where is this 9.1kW going at 37.6c flow?
Heating the water rads and pipes in the whole system from 22c to the flow and return temps
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Letās refresh some details, arenāt your radiators sized(or meant to be) for 9kW at 50C?
If so, to put out 9kW the flow needs to be 50C ish.
Hi Vinny,
I know that but that is just what the heat loss surveyor from Octopus came up with, it doesnāt mean it is true.
No other heating cycle today needed all this electricity and heat output to get going apart from 05.00 this morning.
So why so much now to warm stuff up when it wasnāt required earlier?
Before I spend Ā£5,000 on radiators I need to know for sure that they are the problem, relying on the bloke from Octopus Energy isnāt enough for me.
My radiators appear to put out more heat than they are `supposedā to at times.
Octopus say my heat loss is 7.3kW at 21c/-2.3c it isnāt.
Real life and real data are worth more than guesses on a spreadsheet.
If my water and pipes need 9.1kW to heat now why didnāt they at 10.30 this morning?
You can do your own heat loss survey, there are some free sites online to help you do it, and I think Trystan has a program you can use, see this blog post and part 2. Beware assuming air change rate.
You can also wait for a day that is below freezing and see how much heat you need to put into to the house to keep it warm.
Hi Tim,
We had a day last year that was -5c all day and I heat the house to 21c all the time.
I used 110kWh of gas that day to heat the house, the most gas I have ever used in a day.
That included a tank of hot water at 65c.
As far as I am concerned that is all I need to know, I canāt see how it can be wrong?
If I needed 100kWh (at most) of heat on a day at -5c all day and night my heat loss can only be 4kW an hour.
I can use a multitude of different software packages to calculate the heat loss but they really are just guessing.
Am I missing something?
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If my water and pipes need 9.1kW to heat now why didnāt they at 10.30 this morning?
because they had been preheated at 5am and didnāt drop below 30c after that. You have just jumped the temp up quickly so another high slug of heat has been needed.
Thanks Colin.
So I would assume that if I carry on like this it probably wonāt need such a big slug of heat every time?
Itās been running the fixed lwt for an hour and twenty minutes now. It is not looking good, a COP of less than 3 and about 3 now it has settled down.
Electricty use is 300 watts + more and house is getting warmer albeit slowly
But Iāll stick with it, nothing much to lose by giving it a go.
If we āsimulate heat output using carnot COP equationā, with a heatpump factor of say 0.46, we get a good match on the cycle that starts at 17:00, but the other cycles are showing less than expected heat output (red).
I donāt know why this is, or what could cause it, but it is peculiar and not something Iāve seen elsewhere.
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Thank you Tim.
I am so sorry to everybody. I really appreciate all the help.
Every time I read an explanation or an answer it all makes sense but the it really seems to me like the heat pump or radiators or house go and contradict some of those answers.
You all know more than me but something is telling me that all is not well with this heat pump and I am trying to find something to nail it.
I know it sounds negative, I canāt explain it but I just know it is not right.
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Just looked - itās settling down nicely - stable flow temp minimum flow rate, nice delta T. COP of 3.7 to 3.8.
You might be able to drop the flow temp set point by a degreeā¦ this is fairly consistent with the calculation Iāve just posted suggesting the minimum flow temp for a rad system designed for 50/45/20 giving 9kW
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Yes, Colin, it seems quite happy at this.
It is scheduled to run to 24.00 like this so I hope it makes it without stopping.
It will be interesting to see how it goes tomorrow.
Iāll leave it like this for first thing in the morning and then see what happens when I drop the flow temperature by 1c.
Thanks again for all of your help.
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Thank Colin,
It has worked well this morning, COP of 3,7 for 05.00 to 10.15 and nearly 4 for 06.00 to 10.15
So if I doubled my radiator capacity what could I expect to be able to achieve?
I am waiting for a call from Daikin. I want to find out how low the heat pump can go and whether there are some settings that I am not aware of.
Starting with where you are now- it looks as though 42c is your equilibrium or balance point with an outdoor temp of 8c.
If you notice this morning the flow temp started to rise above this so it was no longer in balance. Two reasons; when your room temp climbed 3c the rad output will have dropped (you would have needed 45c flow to keep the delta t with the room the same) also as your outdoor temp climbed 2c vs last night your compressor efficiency rose with it - rough carnot calcs says it would rise by about 8%. There is a perversity about this - as the outdoor temperature gets higher you will need a higher flow temp to keep the radiators in balance and so use all the heat the compressor can produce at itās minimum setting. ie an inverse weather dependent curve!!
Doubling your radiator capacity will change your equilibrium point (at 8c) from Flow 42, Return 36 down to Flow 35 Return 29. Oddly enough that is exactly where my system is at the moment - giving a COP of around 5 in equilibrium.
Iād pick just 2 or 3 radiators in your main living rooms and change those. Start with the easy ones; for example if you have a 2m wide sofa with a 1 m wide radiator behind it then just double it. Or change a K1 type for a K2 or K3. The other way is what Iāve done - buy one or two daikin HPCS (convector rads) for your main living rooms. These output a whopping 1.6kw each at 35/39/20 on max fan. At that level you can hear the fan but at the lower fan level they are almost silent (good enough for watching TV). They come in at around Ā£1000 each installed so not cheap but you should save around 20% of your running costs so they will pay back eventually.
Mitsubishi sell the same unit albeit in a different case (think VW vs Skoda vs Seat). Iāve looked at teh installation diagrams and they are identical inside and seem to be made by an Italian company which, unfortunately does not seem to supply the UK directly. JAGA also make convectors and I have used them in another house but they are pricier but very quiet. There are some less sophisticated ones you can find on the web - perhaps cheaper?
Good luck!
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Hi Colin,
One thing to note and I have mentioned it before is that the room temperature displayed here does not match reality. The temperature sensor is in our kitchen and not next to the Madoka and it wasnāt picking up the signal there.
I need to move it but the rise you saw is probably because my wife has turned the slow cooker or oven on.
The temperature on the Madoka has been stable at 22.5c to 23c throughout.
Nevertheless I think your summary is about right.
I worked out radiator sizes so that the rooms get the heat they need at whatever low temperature that is, they need to be in proportion to each other otherwise Iāll have hot rooms and cold rooms. For obvious reasons I donāt want to use trvs to control room temperatures.
I understand - if it is more than warm enough then instead of raising the flow temp by a degree you could drop the Madoka set point by a degree. The radiator output will increase by the same amount either way.
With a lower Madoka SP you will start to see more cycling but, hopefully, it will be very efficient cycling.
As mentioned earlier now you have found your system equilibrium you can add some modulation - just a little. Put the set point 2 c above your equilibrium (so 44c?) and add Ā±2c of modulation. Then small adjustments after that.
Incidentally I have been looking at the defrost issue this morning and think that your 9kw unit will struggle to get much more than 7kw at -2 outdoors when it is misty (supersaturated air). So do not rush to change the heat pump before you go through a freezing spell. At 50c flow temp and -2 that machine will be fighting for its life, it will run much better at the lower flow temps that new rads will permit.
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The cheaper solution is to fit some fans to the existing radiators. I bought SpeedComfort ones, which have very neat magnetic fittings that go under the bottom of the radiator, making them effectively invisible and barely audible. They claim to double the heat output of the radiator, and cost between Ā£50 and Ā£100. This is way cheaper and less disruptive than changing radiators, so could be a good way to experiment before making big changes.
Warning: their automatic sensors are configured for high-temperature systems, and only switch on at 37 C, so you may need to come up with your own solution. I use a smart plug that gets told to switch on whenever the central heating pump is on. A timer could also work, or just have them on permanently all winter.
I also have two 140mm PC fans positioned underneath a K3; cost about half as much but mounting it to the radiator is way trickier that I had imagined.
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Wish I had heard about those earlier! Splendid idea.
They seem to come on at 33c - the same as the Daikin HPC and much cheaper than JAGA charge (over Ā£300) for something similar.
Speed Comfort Duo Set - Energy Saving Radiator Fan - Easy Setup - SpeedComfort | eBay
Will these really work?
Downstairs in our house we have the following K2s that I was going to change for K3s
K2s 2@600 x 1200, 1@ 600 x 1600, 1@ 600 x 1400 and 1@ 400 x 2200
I was going to change for
K3s 2@ 600 x 1800, 1@ 700 x 1600, 1@ 700 x 1400 and 1@ 500 x 2200
These fans on my current radiators would have the same effect?
Hard to tell as they do not supply any dataā¦. But forced convection is much better than free convection and if the Daikin data sheet is any guide theirs give 2.5 the output at max fan vs min fan. Iāve ordered one double at just over Ā£100 for one of my rooms where I did not change the radiator. Donāt forget you will need a socket nearby. I think it is worth a try on one radiator to see if you can avoid all those costly K3s.
I left my upstairs rads as they were so they are very undersized but the bedrooms are not any colder than downstairs due to the heat leaking through the ceiling/floor and the staircase. Iād put a big radiator in the hall/stairs ā assuming it opens on to all up and downstairs rooms, then if a room is colder that desired you just leave the door open.
Colin
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There are still questions around the performance of the system, even when it reaches equilibrium:
This has the estimated carnot heat overlaid in red, using a heatpump factor of 0.54. It seems that the dT doesnāt settle into place until 6:30, and is somewhat lower than expected before that time. This would certainly explain why short cycles were failing to show good performance, although it does sometimes perform better when it starts hot, for example after this defrost*:
(*why is it doing a defrost when the ambient temperature is 10 C? Iāve seen other Daikins do the same)
It seems like thereās some lag in the system somewhere, not sure itās the unit itself or the whole system. I had wondered it it might be a metering issue, but I canāt see any patterns that point to that being the case.
Iāve looked over some charts for other Daikin systems, and some donāt show this phenomenon at all; carnot fits the heat output pretty well, except during DHW. Some systems do struggle to match carnot when they are short cycling. Even Colinās system shows slightly worse COP at the beginning of a cycle, short of 3 kW until it reaches a stable temperature.
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