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More on the cost effectiveness of heat pumps

Updated: Jul 16

We posted back in December 2023 about our thoughts on the cost effectiveness of heat pumps. We’re back with an update! If you didn’t read that post, do have a look as it provides some useful background info.

This post focuses on retrofitting heat pumps where the heat pump is replacing an existing heat source. Our recent post about our first Vaillant aroTHERM plus installation generated some interesting debate on LinkedIn on what the design flow temperature should be, so we thought this deserved more discussion.

When designing a new installation, the lowest practical flow temperature should be designed in.  There are good reasons for this – it will minimise energy usage and therefore keep bills as low as possible and minimise the demand on the grid.

When the installation is a retrofit for an existing wet heating system, other considerations come into play. Until recently the generic air to water heat pumps had a maximum flow temperature of around 55°C and specialist high temp units provide flow temperatures of approximately 75°C; these were two-stage units with two separate units. But recently some manufacturers have started manufacturing monobloc units with a maximum flow temperature of 75°C. This may not appear to be a significant development, but actually it’s pretty important. It provides, in some circumstances, the ability to drop in the heat pump without needing to upgrade the radiators and pipework.

Again, if possible, a low flow temperature should be designed in, but if the client is not willing or able to upgrade the existing pipework and radiators, the option to use a low flow temperature is not available. A little while ago, we lost out on a heat pump installation because the client was not willing to upgrade the wet system and so just replaced their oil boiler like for like. I consider this a loss not just in revenue but also in our effort to decarbonise. If I had known about the new monobloc units then, I could have offered to install a heat pump that matched the existing flow temperature.

So how much does running one of these new heat pumps cost? In our post in December, we considered the relative cost of fuel sources along with the effect of flow temperature on heat pump efficiency. The following table uses information from the Nottingham Energy Partnership’s March 2024 update and the SCoP figures published by MCS for the Vaillant and Samsung heat pumps considered.

SCoP stats

So, what is the table telling us? According to the information from the NEP and MCS, if you design a heating system with a flow temperature of 45°C, the Vaillant unit is the most cost effective to run. Increasing the flow temperature to 50°C reduces the efficiency and so increases the running cost. The cost increase is marginal and both the Vaillant and Samsung have reduced running costs versus mains gas but are more expensive than heating oil.

Keeping the flow temperatures down reduces the energy usage running costs.

Now, consider the situation where the retrofit is onto a system designed for a flow temperature of 65°C. Both heat pump units can provide this flow rate, but should we install them? It might be better to turn the question round and ask, “If the only viable option is to install a heat source delivering 65°C, what is the best for the client and environment?” This is exactly what happened with one of our recent jobs where the customer had an old Daikin high temp unit that had failed and was not cost effective to repair. The client was unwilling to replace the existing wet system as it had worked perfectly well for the last 12 years. The options were therefore to install a replacement heat pump and match the original flow temperature, install an electric boiler or install an oil boiler. We considered each option and worked out the following:

  • The electric boiler would use nearly three times the electricity. So considerably more expensive than the other two options to run. According to NEP, this would have the second highest CO² impact.

  • The oil boiler would be more cost effective to run but would produce the most CO².

  • The heat pump running at 65°C would use nearly a third of the electricity of the electric boiler and would have the lowest CO² impact.

We should always aim to reduce CO² production and energy consumption as much as possible but, like many things in life, we sometimes face situations where we must make a decision that is the best possible decision at that time. As a company, Artimus will always strive to provide the customer with an installation that meets their needs (even if that might not always be what they thought they wanted!) We don’t want to install something that we know won’t work or won’t achieve their goals.

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