Is my house ready for a heat pump?

 Heat pumps are hot!  Everybody either talks about them or wants one.  But how do I know if my house is ready for a heat pump?  And what type of heat pump should I go for?

In this blog, we will mainly focus on the first question.  Is my house ready for a heat pump?  After all, we don't want to find out mid-winter that we can't keep our house warm anymore.  So, let's start with the basics.

How does a heat pump work

A heat pump does not create heat, instead, it takes heat from a heat source and transfers it to a heat delivery system.  Heat sources are either air or water (brine) based.  The heat delivery system could also be air but generally is water-based, the water from your radiators or underfloor heating.  The type of heat pump is specified by its source and delivery system.  So, you will find for example an air-water heat pump which means the source is air and the delivery system is water-based.  

the outside unit of an air source heat pump

When your heat pump has an air source you will need an outside unit which looks (and functions) as an air condition unit.

Water-based sources have a closed loop of pipes running either horizontally or vertically in your garden through which brine is pumped to your heat pump.  Note that there exist also systems where the pipes run on your roof underneath your solar panels (PVT panels).

Can I just replace my current heating system with a heat pump?

As heat pumps don't create heat but merely transfer it, they can only do this efficiently at relatively low temperatures.  As a rule of thumb, you can say that a heat pump cannot warm up the water of your radiators to more than 40 °C (or say not much over 100 °F).  Unless you just have underfloor heating, your current heating system is probably set at something like 60-65 °C (140-150°F).  As a consequence, the size of your radiators will also have been calculated with those temperatures in mind.  So simply taking out your current heating system and replacing it with a heat pump might leave you with a cold house in winter. 
 
So, I have an old house, my current heating system uses high temperatures and yes, my radiators were probably calculated with these high temperatures in mind.  Does this mean all is lost?  Well, maybe not.  Chances are that you or a previous owner improved the house over the years.  Maybe the attic got isolated or the windows got replaced with double-glazed ones.  It might therefore be good to redo the calculations.  To know if your heat pump could replace your current heating system you need to do a heat loss calculation of your house and compare that to the power output of your heat delivery system.

Easy check

Doing the calculations isn't easy and often you won't know all the parameters.  So before attempting this I recommend doing the following easy check: just reduce the output of your current heating system so it feeds your radiators with the same low temperatures as a heat pump would do.  Now, wait for it to be a particularly cold stretch and see if your house stays comfortably warm or not.

If you're lucky enough that your house stays nice and warm, you can be confident you can simply replace your current heating system with a heat pump.  

What if the check failed

When you can't keep your house warm with a low-temperature heating system, you basically have three options (which of course you can combine):

• reduce the heat requirement
• increase the capacity of the heat delivery system
• opt for a hybrid system

Reduce the heat requirement

You reduce the heat requirement by isolating.  This should always be the first option you look at.  By isolating you will reduce your energy bill no matter the heat source you use.  Here are some options of what you could do:

• install double or triple glazing
• isolate the roof
• isolate the ground floor (particularly easy when you have a crawling space)
• isolate outside walls (eg by filling up the cavity between the walls) 

Increase the capacity of the heat delivery system

The simple way to increase the capacity of the heat delivery system would seem to add extra radiators.  However, rather than just adding radiators it might be a better idea to replace your existing ones with low-temperature radiators. Placing new radiators allows you to know pretty accurately the capacity of your heat delivery system, which should match up with the heat loss calculations for your house if you did them.

Opt for a hybrid system

So far when I talked about heat pumps I was talking about full electric heat pumps.  When those heat pumps would leave you with a cold house in winter it could be an option to install a hybrid heat pump.  You can compare a hybrid heat pump to a hybrid car.  Where a hybrid car has two engines, an electric and a petrol one, a hybrid heat pump has two heating systems: a full electric heat pump and a gas heating system.  When the temperature outside drops too low the gas heating system switches on to make sure the heat delivery system can keep your house comfortably warm.