The Problem with Eco Hot Water Mode

The Problem with Eco Hot Water Mode

Eco mode is the most efficient way to heat hot water on a Vaillant Arotherm heat pump, but it’s not without its challenges.

I’m a massive advocate of using Eco mode for hot water on Vaillant heat pumps.

Throughout all of 2024 I did all my hot water runs with our 5kW Arotherm in Eco mode, one at 1am and another at 1pm achieving a year long hot water SCOP of 3.7.

You can see my heat pump running stats here:

https://emoncms.org/energystatsuk

We are a family of four with two teenagers and despite having a 250L Mixergy cylinder we still need those two reheats a day.

Between those two reheats a day I 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.  All depends if we’ve all jumped in the shower or not between reheats.

I do this because starting the hot water reheat from an empty/colder state is one of the golden rules to hot water efficiency

The golden rules to hot water efficiency with a heat pump

• Reheat from empty cylinder / lowest temperatures (high hysteresis) – no quick top-ups
• Choose lowest target hot water temperature you can get away with
• Use Eco mode (or some other low compressor / quiet mode)
• Schedule reheats when highest outside temperature

Note: My Mixergy cylinder doesn’t use the same ‘hysteresis’ concept as a conventional cylinder.  It knows how full it is by % state of charge.

So rather than setting your hysteresis to say 10K, you’d say recharge at 50% perhaps.

Personally, I don’t even use that feature on the Mixergy as I just use timed reheats from whatever % state of charge the cylinder is at.

Something you can also do on a conventional cylinder of course.

But regardless of which cylinder you have, you do need to have a good idea you’ve allowed enough hot water between reheats doing it this way.

At least you can track the % state of charge on the Mixergy app.

I detailed within this article how you could work out what cylinder size you might need and what temperature to heat your hot water to.

Hot Water Cylinder Sizing and Mixing

The Problem with Eco Mode

Vaillant Arotherm Eco hot water mode is proven to be more efficient than the two other options; Balance and Normal.

You can read about the three different options in more detail here

Arotherm Plus Hot Water Modes

In summary, Eco mode limits the compressor of the heat pump making the whole hot water process more efficient.  The downside of Eco mode is that heat output is restricted compared to normal mode and so it takes longer to heat the water (around twice as long).

The issues with Eco mode and long hot water run times become more pronounced if you have

1. A small output heat pump (3.5 kW or 5 kW)
2. A large capacity cylinder (200 litres and above)
3. You let the cylinder drop empty/cold before reheat (high hysteresis)

Or like me, you have/do all those things.

I have a 5kW Arotherm, a 250L Mixergy cylinder and as described earlier, regularly allow my cylinder to drop to near empty.

You can see on this table I created that when it is 15C outside a 5kW Arotherm can kick out between 3.5kW and 4.2kW in Eco mode (the right hand “kW output” columns).

But when it’s 0C outside, the output drops to 2.3kW to 2.7kW

5kW, Eco Mode, 50 rps

Table taken from this deep dive article.

Smaller Heat Pumps will never reach Top of the SCOPS

This drop in power output can have a significant effect on reheat times

Heating 200L from 10C to 50C

• 2.5 kW power – 3h 43 mins
• 4.0 kW power – 2h 19 mins

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

I also created this handy table showing reheat times for a variety of volumes and heating power.

Time in minutes to heat water (from 10C to 50C) using various power output sizes

On larger Arotherm models (7kW, 10kW and 12kW), Eco mode presents slightly less of an issue because you’re restricting a higher initial capacity unit.

You can see this in similar tables for the 7kW, 10kW and 12kW models.

7kW, Eco Mode, 50 rps

10kW / 12kW, Eco Mode, 40 rps

In House Temperature Losses

Let’s look at the one specific hot water reheat and the details around it that led to me creating this blog post.

You can link to this hot water reheat yourself here

https://emoncms.org/energystatsuk?mode=power&start=1736123220&end=1736138120

Monitoring by Open Energy Monitor

This was an Eco mode hot water reheat that started at 1am on Monday 6th January, 2025.

Because it was between -0.4C and 1C outside during the run, my 5kW heat pump only managed an average heat output of 2425 Watts (2.425 kW) in Eco throughout the run.

Remember, Eco mode limits the compressor so heat output is also limited and the heat output is further limited the colder it is outsider as per the table earlier.

My 250L Mixergy cylinder was almost empty at the start of the run, being only 7% full.  So we had approx 230 litres of water to heat up.  The target temp was 45C and the bottom of the tank was around 12C and the incoming cold was 7C.

All these factors together, as well as a couple of defrost cycles along the way meant the run took over 3 hours to complete.

Just like any combi boiler, the heat pump can’t heat the house whilst it’s doing hot water.  So the house started to lose temperature throughout the run as you’d expect being around 0C outside.

As you can see from the stats, the dining room, where the temperature sensor is located, lost 1.6C across those 3 hours.

In spring, summer and even mild winter days/nights this isn’t usually a problem as the outside temperature doesn’t affect the internal house temperature as much.

But the house losing so much heat when it’s sub zero outside meant that once the hot water reheat had finally finished the heat pump had to work really hard to get the house back up to the internal target of 20C.

And heat pumps work much better (efficiently) maintaining a temperature than they do reaching a temperature.

In our house, the dining room is a single storey rear extension, so it is pretty exposed.  The room really prefers being trickle fed small amounts of heat to maintain temperature.  Rather than being let to drop and then having to work really hard to get back up.

My daughter’s loft conversion bedroom at the top of the house suffers the same fate.  

But the rest of the house, being more ‘inside’, doesn’t lose anywhere near as much heat as those two.

What are the alternatives?

For the vast majority of the year, Eco mode is probably fine for most people, as long as you’re okay with the longer run times.

I think we are just talking about addressing these cold snaps where it’s sub zero for a week or two.

Use Normal Mode

During really cold times you could always drop back to using Normal hot water mode.

Even around zero outdoor temperatures a 5kW unit can still output 5kW to 7kW of heat in Normal, which should mean much quicker hot water runs than Eco.

Obviously we’ve discussed many times before, efficiency is reduced in Normal mode compared to Eco.

But the house won’t drop as much during the reheat, so the heat pump won’t have to work as hard coming back to space heating.  Which is a win for comfort.

For example, that 3+ hour reheat in Eco from earlier would have taken around 1 hours and 40 mins in Normal.

Revert to Shorter Hot Water Top-Ups

One of the golden rules to high efficiency hot water runs is not to do small top-up reheats.

For efficiency it’s always best to start from as cold a cylinder as you can (high hysteresis), but as we’ve seen in my case with a small heat pump and a large cylinder, this can lead to very long hot water runs.

So you could always take the decision to decrease the hysteresis and do more frequent hot water runs when it’s very cold outside.

Yes, efficiency will take a hit, but the heating wouldn’t be off as long, so the house temperatures wouldn’t drop as much.

You can always increase the hysteresis when the weather warms up again.

Use the immersion?

As a last resort you could consider using the immersion.

Most cylinders have a 3kW immersion heating element in them which is normally used for legionella cycles or backup purposes.

But there would be nothing stopping you using the immersion to heat the hot water whilst leaving the heat pump to carry on heating the house.

Remember, like a combi boiler, the heat pump can do heating and hot water at the same time.  But using the immersion to heat the hot water would allow the heat pump to carry on heating the house.

Now an immersion is only “direct electric”, 1:1 electric, meaning it is only 100% efficient.  

Whereas you’d hope to get somewhere between 2 to 1 and 3 to 1 when using the heat pump to do hot water in Normal mode (200% to 300% efficient) during cold spells.

But in extremely cold conditions outside where you don’t want to lose any heat in the house, perhaps it’s an option?

On my Mixergy cylinder it’s very easy to switch the primary heating source from Heat Pump to Direct Electric (immersion) in the app.

You could also consider setting up some sort of Home Assistant automation trigger if you were that way inclined.