I felt it would be helpful to share some lessons about a heat pump installation from a customer's perspective.  Ours was completed last week.

My wife and I live in a 5 bedroom house of about 350 square metres spread across two floors.  It was built in 1968 and has had several large renovation projects since then.  The heat pump installation has gone well, but of course we won't know how well the central heating performs until the winter.

The house has cavity wall insulation, partial under floor insulation and about 70% of the loft is insulated to current standards.  We will be completing this in due course.  Heat loss calculations were about 15kW, though our gas bills are somewhat lower than this.  Admittedly last winter, we did run the gas heating a bit too low and felt chilly!

Here are the lessons learnt:

1. Preparation for a heat pump installation involves many time-consuming steps. Therefore, it is much better to plan and do it before your old boiler fails for the last time.  This was a motivation for installing our heat pump when the boiler was 10 years old, rather than risking waiting for a few more years.
2. It is important to find an installer who follows good design practice and thinks in a similar way to you.  I found a couple of installers with experience of large properties, but they had a different mindset and I foresaw potential disagreements.  I chose an installer that Mars recommended, which was a good decision.  
3. Even after months of painstaking research, I still missed some important design considerations - see hot water below.
4. In a complex house design, such as a house that has had various extensions at different times, heat loss calculations can be error prone and may give a wide spread of results. An eagle-eyed customer can spot and correct obvious errors.  Still, the margin of error is probably quite high.
5. Different installers may propose significantly different solutions. In our case, the differences included:
   • Number of heat pumps (1 or 2).
   • Heat pump manufacturer.
   • Number of radiators needing replacing (0-12 out of 33).
   • Hot water design (coil in the tank, or plate heat exchanger).
   • Hot water cylinder capacity (300-400 litres)
   • Whether they usually propose buffer tanks or open loop.
   • Method of controlling temperature – room thermostat, weather compensation, a hybrid of the two, or whatever the customer wants.
6. When moving from vented (gravity fed) to unvented (mains pressure) hot water, it is important to check whether the mains can provide enough water throughput for the various outlets.  Our house has three bathrooms and a shower room.  However, my focus was on the central heating and on the water capacity of the hot water tanks.  It did not occur to me that the water main in our house might not be powerful enough to match a gravity fed system with three shower pumps.  Fortunately, only two of us live in the house for the vast majority of the time, so it is rare to have more than one shower running at a time.
7. Heat pumps are much more complicated than boilers:
   • They have many modifiable parameters.  Fortunately, I have a physics degree and worked in IT for 40 years, so I am curious about that sort of thing and happy to experiment carefully.  Other customers might be less comfortable with this.
   • The installation manual and user guide may not give any explanation for some parameters, apart from their names.   
8. Even with lagging, long pipe runs can lose a significant amount of heat. Heat loss is typically quoted in terms of the thermal conductivity of the material, rather than the heat loss per metre of a product with specific dimensions.  Calculating the actual heat loss requires quite detailed maths.  With more thinking time, I would probably have gone for thicker insulation.
9. Some equipment may have a long lead time – e.g. colour-specified Stelrad Vertical Ultra radiators for bathrooms have a lead time of 14 weeks.  This means our installers will have to make a return visit and drain the upstairs radiators, but in the meantime we have a fully working system.
10. A complex installation can take over a week:
    • Hot water can be provided for almost all of the time.
    • However, the central heating is unlikely to be available until the end.
11. There are pros and cons to the season you choose for your installation:
    • Summer has the huge advantage that the house will stay warm. However, you have to wait for months to test the performance of the central heating.
    • A winter installation has the advantage that you can test the performance of the central heating immediately after installation. However, you have to endure a freezing house first, unless you can stay somewhere else during the installation.  Therefore, we chose summer.
12. You need to be confident enough to deal with the concerns of well-meaning relatives and friends, who may warn you that you are making a terrible mistake!

I hope people find this helpful.

Graham