Can oversized Ecodan be swapped for a smaller unit?

Ok, so this is very unlikely to happen but I want to entertain the idea until it runs out of road…

I currently have a 8.5kW Ecodan that appears to be too large. It can’t sustain a steady flow temperature below about 40 degrees without the temp running away and causing it to cycle.
I have found a 5kW model available for quite cheap where someone had the opposite problem and had to upsize and is looking to get rid of the smaller one.

There are many questions around warranties (will I be able to transfer this to me at the new location, might get my original Mitsubishi trained installer to do the swap), electrics (can the 5kW PUZ-WM50VHA be wired up in exactly the same way to my current FTC6), does it effect the BUS grant I got last year, getting the thing collected, selling my current one, and anything anyone else can come up with that would be an issue.

But the main thing I want to know is would the 5kW model be large enough. Is it as simple as looking at my OSM data over this last winter and looking at the coldest periods to see how much the current HP was outputting? My data is here: Emoncms - app view .

January 15th to 18th looks to be the coldest over the last 3 months.
Although at the start of a cycle the heat pump was hitting 6-point-something kW output it was then dropping to around 4.5kW.
Would a limit of 5kW be pushing it a bit and mean recovery time from DHW and defrosts was too long?
I have a limit of 45 degrees flow set which is why it doesn’t go higher.

Around that time it looks like we weren’t hitting target temperature but that was before we had a radiator in the hallway where the main controller is and have upgraded another radiator in the Living Room.

Another interesting day was the 31st of December when we came back from holiday and reheated from setback of 16 to 19.

Thoughts? Great move I would have a Heat Pump well matched to our house or terrible idea it would be too close and struggle at times or one of the other reasons make it a bad idea.

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We’re in a similar position.
If you can get the total heat output on the coldest day, divide by 24 hours, you’ll have the average output you needed. Have a look at the output of the 5kW model at the design temperature (-2C, maybe) and compare the two.
To my mind this is really useful data and better than many heat loss estimates might be!

If you are near the limit, than in practise you’d want to plan ahead to avoid asking too much of the HP (for example when reheating after setbacks).

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@TrystanLea I saw that you installed a 5kW Ecodan, although it looks like a different model to the PUZ-WM50VHA. Do you have any thoughts on whether it would have enough grunt for our system (Emoncms - app view) if we replaced our 8.5kW?

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Do you have it on heatpumpmonitor? Would be interesting to try the heatloss tool on it…

My max output testing on my older r410a model suggests not to rely on more than 4.2kw from my 5 kw, would be interesting to know if that’s changed on the newer R32 model, it is physically larger I think, and datasheet suggests higher output I think?

Good to know, I might take a look at some of the new ones on HPM.
This one (System 16) looks to be hitting the 5kW+ when cold outside Emoncms - app view dashboard

Here’s the heat loss tool which is in development:

I’ve added a very rough area of best fit, there does often appear to be that slight curve off at higher DT, Im not sure why that is, looks like 5-6 kW of heat demand extrapolating a straight line but you have not really pushed close to DT23 yet which is very common:

The important thing is not the max heat output but the average over at least two defrost cycle, that one averaged 3.4 kW over multiple defrosts, I wonder what the flow target temperature was during that period? The target needs to be high enough to really push the unit to run at max and Im not sure if that’s the case in this example?

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This looks very much like my own, clearly oversized 11.2 kW Ecodan, and I’m wondering if I’d be better off with 8 kW, or maybe even smaller…

Measured heat loss is around 5.6 kW at design temp (260 W/K).

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Another thing to look at is how different heat pumps performed on “design day” (Jan 18th) which was the coldest day of the season.

  • your heat pump produced over 100 kWh of heat (including hot water) on that day
  • none of the 5 kW heat pumps delivered more than 80 kWh.

Oh very nice.

Could the curve off be due to either me limiting the flow temperature to 45°C or maybe because I have sometimes had Quiet Mode turned on which would have limited the HP?
Or could it be that my emitters can’t deliver more than that (although I have upgraded some rads in the last few weeks)?

Is this graph over a certain time frame? There was a few days in November-December when the temperature remained cold during the day so I’m surprised there wasn’t some DT20-DT23.

Another interesting observation (sorry) comparing my system to yours - my larger heat pump delivered about the same amount of space heating over the last 90 days.

Heat output for DHW, Heating and Total, respectively.

Oh hey, same installer too :slight_smile:

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Wow we use a lot more Hot Water, really need to have some kind of cut off timer on our shower!

I did the heat calcs myself and told the installer that’s what I wanted. I suspect if I hadn’t I would have ended up with a much larger unit.


It loads every day that is available for each system, I havent implemented a way to limit the window of data yet…

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I’ve been watching your system. I can’t quite understand why my Heating COP is so much worse than yours. In theory you have a more oversized system but it is preforming better.

Looking at the last 7 days it looks like it has been a a degree warmer outside for you but I assume the main difference is your Flow Temp being over 2 degrees lower.
Am I correct that you are still using Auto Adapt? So am I so I assume the difference will be that the room temperature at our Main Controller is lower, or rather takes a higher flow temp to get up to the setpoint temp.

What is your setpoint? I have ours at 19.5 normally 24/7 at the minute


Between 17.5° and 19°, depending on the time of day.

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What’s your total radiator output at ΔT 50 now? Mine’s about 18 or 19 kW. This will directly limit the flow temperature the system can be stable at.

Our system does appear to be underperforming and something else that has only really occurred to me recently (should have done before) is that we have a lot of pipework outside the thermal envelope.

There isn’t a huge amount outside (although I’m going to improve the insulation in a couple of places) but the primary pipework then runs along under the cellar ceiling for the width of the house to the plant area where DHW cylinder, system pump etc are all located. Then all the Ground Floor pipes run in the cellar with only the First Floor and attic ones running within the thermal envelope.

That is a lot of heat loss to reduce the flow temperature before it gets to any emitters.
All the pipework is insulated in standard insulation but none of it is glued or taped. There are many gaps where bends and fittings are. And the old pipes to the rads have varying degrees of falling off or degraded insulation.

All this will mean a higher flow temperature is needed as I’m effectively heating up the cellar.

So maybe now my efforts would be best

Maybe I should look at a nightime setback again. Just need to time it better otherwise in cold weather the internal temperature has to come back from setback as well as a period of off when the water is being heated.

I’m not sure as it got mild at just the wrong time.

I have now added a 1800mm 600mm K2 vertical radiator in the hall (3ish weeks ago). This has definitely helped at night when the internal doors are closed as before the hallway would get cold and so Auto Adapt would boost up the flow temperature.

I’ve also upgraded an old double panel 700mm x 800mm for a K2 of the same size in our Living Room. It surprised me how much of an effect this one had to the room temperature. The old double panel had the panels very close together without any fins so I expected a small effect but have a couple of times felt quite warm in what is the coldest room in the house.

The only steady state I can find since last Thursday when I upgraded the rad was on Saturday morning when it settled on 40 flow temp and gives: