Heat pumps & tariffs: £ saving method 2: Party tricks

5 days ago
8 min read

For the full list of articles demystifying heat pumps click here

Method 2 - Party tricks based on weather compensation

As you may have read in the other article, using thermostats and time schedules with time of use tariffs makes energy bill savings very uncertain and bills can even increase.

Method 2 is MUCH better. It provides high levels of confidence a heat pump will be cheaper to run with time of use tariffs.

This is only the case IF your heat pump is capable of a party trick, details of which heat pumps can do this are below.

It is a way of controlling heating that you may not be used to. More and more people are becoming aware of its benefits.

Method 2, Step 1

The first step is to use your heat pump in a mode called weather compensation mode.

Weather compensation mode means that the heat pump ITSELF controls the temperature in the radiators or underfloor heating. With a little "teaching" the heat pump knows what temperature to send round the radiators to keep the house at a comfortable temperature.

Indoor temperature is NO LONGER controlled by thermostats.

When set up correctly, the indoor temperature remains the same, 24/7, regardless of the weather.

This is brilliant for heat pump efficiency and is explained in this article here (and in the articles that follow it....all easy reads I promise).

Heat Geek recommends all heat pumps are controlled in this way.

I've also published free practical step-by-step guides on how to switch over to this approach for a number of heat pump brands, with more to follow.

Method 2, Step 2

Once you have taken step 1 and put your heat pump into purely weather compensation mode, you will have made a really worthwhile change.

Your heat pump efficiency will now be much better than when using thermostats. Comfort levels will be more consistent, and your running costs will almost certainly be lower.

To get the additional savings from a time-of-use tariff, you now need to take Step 2.

This involves getting your heat pump to do an additional party trick, while also allowing it to do what it does best: pumping warm water pretty much all the time around the heating system.

Side note: this does NOT affect hot water for sinks/showers etc.

The party trick is to get the heat pump to vary the temperature of the water in the heating: different flow temperatures at different times of the day, depending upon the tariff:

Slightly warmer water flowing round when the tariff is cheap
Slightly cooler water flowing round when the tariff is more expensive

This keeps the heat pump in a very happy, efficient place: not stopping it, just tweaking what it does. A very happy heat pump and good for your bills.

This allows the heat pump to always run in what is often called the "low and slow" way of running.

This method means the heat pump just working a little harder during some periods, and less hard during others.

It's rather like the way we walk along the pavement: slightly faster sometimes, slightly slower at other times. Pretty efficient all the time.

This method means:

The heat pump is always operating at flow temperatures that are close to ideal for it. This is great for efficiency
Less wear and tear on the heat pump
Improved comfort levels, as the indoor temperature changes are actually quite small
Very high certainty of benefits from time-of-use tariffs

As far as I know, the only potential downside is the common concern about bedrooms being too warm at night. If this is a concern for you and you want to know what you can do to address this, check out any of my guides. Just do a keyword search for the section on bedrooms.

Heat pumps that can do this party trick

Unfortunately, not all heat pumps can do this super-duper party trick.

If your heat pump isn't one of them all is not lost. Method 3 later offers another, even cleverer way of doing this which may help you. Method 3 also works with lots of heat pump brands.

This Daikin heat pump controller is one of the controllers that can do the method 2 party trick. It’s also a nice easy controller to use.

I believe some Nibe, Viessmann and Vaillant controllers can also do similar tricks, as can Dimplex ground source heat pumps. If you know of any others, do get in touch.

An example: How the Daikin does its party trick

The Daikin controller allows you to create schedules through the day. They are not conventional though.

We normally think of heating schedules as setting different indoor temperatures for different times of the day.

But if you are operating your heat pump in weather compensation mode, there is no thermostat controlling the indoor temperature. The heat pump is in charge, and in control of the water flow temperature.

To do the tariff party trick, the schedule feature in the Daikin now does things differently. When you add a schedule you are NOW setting it to increase or decrease heating flow temperature slightly.

When adding a schedule now you are creating either:

A positive offset or heating flow temperature increase for a period (e.g. increasing flow temperature by, say, 2°C for 3 hours)
A negative offset or heating flow temperature decrease for a period (e.g. reducing flow temperature by, say, –1°C for 5 hours)

This short video shows how to setup an offset schedule using the Daikin controller. You may need installer level access. (For info, you cannot currently use the Daikin Onecta app to schedule offsets).

Now, imagine you've created the above schedule of offsets on a winter’s day when the water is flowing round the heating system at 35°C.

When the first scheduled offset period starts, the flow temperature round your heating will increase by 2 degrees and will now be 37°C for 3 hours.

At the end of that period at 37C the flow temperature for the next 5-hour period will be 34°C (1 degree below the 35C that it would normally have been).

At the end of that second period the flow temperature will go back to where it started, i.e. 35°C.

In a nutshell:

Old method (using thermostat): create a schedule of different house temperatures at different times of day

New party trick method (using weather compensation mode and scheduled offsets): create a schedule that nudges the radiator and underfloor water temperatures a little warmer or a little cooler at different times

A golden rule

Don't forget you are controlling your heat pump in a new way. You are using weather compensation mode, rather than a thermostat.

If you only schedule "plus" offsets, your home will gradually get warmer over a few days! It will settle down, of course, but at a higher average indoor temperature than before you started playing heat pump party tricks.

To prevent this, I suggest that any "plus" offsets you programme are balanced by "negative" offsets in a 24-hour period.

Here is an example of what I mean:

Case study: Octopus Cosy tariff

This is a simple example of the sort of thing you might do if you are on the Octopus Cosy tariff.

The tariff has three different rates: three periods of cheap off-peak energy, three periods at a higher tariff rate, and then an early evening tariff at a very high rate.

I've shown on the bottom row an example of the sorts of offsets you might experiment with, with some set as positive and others as negative.

The golden rule: if you add up all the figures in the pink offset row, the result is zero. This will prevent the house temperature drifting warmer or cooler in the longer term.

How to approach tariffs with high evening peaks

With many time-of-use tariffs the most expensive energy is during the early evening peak from 4pm to 7pm.

Getting the comfort levels wrong at this time of day is likely to lead to most complaints from your household. Your home could end up too cool for the rest of the evening.

To ensure that you are not using more electricity than you need to during this peak period, the best approach is to focus on:

How big can you make negative offsets during peak tariff periods before you get into comfort issues in the evening, and
How much you can gradually prewarm the house using cheaper energy beforehand

What to do if you are on an EV tariff

Many heat pump owners who have EVs are on tariffs designed primarily for EV charging. They have a single low-tariff period overnight. In this situation the overnight rates are incredibly cheap – as little as a quarter of the standard electricity price.

For anyone on one of these tariffs, it makes sense to work the heat pump with quite bold positive offsets during the cheap overnight period.

This pre-warms the house so the heat pump has less to do for the rest of the day. The golden rule above still applies though.

The balance and the challenge in this case is how to avoid overheating upstairs, given that most people feel more comfortable if their bedrooms are cooler at night.

See my comment and link above regarding bedroom temperature control.

Other points and recommendations

Other heat pump controllers will have different ways of scheduling heating flow temperature nudges. Some do it by allowing you to use different weather compensation curves for different times of day. ChatGPT may be a useful help for you to understand how to do this with your specific heat pump.
Start off with small offset figures and learn how the house behaves
Increase any offsets gently and give the house a few days to respond
From my own experience, if you are too bold comfort issues become a problem. From my experience +5C as a fairly large offset
For really fine tuning the benefits avoid steps DOWN in flow temperature that are bigger than 2C, and steps UP that are greater than 4C. The example table above shows this in action
Tune in to issues of comfort more generally. Helpfully, humans usually don't notice gradual change.
Offsets will have different effects on the house and comfort levels in mild weather and cold weather. It is about finding the balance, unless you just love twiddling endlessly!
It may be that you find you need different levels of offset in harsh winter periods compared to milder periods.
A useful very rough rule of thumb: every 2-3 degrees you increase flow temperature may result in an increase of 1°C indoor temperature. Every house will respond differently, though.
If you have underfloor heating, a concrete floor is a superb heat battery for time-of-use tariffs, slowly absorbing cheap heat, and slowly releasing it later.
Monitor your hour-by-hour energy costs if you can. Use a phone app, if you have one. You'll know you're getting your tweaks right when you see only small (or smooth) changes in hourly cost from one tariff period to the next.

Although the above offset method obviously means some judgement and experimentation, cost savings are pretty much guaranteed.

In my opinion, you'd have to adopt some crazy offset arrangements for this method to result in increased costs, but then you'd also have people in the house complaining of comfort issues.

With method 2 it is very much a case of "how MUCH" you save rather than "IF" you save, which is certainly the case when using thermostats.

What is impossible to know though, is whether the costs savings you make are as much as they COULD be. You will never make perfect decisions.

There is, though, an even better approach to take: Method 3 - Automation

The Geeky Zone

I stated that heat pump inefficiency is barely affected by offsets, by which I meant sensible offsets or course. The reality is that a heat pumps efficiency is 2-3% worse or better depending on the type of offset. Positive and negative offsets don't perfectly count each other out of course as efficiency is not linear with flow temperature, but the "net" impact is still small.

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Mark Thompson

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