The Heat Pump Hot Water Golden Rules

What are the best hot water settings to use with a heat pump for both performance and cost savings?

Let’s talk about “The Heat Pump Hot Water Golden Rules”.

I have been tracking my hot water runs, pushing DHW performance and writing many blogs about all things hot water all over the website in the first 3 years of owning a heat pump.

The main blog post that is behind a lot of data here is one where I tracked and detailed around 700 hot water runs across a single calendar year.

A review of 700 heat pump hot water runs in 2024

Whilst recently looking back through many of the previous hot water articles it became clear that any advice and tips and tricks were scattered across too many blog posts.

So I thought it would be useful to consolidate the information into a single article.

The Heat Pump Hot Water Golden Rules

There are five golden rules relating to hot water on a heat pump.

Four of them are related to improving performance and the filth rule is all about cost.

The golden rules of efficient hot water generation with a heat pump

1. Reheat from as empty / cold cylinder as possible (high hysteresis) – no quick top-ups
2. Choose the lowest target hot water temperature you can get away with
3. Use Eco Mode (or some other low compressor / quiet mode)
4. Schedule reheats when it’s warmer outside

The golden rule for the cheapest hot water runs

    5. The money saved by scheduling the re-heats during the ‘dip’ of an off-peak tariff will likely trump any efficiency gains.

Let’s explore each of these 5 rules in a little more detail with evidence to back them all up.

Reheat from as empty/cold a cylinder as possible (high hysteresis) – no quick top-ups

The first golden rule of high performance hot water generation is to start the reheat from as empty/cold a cylinder as possible.

This can be achieved by either setting a high hysteresis (so less frequent automatic re-heats) or just letting the cylinder be used and to empty (cool down), allowing the temperature of the water to drop low.  

I do the latter with my Mixergy cylinder where I then reheat the cylinder in one go, usually on a fixed schedule.  Say 1AM or 1PM.

Between our two reheats a day I just let my cylinder drop to whatever level of emptiness it reaches.  

Sometimes it can be 50% full when the next reheat starts.  Sometimes it can be 5% full.  It all depends whether we’ve all jumped in the shower or not between reheats.

In this article about mixing and sizing I worked out roughly how much water we needed each day.

Hot Water Cylinder Sizing and Mixing

Because the Mixergy works slightly differently to a conventional cylinder in that it constantly records a percentage state of charge (SOC), I was able to record the starting SOC for every hot water reheat alongside all the other data collected.

Using a combination of Open Energy Monitor data and the Mixergy IO platform I was able to collect the following data points for each hot water re-heat.

• AM or PM
• Average Outside Temperature during hot water reheat
• Electrical Input (kWh)
• Heat Output (kWh)
• COP of Run (output divided by input)
• Starting % State of Charge (SOC)

This graph (pulled from the “700 hot water runs” article) shows all 246 hot water runs when it was between 10C and 14.9C outside, along with the starting state of charge (how empty the cylinder was for each one) and the COP achieved for the run.

We can instantly see that this golden rule holds up.

If you start the reheat from an emptier / colder state, then with everything else being equal you will see better performance / higher COP.  The runs in the bottom right hand corner of the graph.

I’ve seen a COP benefit of around 0.5 when the starting SOC is 20% (quite empty) compared to a starting SOC of 60% (not even half empty).

You can also see why “topping up” the cylinder and doing very short hot water runs is to be avoided if possible.  The reheats on the graph where the starting SOC is between 75% and 90% (top left corner) perform significantly worse. 

Unfortunately, it’s really hard to give concrete advice to those of you with a conventional cylinder as to what to set you hysteresis to.

There is so much variability in cylinder size (capacity, width and height), coil size and location, heat pump output capacity, compressor mode chosen and most importantly, where the temperature probe that reads current hot water temperature is located.  The location makes a big difference whether it’s towards the top, in the middle or close to the bottom of the cylinder.

First thing to do would be to look at what your hysteresis is set to now and take it from there.  Can you increase it?

Now look at where the sensor pocket is on your cylinder as this is where the temperature probe is.

Respected heating engineer Ken Bone gave me these ballpark hysteresis figures depending on where the temperature probe is located.

• Towards the top: 5⁰
• Somewhere in the middle: 10⁰
• Towards the bottom: 20⁰

But ultimately, what you set the hysteresis to and when you want to trigger a reheat is solely dependant on your own requirements, i.e. how quickly you want to get back to having usable hot water.  All which can depend on the heat pump you have and the other settings you’ve chosen.

Choose the lowest target got water temperature you can get away with

As described in the earlier linked “mixing” article, choosing a higher target temperature will give you more usable hot water.

But the hotter you choose for your whole tank to reach by the end of the hot water run, the lower the performance will be.

As you can see from this screen grab from Open Energy Monitor.

This reheat was targeting a whole cylinder temperature of 55C.

The red line (flow temperature) starts at 22C and goes all the way to over 67C

A direct link to the data behind this run is available here (along with all my heat pump data back to November 2022).

https://emoncms.org/energystatsuk?mode=power&start=1668951040&end=1668955950&cop=1

The blue line is instantaneous COP (coefficient of performance) showing how the performance drops off as the flow temperature rises.

The COP for the whole of this “Normal Mode” run was 2.97 (when 8.4C outside).

If the reheat had stopped when flow temperature was around 57C (targeting 45C whole cylinder temperature) the overall COP would have been closer to 3.25

COP benefit in this example from choosing a target of 45C rather than 55C target is around 0.25 COP

Use Eco Mode (or some other low compressor / quiet mode)

The harder you work the compressor inside the heat pump whilst doing a hot water run the lower the performance you will achieve.

In this graph you can see a comparison of Eco mode and Normal mode across two winters from my 5kW Vaillant.

Graph taken from the following article.

Mixergy Heat Pump Performance

There are two main hot water modes on the Vaillant, “Eco” and “Normal”.

• Eco mode reduces and limits the compressor
• Normal mode puts no limits on the compressor and so it goes flat out

You can see the result in the graph that performance is much improved in Eco mode.

The only downside of Eco mode is that it can take longer to complete. i.e. reheats are slower than they would be in Normal.

You can read more about the different Vaillant hot water modes here:

Arotherm Plus Hot Water Modes

If you don’t have different hot water modes available on your heat pump then you might want to look at invoking quiet mode.  

On many heat pumps quiet mode limits the output of the compressor, so if you schedule the timing of quiet mode at the same time as you do the hot water reheats, you might be able to take advantage of the lower output of the compressor

This is the technique I use to invoke “sweet spot mode” on my Vaillant.

Vaillant Arotherm Sweet Spot Hot Water Mode

The benefit of choosing Eco mode rather than Normal mode is around 0.75 COP

Schedule reheats when it’s warmer outside

Through the whole of 2024 I did all my hot water runs in Eco mode on my 5kW Vaillant.

The twice daily schedule of re-heating our 250L Mixergy cylinder is that one runs at 1am and another at 1pm.  We are a family of four with two teenagers, so we have a high hot water demand.

With the fourth of the golden rules being “Schedule reheats when it’s warmer outside”, I wanted to see the difference in performance based on the difference in outside temperature each day.

I already had all the data from “700 hot water runs” deep dive.

• AM or PM
• Average Outside Temperature during hot water reheat
• Electrical Input (kWh)
• Heat Output (kWh)
• COP of Run (output divided by input)
• Starting % State of Charge (SOC)

This graph shows all 700 hot water runs in 2024 along with the COP and outside temperature.

As you can see that with all things being equal, COP rises in line with the outside temperature.  Although performance gains seem to plateau from around 10C outside.

One thought for this plateu is that the incoming water temperature rises in summer so you don’t get as cold a starting point.

My incoming mains water can be lower than 10C in winter but closer to 20C in summer.

When looking into the AM versus PM data from the 700 runs I was able to pull out the average temperature during each run as shown in this graph.

With the blue line being the 1AM runs and the red line the 1PM runs.

My heat pump is in Sheffield, so your outside temperature range may vary through the year.

By looking at the daily details within the data I was able to work out the temperature difference between the AM and PM runs.

Here is a high level comparison of every run in 2024, summarised and averaged by AM and PM.

First observation is that the outdoor temperature golden rule holds true; a warmer outside temperature does lead to better COP.

Let’s also go a bit deeper and look at month to month averages.

Are there any advantages to switching approaches for AM or PM at different times of the year?

Again, the data says there’s not that much difference between AM and PM COP when we look month to month

As we saw in the earlier temperature graph, the overnight and afternoon temperatures rise and fall with the seasons but the differences between AM and PM each day are not that much..  

Certainly not enough to warrant changing habits in different seasons?

Average COP benefit from choosing PM rather than AM is around 0.25 COP

The money saved by scheduling the re-heats during the ‘dip’ of an off-peak tariff will likely trump any efficiency gains.

Now that all the performance tips and tricks are out of the way, let’s look at the financial golden rule.

As we saw in the previous tables, there isn’t much to choose between the AM and PM outdoor temperatures when viewed across the whole year.

And upon investigation, that COP improvement using the warmer PM temperatures is not enough to outweigh the financial gain by running overnight on cheaper electricity.

For example, say we need 10 kWh of energy to heat the water (this would be approx 215 litres heated from 10C to 50C) and we use the year average AM and PM COP from earlier

• AM: 10 kWh / 3.63 COP = 2.75 kWh electricity x 10p per kWh (cheap rate) = 27.5p per hot water run
• PM 10 kWh / 3.86 COP = 2.59 kWh electricity x 25p per kWh (Winter 2024/2025 price cap) = 65p per hot water run

Obviously, you can adjust the figures in the calculation for your own off peak / peak pricing / water demand.

You can use this calculator for water heating times

https://gettopics.com/en/calc/water-heating-calculators

So that financial calculation suggests that time of use tariffs, with cheaper off-peak periods can be used to significantly reduce the cost of hot water runs.  And no amount of afternoon warmer temperatures can bridge that price gap.

Let’s look at another example.

Let’s say you didn’t follow any of the performance rules and you achieved a very poor COP of 2 for you hot water run.  But you did schedule the reheat overnight using a 10p per kWh off-peak tariff.

And let’s compare that to you having followed the performance rules and you achieving a much more respectable COP of 4 for hot water.  But you decide to schedule this run for the more expensive (but warmer) afternoon slot.

In this very extreme example, even a COP of 2 scheduled overnight using 10p per kWh of electricity would still be cheaper than a COP of 4 at 25p per kWh of electricity

• AM: 10 kWh / 2 COP = 5 kWh electricity x 10p per kWh (cheap rate) = 50p per hot water run
• PM 10 kWh / 4 COP = 2.5 kWh electricity x 25p per kWh (Winter 2024/2025 price cap) = 62p per hot water run

That shows how much influence those cheaper electricity slots can have on running costs.  

Certainly more beneficial than warmer outside temperatures with slightly better COP and any mixture of the other golden rules.

If you have solar and you’re not paying anything directly for the electricity you might be better off sticking with the warmer afternoon slots if you’re able to stick to just one reheat a day.  Using the heat pump and solar is a way better solution than using any sort of solar diverter into the cylinder where you only get a COP of 1.

So the recommendation is absolutely re-schedule your hot water runs into off-peak and cheaper periods.

If you are unable to do this yourself, perhaps like a remote heat pump optimser product like Havenwise do it for you.

All you need is a supported brand of heat pump and Havenwise can automatically schedule your hot water in those cheaper times.

Havenwise can also optimse your heating schedule too and do away with the need for tinkering with heating curves and such like.  I’m finding it do a far better job at hitting indoor target temperature than the manufacturers controls.

It can even handle the complex time of use tariffs like Agile where the price is changing every 30 minutes.

Octopus Agile Yorkshire

You can also extend this “off-peak” thinking to normal running of the the heat pump for space heating too.  Perhaps using batteries to reduce the average import costs of your electricity.

I did a deep dive into the link between COP and the unit price of electricity in this article.

SCOP versus Pounds and Pence

Summary and Round Up

Aside from the “low compressor / Eco Mode” rule, each of the golden rules on their own don’t individually add too much performance.  But if you can combine the benefits from some or indeed all the rules you can achieve much better performance.

Let’s recap the performance gains I achieved from each of the rules.

• Starting at 20% (quite empty) rather than 60% full : 0.5 COP
• Average COP benefit from choosing PM rather than AM is around 0.25 COP
• The benefit of choosing Eco mode rather than Normal mode is around 0.75 COP
• COP benefit from choosing 45C target rather than 55C target is around 0.25 COP

So select the easy wins.  Combine some or all of the rules to achieve better performance.

But remember, be minful that there are consequences to each of the actions and rules; Eco is slower, lower target temp means less usable water etc etc.

And as we showed with the financial calculations, it’s unlikely that any of the performance benefits achieved from the golden rules can match the cost saving from moving your hot water runs to an off-peak period within a time of use tariff (or using your solar and/or batteries).

Obviously, the best recommendation is to use as many of the tips from the performance golden rules that you’re able AND also shift your usage to an off peak period.

So have your cake and eat it.

One final reminder though.  There are ceilings and limits to the amount of performance and COP you can get from hot water, especially when compared to what you can achieve when space heating.

Under the same flow temp / outside temp combo, you’ll almost always get better COP during space heating than hot water.  Likely because the coil or plate exchanger during hot water has way less surface area than all your radiators or underfloor.  So it’s much harder to shed all that heat efficiently.

This was something we dived into in this article if you want to read more about theoretical heat pump performance.

Smaller Heat Pumps will never reach Top of the SCOPS

Big shout out the Open Energy Monitor for their amazing monitoring package.  I couldn’t have created this article without it.

And also kudos to Mixergy for recording and holding all datapoints as well as exposing this information via an API so we can run Tomas McGuiness tremendous Home Assistant integration.

Available at: https://github.com/tomasmcguinness/homeassistant-mixergy

Hope you’ve found this article useful.

Below are some other recent articles you may be interested in

The Problem with Eco Hot Water Mode

Third Winter with our Vaillant Heat Pump