Tuesday, 10 February 2015

Do heat pumps deliver?

This post is slightly out of date in that you can how get high temperature heat pumps to run at normal radiator temperatures . However there is still a performance hit compared to lower temperatures - see 'High temperature heat pumps go better with weather compensation'.

A colleague said to me recently that heat pumps are pretty useless really, which I thought was a pretty broad statement and hopefully wrong. Many people think heat pumps are the best option for weaning us off fossil fuels - but they don't deliver heat in the same way as a gas or oil boiler. It seems to me that some of the cynicism about heat pumps is due to misconceptions of how to use them.

Heat pumps are sized to provide your full heat demand 99% of the time
The guidelines for specifying heat pumps require the installer to calculate the heat demand for the home and work out what size of pump is needed to deliver all your heat 99% of the time. The expectation is that you will have it on all the time, not just a few hours a day like your old boiler, and even then there will be a few days a year when it isn't quite up to it and you will need a backup system (or put an extra jumper on). Also, there won't be much spare capacity so if you do let the house go cold it will take a while to heat it back up, especially if it is a very cold day.

The limiting factor is the radiators
Eh? Why is this? If it isn't powerful enough, why can't I have a bigger one please? Well the trouble is, that your boiler is only half of your heating system. The other half is your radiators, that deliver the heat into your rooms. Normally you can increase the heat coming out of your radiators by running them hotter. A traditional boiler can easily generate heat at 60C, 70C or even 80C if necessary. However, heat pumps only work efficiently up to about 55C, tops. So you could have a bigger heat pump but you would need bigger radiators too.

Why can't heat pumps supply high temperature heat?
A heat pump works on the same principle as a refrigerator. The refrigerator extracts heat from inside the fridge and dumps it into the room outside. A heat pump heats your home by extracting heat from outside (usually from the air or the ground) and 'dumping' it into the house. In each case, the pump forces the heat 'uphill' from a low temperature to a high temperature. The bigger the temperature difference the harder it has to work.

The COP (coefficient of performance) of a heat pump tells you how much heat it is shifting, compared to the electricity it uses. High values are good. If COP is 3, then you can deliver 3 kWh of heat using 1 kWh of electricity. Oil and gas are lower in carbon emissions than electricity and also cheaper to buy so you need a COP of at least 2.5 to be worthwhile. Generally speaking, the COP is high for delivering low temperature heat and when it isn't too cold outside. The chart below shows how the COP goes down with increasing delivery temperature, for a typical heat pump. Obviously one unit won't be quite the same as another but they all work on the same principles.
From http://heatpumps.co.uk/graphs.htm
Interestingly, if you apply for the Renewable Heat Incentive, the renewable energy calculation lumps heat pumps into two categories - ground source and air source -  and then calculates SPF (seasonal performance factor)  from a star rating system based on the radiators and hence the heating circuit temperature [1]. You need to get an SPF of 2.5 or above to qualify for the RHI: for an air source heat pump you would need to run at no more than 50C to meet this requirement.

How can I get more heat at a lower temperature?
New homes with heat pumps often have underfloor heating. This works at much lower temperatures because you have a much larger radiator - the whole floor. Some homes use warm air heating, and that can be effective at fairly low temperatures too.

If you want to install a heat pump to use with your existing radiators, then you will be OK if they are sufficiently over-sized - for example if you have put in double glazing and lots of insulation then you don't need as much heat as you did before. Otherwise you need to put in larger radiators or forced convection radiators or fan coil radiators.

Why are heat pumps often not as effective as they are supposed to be?
The EST and DECC have done a lot of work on heat pump trials, seeing what they can do, and what factors lead to poor performance[2]. Here are some of things they found.

Factors to do with the heat pump

  • Heat  pump too small - there is often an auxiliary heater which cuts in if necessary. This should be rare but if the heat pump is too small it can happen a lot. Air source heat  pumps are rated for their capacity at 7C but in very cold weather the capacity will be reduced. So they need to be sized based on their capacity at the coldest design temperature. 
  • Ground loop too small - for ground source heat pumps only - if the bore is too shallow or the loop too short then the ground can get very cold and the heat pump has to work harder
Factors to do with the plumbing

  • Insufficient insulation in the pipework - if your pipes aren't lagged well then you will lose a lot of heat before it even gets to the radiators. This is more critical than before because of the lower temperatures - a drop in 5C from 70C is no big deal, whereas 5C from 55C will make a big difference to your radiator performance.
  • Siting your pump too far away - they found one site where the pump was at the bottom of the garden. You can imagine this leads to even more heat loss from the pipes.

Factors to do with the radiators

  • Running the radiators at a higher temperature than necessary makes the heat pump work harder. It is generally better to run the system all the time at a lower temperature than half the time at a higher temperature. Also, having weather compensating controls helps - these adjust the radiator temperature lower when it is warm outside so heat demand is low.
  • Handicapping your radiators so they don't work so well means you will have to run them hotter to make up for it. So don't put carpet over your underfloor heating system and don't hide your radiators behind the sofa.

Factors to do with the hot water tank

  • Having too small a hot water tank is a problem because it means you need to run it hotter to store more heat - and that means the heat pump has to work harder. There was one home where a family of five had a 140 litre water cylinder and in order not to run out of hot water they set it to heat to 70C. The heat pump was not coping and an auxiliary immersion heater cut in.
  • Too large or insufficiently lagged hot water cylinder, leading to high heat loss. The trials found several homes where less than 20% of the heat going into the hot water cylinder was actually used as hot water - the rest leaked from the tank.

The trials show that heat pumps can work well - but they aren't the same as traditional central heating systems and if we treat them the same we will pay for it - through high bills. You won't be able to run a heat pump through your existing radiators unless you have already reduced your heat demand by other measures. Also, you should have a backup system for very cold days, such as a nice wood stove.

Heat pumps are a relatively new technology for us, and they will get a bad name if we don't recognise and fix poor installations or if we misuse them through ignorance. I found similar issues with another new technology: MVHR (see What can go wrong with MVHR). We just have to take a bit of care, and make sure we understand how to get the best from these new systems - so that they do a proper job of taking care of us.

This post was amended on 11th Feb to add a few points on advice from Peter McKeown of Cernunnos:
ASHPs not qualifying for the RHI if the heating temperature is 55C or more
Coil fan radiators being useful
ASHP reduction in capacity in colder temperatures.

[1] Heat Emitter Guide for domestic heat pumps (MCS)
[2] Detailed analysis from the first phase of the Energy Saving Trust’s heat pump field trial (DECC/EST) March 2012


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  2. Be careful talking about air-source and ground-source heat pumps together. (Outdoor) air temperature fluctuates a lot (-5 to +30C), while the ground temperature is quite stable (10-12C). On cold days, when we need most heat, it's quite easy to take energy from the ground (at 10C), but much harder to take energy from the air (at 0C). This means the heat pump has to work much harder, using much more energy.
    And it's all very well to run heating continuously if you're in all day, but this wastes heat and increases heat-loss if you're out for long periods. This strengthens the case for traditional, fast-response heating systems.

    1. I agree, GSHP and ASHP are rather different and GSHP should give better efficiency - but the trials recorded sub-zero ground temperatures in a number of cases where the ground loop was too small.

      Also I agree that there is a potential waste of heat in leaving the heat pumps on all day. It may be this is only necessary on very cold days when the heat pump has not got enough oomph to warm the home up quickly in the evening. An optimums start heating control would help with this, - you programme it for when you need the home to be warm and it works out how much in advance it needs to turn on.

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