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Air Source Heat Pumps, SCOP and humidity

AJJewsbury

My gas fired combi is getting a bit long in the tooth now, so I've been vaguely looking at the possibility of replacing it with a heat pump.

SWMBO isn't keen on the idea of ground source - we've probably got enough land for our relatively modest heating load, but having a mature garden on the top of it makes trenching a hard sell, so I've been looking at air source...

I realize that at times (very often in the cooler months I imagine) that the outdoor evaporator coils will be below 0°C - and so will ice up from condensation. Ice will not only block the airflow but insulate the coils, so needs to be removed. No problem I understand, the heat pumps detect this and automatically go into a defrost cycle where either the refrigerant flow is reversed (taking a bit of heat back from the heating water circuits and using it to melt the ice) or by direct electrical heating. From what I can tell all that's all included in the seasonal co-efficient of performance (SCOP) figures, so I can in theory still work out (roughly) how well the system should work overall.

But thinks I, having spent the last couple of days in fog, the SCOP benchmarks for EN 14825 are done according to the climates of the likes of Strasbourg, Athens and Helsinki; and it occurs to me that the British climate is often somewhat damper, so even if the temperatures match I would have thought we'd likely get a lot more condensation, and therefore a lot more ice - so it'll have to have much more frequent defrost cycles- which is going to consume more energy for no increase in output. So the SCOP figures provided by manufacturers may be well off what I could achieve in reality - which makes me a bit nervous.

My research so far seems to suggest that the test conditions only have to reflect the temperature profiles of Strasbourg, Athens and Helsinki, so far I've found no mention of humidity, so it might be possible that manufacturers could run the tests in a relatively dry atmosphere and get improved results and still comply with the standard.

Has anyone got any ideas as to how significant the defrost cycles might be on the overall SCOP?

   - Andy.

  • 0 in reply to alanblaby

    There is a lot of misinformation in this thread....

    Thanks - that was useful reply!

        - Andy.

  • 0 in reply to Eur Ing Graeme Craib CEng MIEE
    Eur Ing Graeme Craib CEng MIEE said:
    The simple fact is that heat pumps are nowhere near as effective as gas boilers.

    What do you mean by, "effective"? On the face of it, heat pumps are more efficient than gas boilers, but if the electricity which powers them is generated from gas, the efficiency is much lower. (That is why electrical energy is more expensive than gas.)

    The thing is that on a good day, a 3 kW heat pump might put out 9 kW of heat, but what if you need 72 kW. You need 8 pumps and where will you put them all?

  • 0 in reply to Chris Pearson

    If you need 72 kW to keep the house warm, then you need to put some glass in the windows in your house, to stop the icy cold wind blowing straight through.

    Unless you live in a big mansion.  In which case, buy a bigger heat pump.

  • 0

    A quite positive article on heat pumps in Norway. It does contain the key comment 'The Norwegians also benefit from well-insulated houses.' 

    ‘You can walk around in a T-shirt’: how Norway brought heat pumps in from the cold | Energy | The Guardian

  • 0 in reply to Simon Barker
    Simon Barker said:
    In which case, buy a bigger heat pump.

    That's my point!

  • 0 in reply to alanblaby

    Alan, simple thermodynamics says he was lying! You have been scammed big time. To get a COP of 9 the temperature differential between input and output needs to be about 10 degrees C. Fine for a fridge on a cold day, but for heating (particularly with water radiators) it is impossible. About 2.5 to 3 C is possible, but then do you want radiators as large as a room wall? Basically heat pumps are not the way to go, because most electricity comes from gas! Burning gas to make electricity to run a heat pump is less efficient than burning gas directly. This guff is driving me to distraction, the whole lot is political lies. If you disagree provide the genuine data, and I will refute the lot with basic physics. Theremodynamics is basic physics which has yet to be proved defective in any way.

  • 0 in reply to Roger B

    You are missing the fact that Norwregians also have large wood burners for heating when required! Have you ever been there, well I have? If you read the Guardian you will be deluded and lied to forever! Norway also has much cheaper electricity than we do!

  • 0 in reply to David Stone

    Yes I have been there, my wife worked for three months in Bergen and we have visited several different areas on holiday. The smaller hotels in the north certainly used electric resistance heating.

    Your statement does not seem to match the Norwegian view:

    How Norwegians Heat Their Houses - The Norway Guide

    Norwegians typically use either heat pumps, a wood stove or regular electric heaters to hear their houses. The heat pumps are by far the most efficient and cheapest to use, but they are pretty expensive to install.

    Current status of heat pumps in Norway and analysis of their performance and payback time - ScienceDirect

    According to a report by Asplan Viak [4], 80% of the heating energy for buildings in Norway comes from electricity (used mainly through conventional electric heaters), around 15% comes from biofuel (wood, ethanol, biodiesel), 3% from district heating systems, and less than 2% comes from fossil fuels [4]. Although almost all of the electricity produced in Norway comes from hydropower and wind farms which are renewable sources of energy, it is yet important to reduce the electricity consumption for heating and cut the costs on households by using heat pumps. 

  • 0

    We went on holiday last week for four nights, so I turned the heating down for frost protection and the hot water off. So effectively the system was off for for days and would not fire up when we got home last Sunday evening. 

    It took until Wednesday afternoon to get the parts and we don't have an immersion heater as back up for hot water, only a fan heater for heating a room.

    It's definitely time to replace the ancient Glow Worm boiler, my plan is a new heat only gas boiler to the existing cylinder. 

    Then hopefully, in the long term replace the cylinder with a new dual coil cylinder with solar thermal connected to the bottom coil and the gas boiler connected to the top coil, as well as an immersion heater for back up and/or connection to PV panels. 

    That should do it, there's always the option to retrofit a heat pump anyway if we replace the gas boiler and still have a cylinder. 

  • 0 in reply to David Stone
    David Stone said:
    This guff is driving me to distraction, the whole lot is political lies. If you disagree provide the genuine data, and I will refute the lot with basic physics. Theremodynamics is basic physics which has yet to be proved defective in any way.

    David, I have missed you. Kissing heart

    Remember gas-powered heat pumps?

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