How heat pumps are usually used in the wrong way

For the full list of articles demystifying heat pumps click here

Thermostats and timers: bad news for heat pumps!

Traditional heating systems rely heavily on timers and thermostats. They allow us to keep our heating and costs under control.

Over the years these controls have become more complicated — and more confusing.

Many homes now have:

• Central thermostats

• Portable thermostats

• Different timers and schedules

• Thermostatic radiator valves (TRVs)

• Underfloor mixing thermostats

Some homes have all the above. Paracetamol time!

This might surprise some people: With certain exceptions, using thermostats and timers of any sort is often the most inefficient way to control a heat pump*.

This means higher costs and/or poor comfort, and often worry too.

I regularly come across households who daren’t have their home at the temperature they would like or for as long as they would like, simply because they are using the heat pump in the wrong way.

Without exception, I’ve seen that once householders learn HOW they should use their heat pumps their bills immediately reduce and they have the comfort that they really want.

Why thermostats and timers are bad for efficiency and bills

Imagine a home with one central thermostat set to 20°C. It controls the heat pump, telling it to switch on.

The heat pump will heat the radiators until the thermostat decides the house is warm enough, which will be a little warmer 20C, (usually around 20.5°C). The thermostat then tells the heat pump to switch off.

The radiators cool.

The house gradually cools.

An hour or two later, when the temperature has dropped, the thermostat will decide it wants to turn the heat pump on again. This will might typically be when the home has already dropped down to around 19.5°C

The heat pump now has to start from cold and generate fairly hot water to get the home back up to temperature in a reasonable time. 

So the heat pump was on for a while....then off for quite a while....then back on again.

The same is true if use a heat pump on a timer.

When we use a timer the periods of "off" are typically even longer.

It means we are trying to use a heat pump a bit like a boiler...on for a bit....off for a bit...on for a bit.

Controlling a heat pump in this on/off/way means asking it to provide a LOT of heat occasionally, a bit like how we use a boiler. This is very hard work for a heat pump. This means it is costly.

Controlling a heat pump in this way is like driving a car at full speed, then stopping for a while, then going back to full speed. 

You could take the same car on the same journey a second time, but drive it at a steady pace. The car will still get you to the destination in the same amount pf time but it will use much less fuel and with less wear and tear.

Heat pumps are happiest and most efficient when working at this steady gentle pace. This means circulating warm water around the heating system as freely as possible, for as long as possible.

Timers, schedulers, thermostats and thermostatic radiator valves are all barriers to a heat pump doing that. The result is usually higher costs and poorer comfort.

This will even be the case if you simply keep your thermostat at a constant setting all day. The heat pump will be on for a bit…off for a bit….on for a bit....off for a bit...

One of the heat pump experts in this podcast refers to thermostats as "the devil" for heat pumps!

Why installers fit thermostats and timers

There are several reasons:

• Installer habits, from a history of fitting boilers

• Installers fit controls they think households will want to keep things familiar

• They don't feel confident or willing to educate customers in the way a heat pump SHOULD be setup and used. I'll explain in the next article.

  
  

The problem is that using thermostats and timers increases the risk of customers calling installers back because of high cost or comfort issues.

Most householders will not realise that their systems are not being setup and used in the right way.

This is a strong motivator for me to write these blogs.

To summarise

Directly repeating the line above: With certain exceptions, using thermostats and timers of any sort is usually the most inefficient way to control a heat pump.

If a heating engineer of any type tells you that continuing using thermotats and timers is the right thing to do and that the heat pump is ‘set up right, and you don’t need to concern yourself with it….they are almost certainly wrong!

The following articles explain a much much better way to use a heat pump, with step by step guides on how to make the switch.

Next: Article 6: The Goldilocks secret to great heat pump results

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*The geek zone

Where schedules and thermostats MIGHT make sense

There are some exceptions to the statements made earlier. They will be very dependent on circumstances.

:

1. In situations where room overheating is an issue in winter, such as bedroom temperatures or rooms with significant solar ganUse of thermostats make sense. In these situations thermostats should be used as "limiters of MAXIMUM temperature", not as day to day controllers
   

2. Use of timers or thermostats or timers to switch off or throttle back heat pumps where there are very high tariff periods MIGHT make sense overall. An example would be when the energy tariff is Octopus Agile with a very expensive early evening tariff period. Even then, it is a judgement call based on knowledge of the house, heat pump and how it all behaves as to whether this gives overall benefits. There is also often a comfort impact doing this.

Where schedules and thermostats make much less sense

When using with tariffs such as Octopus Cozy

Tariffs like cozy have multiple tariff rates in multiple windows across very 24 hour period. Thermostats and schedules are often used by households to setup routines that vary the indoor set temperatures up/down to match the various low/high tariff periods. An example would be Octopus Cozy.

In these situations the benefits of using a thermostat/schedule control approach are unprovable. This approach also carries the risk of INCREASING overall energy costs.

Using timers and schedules in this way means setting the heat pump up with higher weather compensation flow temperatures than would be used if using pure weather compensation mode.

The trade offs between this inefficiency vs any benefits from pure weather compensatin can only be guessed at by most householders. Making comparisons between different settings day to day using these methods is also extremely difficult because of weather variability.

With overnight cheap rate EV tariffs

My views on this situation are very similar.

In this situation:

• Use of conventional thermostats and timers is even more problematic requiring more guess work
  

• Using flow temperature offsets is still a much better alternative and more likely bring beneficial results in terms of energy savings compared to pure weather compensation, especially with wet underfloor heating

The only way to be certain of benefits with time of use tariffs

Only by using weather compensation curve offsets to modify FLOW temperatures up/down to suit different tariff low/high periods gives any certainty of benefits using a time of use tariff.

By offsets I mean allowing the user to program the controller with a fixed increase or decrease in flow temperature above or below the "baseline" weather compensation curve at different timese of day. An example. Take a situation where the flow temperature for a particular day using weather compensation is 32C. Adding an offset of +2C during a cheap tariff period would artificially lift the weather compensation curve up by 2C, and the flow rate would increase by 2C during that peirod. The flow temperature target would be 34C for this period.

This way, the heat pump keeps flowing warm water into the property without the ON/OFF issues that thermostats and schedulers cause. The heat pump just works a little harder during the cheap tariff periods, and can cruise a bit when tariffs are high.

This is possible to configure with some heat pumps (such as Daikin, Nibe and possibly Samsung). BUT, this needs manually defining. It still requires some skill and judgement. It is also likely to need to experimentation and changes to suit different weather conditions.

 It also means juggling overall weather compensation settings to suit. Boosting and reducing flow temperatures with offsets distorts the overall amount of heat going into the property.

This might well bring useful and credible savings but isn't the type of intervention but is probably beyond most households to understand and configure well. In my opinion, only useing an intelligent third party control system such as Homely or Havewise can do this in a way that you can gurantee savings.

For homes with batteries

It is of course possible to use timers and schedules to reduce overall costs in the above situations.

It is very much for geeky householders to work out and have confidence what benefits can be realised.

The thrust of all my articles though is to help the vast majority of heat pump households who simply could not afford batteries or wouldn’t be able to sensibly engage with them if they had them. This includes many in social housing and fuel poverty.

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Thanks.

Mark Thompson

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