21 June 2021

Four reasons not cool your house using a heat pump

Air source heat pumps are clever little things. Not only can they heat your house, but did you know that they can also cool? In any refrigeration system reversing from cooling to heating is really simple and we use a 4-way valve to do this. Simply by moving a valve we can make the hot bit cold and the cold bit hot – it’s a simple trick of reversing the direction the refrigerant goes round the system. But I don’t want to get bogged down in fridge theory today, because I want to talk about heat pumps and cooling.

If you have a heat pump installed, it will heat the hot water tank and supply all the heat for your radiators and underfloor heating. Essentially all it is doing is cooling the garden by sucking the heat out of the air and dumping that heat into the water going round your house. If you reversed the refrigerant direction (easy when you know how) you could cool the water in the house and dump the heat in the garden.

But I’m going to be a spoil sport and share four reasons why you could do it and explain why it would be silly if you did.

#1 If you cool the house using a heat pump the RHI police will take your grant away so your cooling potentially costs you thousands of pounds in lost grants. The government and the environmentalists don’t like cooling. They think you should open the windows if you get too hot.

#2 If you pump cold water around your system, every single bit of it that’s made of metal will sweat. Think of a cold can of beer on a warm day; it sweats because the water in the air condenses on cold surfaces. So you are going to have wet pipes and dripping radiators. You could lag the pipes and put rags and towels under the rads but that’s not great is it? Underfloor heating is even worse, and you could get to a point where you have condensation on the floor. Again, not great.

#3 Cold air doesn’t rise. So you’ve got rags under the rads and the cooling on, the cold air pours off the radiators and sinks to the floor because cold air sinks. So if it’s hot you have to lie on the floor to keep cool.

#4 Biggest reason of all: the cooling won’t be powerful enough.

Your heating is designed to supply heat in the coldest weather, when we do a heat loss we assume it is cold and dark outside and you want the rooms at 21C. We don’t take into consideration all the people, dogs, cats, ovens, kettles, TVs, etc. which you could have in the house. These all help by adding heat so we see them as a bonus. But if you do a heat gain calculation for cooling you have to fight all the heat coming into the room from the sun, your pets, people and appliances.

As an example, a standard house built in 2000 would need about 40 watts of heat for every square metre of floor area, but in cooling it would need over 100 watts per metre square. So the cooling load is more than twice the heating load.

Your system has a design flow temperature typically around 50C for rads, that’s 30 degrees higher than the room temperature, and 45C for underfloor heating, or 25 degrees above the room temperature.

The heating / cooling load is proportional to the difference between the water in the rads and the room temperature. Hot rads emit more heat than cold ones. Its the same in cooling. Cold rad = good cooling, tepid rad = poor cooling.

If you want double the capacity out of the radiators or UFH you have to double the temperature difference between the room and the water in the radiators and underfloor heating. So that means running the water through radiators are 21 – (30*2) = -40C. You will be making ice on the radiators if you did this and the water in the system would be like slush puppy even with glycol. I implore you do not try this at home if you have been tempted because you won’t be covered by warranty.

So when you are offered cooling using a heat pump, it is possible but it is not going to be real cooling like you get with the air conditioning in your car, its just going to take a tiny bit of heat away. On some heat pumps we can offer a dew point sensor which means the radiators and UFH never sweat. To do this we can only drop the water a few degrees below the room temperature. The result is you only get a tiny cooling effect which you will only feel on your ankles. It’s not very good, but it is cheap to do (well free actually, if you dont tell the RHI).

But it’s not all bad news. If you want cooling using your air source heat pumps, we can put a separate, insulated cooling system onto a heat pump like this below. The cooling zone would use fan coils designed for cooling and normal rads in heating.

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