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Mike the Boilerman -

Your Gas Safe Registered boiler 

and central heating repair technician in west Berkshire. Willing to travel :)

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Last updated 11th January 2021.

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Heat pumps - some things they don’t tell you.


I’m imagining you’ve already worked out how heat pumps work, in that they use fridge technology to collect heat energy from outside, raise the temperature and use it to heat your home. Here is my summary of the information I think you need to know when considering ‘going green’, and replacing your gas or oil boiler with a heat pump:


Heat pumps come in two broad types. Those that generate warm air, and those that generate warm water. 


Those that generate warm air won’t heat your hot water tank - something easily overlooked initially.


Heat pumps either collect their heat energy from the ground, or from the outside air. The latter have a large air fan which does not run silently. Beware.


The benefits of warm air heat pumps are they cheap to buy and most provide cool air in summer too. They are mostly modular for fitting by non-refrigeration technicians but generally heat only one or two rooms. Good for summer cooling, poor as a main source of heating for a whole house. One possible exception is when your existing gas heating is the warm air ducted type.


The fuel efficiency of hot water-producing heat pumps is poor if they are set to produce water as hot as your existing gas boiler produces. They work best producing water at about 50 degrees Celsius. This means your radiators will not get so hot, so you will need bigger radiators when retro-fitting one to replace a gas boiler. 


Heat pumps are an excellent partner for under floor heating systems as underfloor heating uses the low flow temperature that gets the best from a heat pump. Heat pump salesmen will often suggest partnering heat pumps with underfloor heating.


Underfloor heating relies on heating a large and heavy floor slab, so the response time to you turning up the heating can be very poor. This means it works well if you need your house warm all the time, but not so well if you like to turn it OFF when you go out in the daytime and back ON when you return in the evening. Also might seem unresponsive if you like to turn the heating OFF at night and ON for a short period in the morning before going off to work. 


Underfloor heating is best not installed in upstairs rooms as it make access to under the floorboards (for occasional maintenance) very difficult.


It is my view that a hot-water heat pump with it’s low flow temperature is best installed in addition to your existing gas boiler, as a hybrid system. This means you can connect it to your existing radiator system and most of the time, the 50c flow temperature from the heat pump will heat your house perfectly well using the existing radiators (as the weather is not always that cold). When the outside temperature dips below freezing, the gas boiler comes ON to supplement the heat pump output. This method is far more environmentally friendly than using gas to heat your house all the time and seems to me to be a good compromise as it no longer requires you to replace all the radiators with larger. 

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Copyright Michael Bryant 2020

Site first published 16th January 2004

Last updated Monday 11th January 2021

Gas Safe Register 197499, CIPHE registration number 56207

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