samohtom
05-02-2012, 01:04 PM
Hi. I had an 8KW heat pump put in back in mid December. For a while it was sort of working though I never really got all the rooms up to the temperature I'd like them - I'm now wondering if it's ever been working properly. My flat is a 500 square feet ground floor flat (with flats either side and another one above it). Fully double glazed, cavity wall insulation, 50mm slab insulation in the ceilings. We've recently had some very cold weather, and I haven't been able to get the room to temperature. I'm running Dimplex smartrads and a single ordinary oversized rad in the bathroom, so I can use low flow temperature. I contacted the installer saying I couldn't get the rooms to temperature and was told to try running at 45. Now the pump can get the tank to 45 but I don't think it's working correctly. I've swiped the below from another post by desA (thanks that man):-
To calculate the delivery from the heat-pump do a simple test on the water buffer tank:
1. Make sure that water circulates only heat-pump to/from buffer tank;
2. Measure the time taken to heat up the water from start temp to top temp;
3. Calculate the average heating power into the water as follows:
Q = 1000*(Volume [m3])*(4186.8)*(T,end - T,start)/(time taken in seconds) = heating power in Watts (/1000 => kW)
(temps in 'C, or K)
I did exactly that today. Results as follows:-
starting at 9:23 am tank was at 33 and it was -1 outside. Finished getting to 45 at 11:21 am, now read 6 degrees outside. It's a 150L tank, plugging in the numbers I make it 1064W delivered over just under 2 hours. Now is it me or is that much lower than you'd expect for an 8KW pump? I could probably do better with an immersion! I should be oversized if anything (the company deliberately oversizes and provides a buffer tank so the pump doesn't multicycle - and it certainly doesn't do that as the pump's on all the time at the moment!). Back when they were installing it they did have an issue and had to call an engineer out at the time, something to do with a reversing valve I think? Would that explain anything perhaps?
I'd appreciate feedback from anyone who understands this stuff more than me - I know heat pump capacity drops in cold weather but surely not that much? The quoted figures for the pump are COP 4.27W/W (whatever that means?) STC AIR 7 degrees celsius, water 35 deg Celsius.*
Thanks*
Tom
To calculate the delivery from the heat-pump do a simple test on the water buffer tank:
1. Make sure that water circulates only heat-pump to/from buffer tank;
2. Measure the time taken to heat up the water from start temp to top temp;
3. Calculate the average heating power into the water as follows:
Q = 1000*(Volume [m3])*(4186.8)*(T,end - T,start)/(time taken in seconds) = heating power in Watts (/1000 => kW)
(temps in 'C, or K)
I did exactly that today. Results as follows:-
starting at 9:23 am tank was at 33 and it was -1 outside. Finished getting to 45 at 11:21 am, now read 6 degrees outside. It's a 150L tank, plugging in the numbers I make it 1064W delivered over just under 2 hours. Now is it me or is that much lower than you'd expect for an 8KW pump? I could probably do better with an immersion! I should be oversized if anything (the company deliberately oversizes and provides a buffer tank so the pump doesn't multicycle - and it certainly doesn't do that as the pump's on all the time at the moment!). Back when they were installing it they did have an issue and had to call an engineer out at the time, something to do with a reversing valve I think? Would that explain anything perhaps?
I'd appreciate feedback from anyone who understands this stuff more than me - I know heat pump capacity drops in cold weather but surely not that much? The quoted figures for the pump are COP 4.27W/W (whatever that means?) STC AIR 7 degrees celsius, water 35 deg Celsius.*
Thanks*
Tom