Heat pumps & tariffs: £ saving method 1: Thermostats

5 days ago
6 min read

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Method 1: Thermostats. The most common method

This method of controlling heat pumps with time of use tariff is the least likely to provide energy bill savings.

Most households use traditional room thermostats and schedules to control their heat pumps day to day. This is of course the traditional way of controlling central heating.

This is fine for controlling boilers. But most households are unaware that regardless of what tariff they are on, this method is usually the least efficient way to control a heat pump.

This is even usually the case if a thermostat looks pretty modern or whizzy or describes itself as smart (though there are some exceptions).

When used with time of use tariffs these same thermostats and schedulers are typically used to:

Increase indoor temperatures at times of the day when energy is cheaper.
Reduce (or set back) indoor temperatures at times of the day when energy is more expensive.

The expectation is that this will reduce overall running costs. This is far from certain.

Method 1, Problem 1

Firstly, this method often has a basic weakness: Thermostats are often located in less than ideal locations.

A poor thermostat location....means poor information for the heat pump to work with....means poorer heat pump decisions.

Decisions about how to heat the whole house, are made on knowing the temperature in just one part of the house.

This reduces the chances of benefits from time of use tariffs and often means poorer comfort.

Method 1, Problem 2

The second big problem with relying on a central thermostat is what happens when a scheduled temperature reduction or setback period starts.

For example, let's say that we have a home where:

The time-of use tariff has a cheap rate in the afternoon from 1pm until 4pm, and a more expensive evening rate from 4pm until 10pm
Let’s say the thermostat is programmed to stay at 21°C from 1pm till 4pm and then drop the set temperature to 18°C from 4pm till 10pm.

At 4pm, at the end of the 21C afternoon period the thermostat will say:

"Ah, I have just been asked to cool the house to 18°C. But the house is 21°C. So, no more heat is needed."

The thermostat will then switch the heat pump off. The heat pump will stay off until the indoor temperature falls to below 18°C.

This means the heat pump will sit there doing nothing. Some money will be saved because the heat will be off (which sounds good, right?) but the heat pump and the heating system will all go cold.

This off period can last for several hours. At some point the temperature will drop below 18°C, at which point the thermostat decides:

"OK, the house is now too cold. I'll request more heat from the heat pump”.

Householders naturally then expect:

The radiators and any underfloor heating to warm the home back up reasonably quickly, and
The indoor temperature to reach the temperature they have requested for the next period.

The problem is that to do this the heat pump needs to quickly circulate really warm water to send round the heating system.

It basically has to work really hard to catch up. This is not good for heat pump efficiency.

It's trying to use the heat pump a bit like a boiler. But it isn't a boiler.

It is quite possible that the heat pump has to work so hard in catch up mode when the tariff is cheaper, that it wipes out most or all of the savings from when heat pump was "off" earlier.

This method also means big swings in indoor temperatures which is poor for comfort.

Are there settings that definitely DO work? What settings should people use?

Unfortunately, there is no silver bullet magic answer. I wish there was.

No-one can tell you for sure what settings to use. There are simply too many uncertainties and variables.

Heat Geek and other experts recommend that you should only use modest setbacks: 2°C at the very most.

This is very limited guidance and isn't a guarantee of success.

So savings using this method are...?

In summary, if you are on a time-of-use tariff and use thermostats and time schedules to reduce energy costs:

Savings are far from guaranteed
When savings are possible, they are likely to be small
Heating costs can even increase

This analysis draws a similar conclusion.

Fear not! All thermostats are not the same

Some manufacturers’ own thermostats are more advanced. They help control the heat pump in more intelligent and efficient ways than thermostats from other sources.

Despite this, most of them only offer a small advantage when it comes to using thermostat methods with time-of-use tariffs.

I believe there are two exceptions, some of the products from Mitsubishi and Vaillant.

Mitsubishi have a feature called "Auto adapt"

Vaillant have a feature called "Normal setback". They don’t turn simply turn the heat pump “off” when a setback period starts.

They will give much more certainty that thermostat schedules will bring time-of-use tariff cost benefits.

That depends though on IF the relevant features are enabled, other relevant settings have been setup appropriately, and the central heating system in general is set up in the right way.

Like any thermostats though and explained above, where they are physically located is REALLY important.

If you have wireless versions of the relevant Mitsubishi or Vaillant products they really need to be nailed down...in the right location of course.

Method 1: conclusion

It is very hard to know what heating settings actually work best, let alone be sure if any actual savings are being made: every home and its heating system is different.

The weather is also different every day. This means for anyone experimenting with thermostat settings it makes it virtually impossible to make reliable day-to-day comparisons.

Unless you are pretty geeky on the subject, understand how your house and heat pump behave and are prepared to experiment a lot, choosing settings is something of a guessing game and a matter of trusting to luck.

There are a limited number of heat pump controllers that offer functionality which may provide some confidence of cost savings using thermostat control methods with time of use tariffs.

Much better methods

There are two much better methods for both controlling a heat pump in general, and giving confidence of cost savings with time of use tariffs.

They are both better for comfort too.

Method 2 - Party tricks with weather compensation - applicable to some Daikin, Viessmann, Vaillant, Nibe and Dimplex heat pumps. Very useful tricks if your heat pumps can do them.

Method 3 - Automation - applicable to a broader and increasing range of heat pumps. This is the future direction for comfort and cost minimisation with heat pumps.

The Geeky Zone

Re the statements above regarding a time of use tariff can actually INCREASE costs: For simplicity one of the factors I deliberately didn't mention is other energy use in a home. Cooking on electric during a peak tariff period is an obvious example of energy use that will increase overall energy costs.
Referencing my comments about manufacturers own "more sophisticated" thermostats: I stated they would give only moderate advantage with time of use tariffs. Of the ones I am familiar with, they still turn the heat pump "off" when a drop in set point event occurs, just as a traditional thermostat. Daikins Madoka is an example of this. My understanding of the Mitsubishi and Vaillant functionality is that they keep the heat pump engaged at a lower flow temperature when a setback period starts. If this is incorrect, or there are others that do this please let me know.
Regards the point about Mitsubishi and Vaillant effectiveness needing the heating systems to be setup in the right way: I am referring to the things you would expect in open loop weather compensation mode, such as minimal or no other third party thermostat interventions, no radiators turned off etc.

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