any idea why pretty much regardless of mode my 7kw unit will beat heating scop when heating hot water?
about 3 for heating since install last year and 3.72 for hot water.
250L Vaillant heat pump tank.
Adam,
I’m assuming you mean the Vaillant reported COP in the MyVaillant App rather then an OEM calculated COP. If so have a look at this thread:
and follow the instructions in this part for checking if your Vaillant flow and return sensors are mis-reporting
As the thread describes it’s a really common problem with Vaillant Aerotherm systems. That said in theory you should also see that on you hot water too. MyVaillant App is misreporting my COP by over 33% due to temp sensor issues
Ian
What if your
- DHW tank always have the coil totally covered by 10c water when reheat starts
- and you don’t have a super low CH flow temperatures,
- and your coil keeps the flow temperatures within a few c of the DHW tank temperature while heating.
Hi Adam, The known variability in the accuracy of the Valliant COP results does not resolve a key part of your question.
The heating cop is ~3 and the DHW cop is 3.78. I assume it is over a full year.
This is indeed curious, since my system has a DHW cop of 3.5 but the heating cop is much higher. However, I run with low water flow temperatures in winter ( flow temp <30C at 0C).
The DHW is used all year, with improved performance in the summer months to balance out the colder months. I think the DHW cop around 3-ish feels about right. Your 3.78 can easily fall within the inaccuracy band. Likewise your heating cop of ~3 is probably closer to 2.5-2.75.
I can only speculate that your lower heating cop is due to a poorer house heating performance over the cold months with no heating necessary in summer.
I would guess you have radiators and are running at a higher flow temperature above 45C on your coldest day.
More information would be helpful on your house and heating system, especially over winter.
i have the flow return sensor issue that means my cop is underestimated. the period is since June last year. the DHW cop i was getting back in the heat wave was 4.9, which was heating at 2pm each day in ECO mode.
the heating is running about 35c flow this time of year where outside is 5c, thats with a 19.5c target.
Okay, so it’s an under-estimation from the temperature sensors. So the partial annual hot water COP is more likely at or above 4 on ECO mode and your heating COP is at or above 3.5. Your heating information looks like ~40C water temperature at 0C. That looks pretty good to me for a radiator system.
I understand that ECO DHW mode limits the tank heating to a compressor speed of ~60rps. For a 7kW machine, that is a heating limit of 6.3 kW for A7/W45. The associated COP is 4.8. If you had used Normal mode, then it would run toward 120 rps with a heat output of 12.3kW and a COP of 3.9.
It now looks like your getting much better DHW COP because your in ECO mode.
As a side note. I would be more inclined to look at your running costs to see the actual benefit of ECO mode versus Normal mode. The ECO does use roughly half the electrical power, but typically takes much longer. So the kWh difference may be small. For your 7kW machine is roughly 57 minutes in Normal versus 110 mins on ECO. That is taking water from 10C to 50C with outside at 7C. So the actual cost saving may be a few pence per tank!
If you can reheat your tank on a night tariff, then it’s much cheaper using NORMAL mode at night than ECO in the day.
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i heat the tank twice per day to 40c. once at might on the cheap rate then again about 8pm from my batteries. i have 27kwh of batteries so after 5am i run on those till midnight. i tried heating to 50c once per day on the cheap rate night period but found we used up the 250L tank on some days. so had enough of the missus complaining and did a split heat up time.
it would be handy if it could do one at night to 50c and then another at 40c around 8pm if needed.
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As you say, heating to a target temperature of 40C can be problematic. If the temperature falls to 37C or less, then most folk feel it’s not warm. You will lose heat from the tank during the night/day, so it does not take too long to fall below 37C and lose that magical family harmony ( been there and done that).
I guesstimate that I lose about 0.5C/ hr from my 190 litre tank. So that’s my rough guide to ensure hot water for the family. In my house, we target 40C as our minimum and work backwards. That sets a temperature of ~48C, but we actually use 50C to get our heating during the low night tariff.
Have you considered a mid temperature of 45C. That buys you more time (~10 hrs ) before the tank cools too much.
PS. Your target water temperature of 40C also explains your high DHW CoP.
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i did have it set to 45c in the summer when i was getting the high 4.9 cop but then dropped went through the experimentation session recently as had a few days where we ran out. i find it crazy as its a 250L tank and has a very low heat loss, but 3 people one who washes there hair separately from the shower is what seems to trip it.
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