal temperatures at all - which is the strange bit if you are trying to estimate your likely future usage. I guess this is similar to the EPC SAP calculation.
We actually had a new EPC done last week, which estimated around 26,000kWh per year (although that incorrectly assumed insulated cavity walls in 2 rooms)

[richbee]

Rich, I second Joe’s view that this will be a very informative thread - your ongoing musings and experience plus the input of others.

We are in the position of having a seldom used, 9 year old combi - tend to use WBS as main heat.

My biggest concern is provision of DHW - we don’t have a ‘hot water tank’ and due to using space afforded by not having one/having a combi boiler, would need some disruptive changes to provide one, that, from my
‘Survey’ would be relatively small and struggle to provide enough HW for multiuser showers and/or a bath.

Not having researched the grant T’s&C’s, presuming having a secondary form of heat (WBS) precludes qualifying for the grant ?

I'm hoping not as we have 2x WBS
I know you can't have an eg. combined gas and heatpump combi system.

We also don't have a hot water tank currently - I think we can fit one in where the old one used to be many years ago - fingers crossed- will have to relocate the junk (important stuff)!
I think the standard heatpump specification for the grant includes a certain size of water tank depending on house size, possibly number of bedrooms

[Countrypaul]

The excel calculation sheet also calculates heat loss from chimneys, air change per day (more air changes for old rooms vs new rooms) etc - it gives a value of W per room based on walls, doors, windows, chimneys, air circulation etc, which is based on the temperature difference between inside and outside - eg. 21 degrees inside and -4 outside.
It also works out a total annual kWh usage for each room and each contributing part - but that is wholly based on the W/m2 and the degree days (based on 15.5 outside) it does not reflect the actual internal temperatures at all - which is the strange bit if you are trying to estimate your likely future usage. I guess this is similar to the EPC SAP calculation.
We actually had a new EPC done last week, which estimated around 26,000kWh per year (although that incorrectly assumed insulated cavity walls in 2 rooms)

Sorry Richbee, but I don't quite understand the above. You say the calculation for each room uses walls, windows etc. and the temperture difference between inside and outside giving a W per room, but then say the total kWh for each contributing part using W/m2 and degree days. I would expect the first figure for the rooms to be W/m2K (or degrees C) which when multiplied by the degree days would give the total annual figure. Can you give us an example of the figures for one of your rooms to see if we can better understand?

[Ken]

I think most people think that SAP and EPC rating etc has something to do with the way they live, it actually applies to the building being heated to a quoted temp of 21C so that all buildings around the UK can be compared nothing to do with the people living in it.

I thought that only HP fitted to meet the standard could get the grant but in reality there is wiggle room from some installers ie was talking to an installer whose attitude was will fit HP to satisfy EPC and come back later to replace any rads, as the occupants see fit. A house with too small rads does not really work and is the route of most of the discontent.

One problem i have with HP is that it might not be the best idea to have a capacity that will meet the "beast from the east" moment but have a slightly smaller unit to cover a avg winter and then reserve heating or just more clothes etc for the worst.

If i did not have a tank i would fit an electric shower and a undersink heater. but i guess thats not so good for those who like baths.

[richbee]

The excel calculation sheet also calculates heat loss from chimneys, air change per day (more air changes for old rooms vs new rooms) etc - it gives a value of W per room based on walls, doors, windows, chimneys, air circulation etc, which is based on the temperature difference between inside and outside - eg. 21 degrees inside and -4 outside.
It also works out a total annual kWh usage for each room and each contributing part - but that is wholly based on the W/m2 and the degree days (based on 15.5 outside) it does not reflect the actual internal temperatures at all - which is the strange bit if you are trying to estimate your likely future usage. I guess this is similar to the EPC SAP calculation.
We actually had a new EPC done last week, which estimated around 26,000kWh per year (although that incorrectly assumed insulated cavity walls in 2 rooms)

Sorry Richbee, but I don't quite understand the above. You say the calculation for each room uses walls, windows etc. and the temperture difference between inside and outside giving a W per room, but then say the total kWh for each contributing part using W/m2 and degree days. I would expect the first figure for the rooms to be W/m2K (or degrees C) which when multiplied by the degree days would give the total annual figure. Can you give us an example of the figures for one of your rooms to see if we can better understand?

OK sorry. I'm not sure how to add an image from my excel sheet, but i'll try to explain.
For the dining room floor. the area is 19.8m2, the U value in W/m2K is 0.55, and the temperature difference is 8.6 (18-11). Multiplying these 3 things together works out as a heat loss of 93.65W.
It also works out the annual usage in kWh - which comes from Area 19.8 x U value 0.55 x degree days (2370) x 24 / 1000 = 619kWh. This does not change if you change the room temp, it is solely based on the degree days, which are based on how many days per year the outside temp drops below 15.5 degrees.
I sort of understand it is a standard calculation to enable comparison, but not so great if you want to work out how much actual energy you are likely to use.
It is also flawed (maybe just in this spreadsheet), even if it is based on a temp difference between inside and outside - for the walls, the temp difference will be the same, but the floors have an assumed ground temperature of 11 degrees, which gives a much lower heat loss in W - but the energy usage in kWh is at the same rate

[richbee]

One problem i have with HP is that it might not be the best idea to have a capacity that will meet the "beast from the east" moment but have a slightly smaller unit to cover a avg winter and then reserve heating or just more clothes etc for the worst.

Yes, indeed - I'm not sure how to decide what is sensible in terms of capacity - eg. size to meet 90% of weather, and then use WBS to top up for the other 10% (or add extra jumpers)

[Countrypaul]

The excel calculation sheet also calculates heat loss from chimneys, air change per day (more air changes for old rooms vs new rooms) etc - it gives a value of W per room based on walls, doors, windows, chimneys, air circulation etc, which is based on the temperature difference between inside and outside - eg. 21 degrees inside and -4 outside.
It also works out a total annual kWh usage for each room and each contributing part - but that is wholly based on the W/m2 and the degree days (based on 15.5 outside) it does not reflect the actual internal temperatures at all - which is the strange bit if you are trying to estimate your likely future usage. I guess this is similar to the EPC SAP calculation.
We actually had a new EPC done last week, which estimated around 26,000kWh per year (although that incorrectly assumed insulated cavity walls in 2 rooms)

Sorry Richbee, but I don't quite understand the above. You say the calculation for each room uses walls, windows etc. and the temperture difference between inside and outside giving a W per room, but then say the total kWh for each contributing part using W/m2 and degree days. I would expect the first figure for the rooms to be W/m2K (or degrees C) which when multiplied by the degree days would give the total annual figure. Can you give us an example of the figures for one of your rooms to see if we can better understand?

OK sorry. I'm not sure how to add an image from my excel sheet, but i'll try to explain.
For the dining room floor. the area is 19.8m2, the U value in W/m2K is 0.55, and the temperature difference is 8.6 (18-11). Multiplying these 3 things together works out as a heat loss of 93.65W.
It also works out the annual usage in kWh - which comes from Area 19.8 x U value 0.55 x degree days (2370) x 24 / 1000 = 619kWh. This does not change if you change the room temp, it is solely based on the degree days, which are based on how many days per year the outside temp drops below 15.5 degrees.
I sort of understand it is a standard calculation to enable comparison, but not so great if you want to work out how much actual energy you are likely to use.
It is also flawed (maybe just in this spreadsheet), even if it is based on a temp difference between inside and outside - for the walls, the temp difference will be the same, but the floors have an assumed ground temperature of 11 degrees, which gives a much lower heat loss in W - but the energy usage in kWh is at the same rate

This is the problem I was mentioning in setting the base temperature for obtaining the degree days figure. The base figure is the temperature outside that requires no heating inside to maintain the internal temperature, obviously that base temperature will vary depending on what temeprature you require inside. So if you raise the base temperature to 16.5 degrees for example you will get a significantly higher number of degree days and a higher kWh figure for the per year heating. Determening the base temperature is therefore crucial to obtaining an accurate figure for the amount of heating that will be reuired.

If you are looking at changing to a heat pump from oil CH, then you can work backwards and get what is probably a more accurate fgure for the amount of heating required and then what base figure you should be using for the degree days. For example, if you use 2500L of oil pa and your boiler is 80% efficient in getting heat into the house, that would be about about 20,000kWh.

[Countrypaul]

One problem i have with HP is that it might not be the best idea to have a capacity that will meet the "beast from the east" moment but have a slightly smaller unit to cover a avg winter and then reserve heating or just more clothes etc for the worst.

Yes, indeed - I'm not sure how to decide what is sensible in terms of capacity - eg. size to meet 90% of weather, and then use WBS to top up for the other 10% (or add extra jumpers)

I would agree wih Ken on this, even if you have to use direct electric heating in the extreme weather case. The WBS would be safer should the extreme weather also cause a power cut mening the HP would not be functioig at all. Better to have an independant backup - wish we had, somethng I am still working on!

[richbee]

Heat Geek seem to specialise in training plumbers in the design and installation of heat pump systems, they don't install systems themselves any more but have a list of people that they've trained who should be competent, but are likely to be expensive as they will try to make the system as efficient as possible which may mean more or less complete replacement of the system. There's lots of good info on the Heat Geek site though.

The Build Hub forum has lots of useful discussion about heat pump systems - https://forum.buildhub.org.uk/forum/119 ... umps-ashp/.

I found Glyn Hudsons' video of installing an ASHP himself interesting - His demonstration that an ASHP could operate very efficiently was one of the triggers for my conversion to ASHPs and doing my own install.

Graham Hendra has some interesting posts including a few where he's installed an ASHP in an old house without changing any of the radiators or pipework. https://www.linkedin.com/pulse/project- ... am-hendra/

Octopus may be cheap, but when I asked for a quote they wouldn't look at anything other than standard estate type houses, so 1830 would be out. That was a few months ago and they may have changed.

Thanks for the links, I will have a look.
Octopus have given the initial price, which suggests they are branching out a bit. Although when I spoke to them on the phone they did say that the max heat pump size they will supply would be 11kW - so I might miss out on that front, if my calculations of 12.5kW are accurate

[Ken]

I think the only sensible way of working out how much heat energy is to record what you use (as stated above)

Take a measurement on the 1st of each month or every week during the real winter and then you have YOUR need. Oil is 10kwh/L and burns with at best a 80% efficiency. Without those figs i would not know where to start sizing the HP.

Another problem to consider is that if the HP is only just big enough and on 24/7 then there is no capacity to heat up the building eg after a holiday.