Balancing hot water and heating requirements with heat pumps part 2

This article is a continuation of an article we shared some months ago on balancing hot water and heating requirements with a heat pump system. 

In our previous article we focused on the risk that sizing heat pumps for well insulated new build homes can lead to slower response times for the replenishment of hot water. If you haven’t read that article or want to refresh your memory on the suggestions for mitigating this risk, you can find it here. But in short, better insulated homes which have lower heat losses often have relatively small heat pumps to supply their needs. But whilst this increases efficiency, it can lead to slower response times for heating hot water.

In this second part of this discussion, I want to look at the issue the other way round and focus on the risk that failing to account for hot water demands can have on room temperatures. Let’s look at example:  a new-build 130 sqm home with a 4.5 kW heat pump with 2 bathrooms and a 200 litre tank. This hypothetical house has a typical heating programme that is on frost protect only during the day. Our resident comes home half an hour after the programme begins, and has a bath. The heat pump will then switch to hot water potentially not having achieved the comfort temperatures for heating. The heating will then remain off as hot water is prioritised for however long it takes to recover the heat in the tank. With the amount of water taken for bath this could be an hour or more during which time the house remains below target temperature. Maybe during this time another family member has a shower and our original bather washes a sink full of the good glasses that don't go into the dishwasher. Now our heat up time is extended by another half an hour.  By the time the heat pump switches its effort back to room heating, it may be late in the evening and there is no time for the target temperature to be reached before the heating switches off again. This can lead to a vicious cycle – which is exacerbated by cold outside temperatures -where the house cools and never has enough time to reach its target temperatures. 

The problem can be overcome retrospectively with scheduling. There are domestic hot water time and temperature programmes built into the heat pump controller. Where you have a well insulated property with a relatively small heat pump required for the heating load, use these programmes to schedule the heating and hot water to come on and off at different times. You can, alternatively, drop the target temperatures in the tank during the peak heating hours of the day, so that the heating switches back on faster if hot water is taken from the tank. 

Some education of end users in longer/continuous heating programmes for systems with low flow temperatures to the emitters is also advisable. Instead of switching heating off completely during the day and expecting the kind of rapid response we see from radiators, it can be advisable simply to set a setback temperature that maintains a basic level of heat, from which the comfort temperature can be reached more easily. 

To avoid the issue more comprehensively, take the following into account at the design stage of a project:

  • Consider a slightly larger heat pump to allow for faster response times for hot water heating
  • Consider a larger buffer tank to supply the heating whilst the heat pump recovers the temperature in the water tank
  • Consider the use of point of use and instant water heaters to avoid to avoid this conflict – see our previous article to read more about the benefits of this approach