….which is oddly close to the 50C design flow temp….that Octopus standardise on it seems
Or is that a happy coincidence?!
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Hi Tim,
Thanks.
I don’t want the house to be 23c particularly, I am only doing it now to help match the house to the heat pump so at a 21c difference my heat loss is about 4.2kW an hour.
I have been running at a fixed flow temperature of 42c for a couple of days now.
The house is warm enough so everything is good in that respect.
COP is not great though, which is OK in itself.
But I am using more electricity than I could be, quite a lot more possibly?
I’ll stand correction this but your ‘net’ heat loss was 3.125kw at 6kw (excuse me for being pedantic but no need to say ‘an hour’ as kw is already a rate, like speed). Your ‘gross’ heat loss from the building to outside would have been higher but it is offset by other energy sources and people and solar gain to get this net figure. This is why, at the probable base temp of 16c, your net heat loss is Zero and you have to extrapolate down from there and not from 23c.
Clearly the heat pump input required is the net figure but as I said in an earlier post, if you need zero at 16 and 3.125 at 6c then at -2 you will need = 5.6kw from the heat pump. Hope I’ve remembered my quadratic equation solution from school.to get this number;
6 a + Constant = 3.125kw
16 a + Constant = 0 kw
Solve for a and the constant then calculate -2 a + Constant = 5.6kw ?? I hope!
Indeed, as Michael de Podesta’s explains in “Cold Weather Measurements of Heat Transfer Coefficient”:
So to estimate the actual amount of heat dissipated in the house I should really take the heat pump output and:
- Add 2.4 kWh/day for each person in the house
- Add 10 kWh/day for all the electrical items
- Subtract 3 kWh/day for the hot water lost.
Another way of understanding the gross vs net difference is to think of an empty church. Even if the outside sits at -2c for a month the inside of the church will be a few degrees above this. This is due to a) some solar gain and b) the ground underneath is much warmer than -2c which is why ground source heat pumps exist.
So when calculating your Heat Pump energy at -2c you have to factor this ‘hidden’ heat gain into the equation and the base temp assumption is the easiest means to do this.
When I did this exercise for my house, I calculated a gradient of 270 W/K with a base heat of 600 W from electrical sources, plus 400 W from humans. So, in theory, that’s 1 kW less heat I need from the heat pump.
The degreedays calculation method is the standard way to make these adjustments. I admit to have never gone as far as the detailed regression analysis but here it is;
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OK, it gets a bit more complicated I agree.
I am not sure agree with extrapolating from 16c as I have calculated this at a range of outdoor temperatures and I always come back to a heat loss of about 4kW at 21/-2
We don’t even use 10kWh of electricity a day on other electrical items but even if we did they are always there although it varies from day to day.
However we calculate this my heat loss is never anywhere near 7.3kW and all this has been done whilst heating our conservatory.
Most compelling to me is our previous gas use over two winters. I have never tried to save money and turn the heating down. The most gas we ever used in a day was 110kWh. If that is true I don’t see how my heat loss can be 7.3kW unless my gas boiler was more than 100% efficient!
The conservatory is an issue at it adds a lot of heat loss but isn’t actually a room that should be considered when looking at the worst case.
We would shut the double glazed doors to it and pull the thermally lined curtains, we wouldn’t leave it all open sucking heat out of the house when we were struggling to keep warm with the heating system.
I am going around in circles a bit, I know but I am struggling to find a solution to my unhappiness with what I have got.
Radiators would help with any heat pump I am sure but there’s a limit to how much money I want to throw at this.
The heat pump has a minimum electrical input that is not consistent with my heating requirements.
I am scared that I will replace the radiators and then end up with something that cycles a lot using more electricity and a house we still have to run hot.
Not very technical but I also have a wife who is very understanding and will let me do whatever I want but I feel bad about wasting what is her money too.
Here is my heat input for well over a year - both gas boiler (blue dots) and heat pump. (orange). Each dot is a day.
As you can see the daily kwH at -2 is approx double that at 7 to 8c although my heat pump does struggle at -2 due to the defrost issue so the internal temp was lower than desired. So I would say that a good method is to take the heat required at 7c and double it.
You will see from this that the 150kwh I would like to put in at -2c equates to a rate of 150/24= 6.25 kw but as it is set back at night my heat pump operates for a max of 18hrs and so has to put heat in at a rate of 6.25 x 24/18 = 8.33 kw. During defrosts my 10.6kw heat pump only averages about 7.5kw which is why it is struggling at -2c.
At some point I’ll start a separate thread on the defrost issue as I think I know how to calculate it now - there is nothing in manufacturers details or from MCS and any papers you find when searching are purely academic and not practically based.
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If I accept that my heat pump capacity will be needed at some point as suggested by Colin then I am still wondering what sort of house this heat pump is designed to fit.
When it is sub zero it will give about 7.5kW and when it is +7c it gives 4.6kW if I wanted to run at the best efficiency. Colin has just said that his struggles to give 7.5kW at -2c
What sort of house fits these heating requirements?
Mine doesn’t.
What were Daikin thinking when they created this range of heat pumps?
Is it normal to need 40,000 kW of radiator output for a 98m2 house with an EPC rating of B/89 when you install a heat pump?
Whatever way I look at this my installer has got it terribly wrong.
It sounds like an awful lot to me
Your HP is clearly too big. I’ve just spotted an easy way to use my data to get your sizing correct.
My maximum daily gas input was 158kwh at 90% = 175.5 kwh actually purchased
Your is 110kwh so mine is 1.6 x yours.
My heat pump is slightly undersized at 10.6kw so your will be slightly undersized at 6.625 kW
We keep coming back to that 7.5kw unit … and some bigger rads to get the flow temp down
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Thanks Colin, again!
I just looked at my DHW run, it is almost the same COP as my heating today!
Somehow I need to express my concerns to Octopus when they respond to my complaint and I am a bit scared of what I will end up with if it is changed.
I think my feeling is now that I just have little confidence in Daikin heat pumps, I don’t feel confident that any of them will work very well.
What is it doing?
It is set at a fixed flow temperature of 42c but it just sits there building up slowly at 32c which we all know my radiators cannot cope with. Why doesn’t it just heat the water to 42c as requested.
I guess the answer will be something to do with radiators and room temperature but if the room is hot enough I wish it would just stay off until conditions allow it to run as it has been asked to.
I cant say I understand why the cycle that started at 14:40 should be different from the later one - same flow but no boost in compressor power at the start. Also the 14:40 COP was better. Could you try dropping the flow set point to 39c to see if it settles there?
In testing mine with Trystan yesterday there was a massive difference in performance when I simply raised the fixed temp 2c and took of my ±3 modulation. I wonder if yours, with limited radiators is in some kind of unstable equilibrium.
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At about 15.35 the flow temperature was still only 35c
The Madoka is set at 22 and the displayed room temperature was 23c.
I turned the requested room temperature up to 23c to see if I could get the flow temperature to increase but of course the heat pump turned off because it was already 23c in here.
I then increased the requested room temp to 24c to get the heat pump to start again.
That used a lot of electricity.
I may be better removing the Madoka influence and just running a fixed flow?
It has taken the best part of two hours to settle down into some sort of reasonable operation.
You are putting in 3.8kw at min compressor power when the house loses 2.7kw - so it is going to have to cycle.
Your flow temp is stable at 42 and so you are not radiator limited … which is a good result!
I would not remove the Madoka influence as your house will just get warmer and warmer until the radiators become limited again and your COP will get worse. The key is to get the lowest flow temp that will give you the flatlining flow temp profile. So I would drop the ‘fixed’ temp to 40c and add ±1 Modulation to see if it can find the optimum flow temp. between 39 and 41. The Madoka will switch the unit off at room set point +0.5c and off a degree below that.
It’s not clear yet if the Daikin detects that it is cycling and changes its behaviour to optimise that but it might do.
Thanks Colin,
I have just added the modulation Colin.
I was just checking the electricity usage and note that my back up heater was used this morning. Is that necessary?
I have a confession to make to Colin.
When I changed to fixed flow rate a few days ago I was sure I switched off modulation but I don’t think I did it properly as when I went to turn it back on it was already on at 10.
Explains a lot.
Sorry Colin!