a 5kW Mitsubushi EcoDan
Your 5kW scales down way better than our 14kW one. It’s amazing to see yours being gentle and warming over a two hour stretch and hitting a CoP of over 4. Ours is like a tiger waiting to be unleashed and it races off to heat the water really quickly, so mostly it’s doing hot water at around a CoP of 2.5 even on a warm Summer day. I’m reluctant to tinker as much as you whilst I’m still getting the RHI because it cost us 15k for the system and I won’t save that much more by taking the bigger risk of playing inside the flow-temp controller (FTC).
I’ll talk through this example so you can feel even better about your 5kW one.
During this run we had 1.4kW solar (the yellow bar at the top) but the heat pump got as high as using 5.5kW at the end. I only have coarse-grained control using the web-based interface so I can’t tell it to operate more gently. It’s actually much worse with the hot water, when doing the space heating I have more control by fiddling with the target temperature and making it rise gently but if I set the hot water to a lower temp it gives up and turns it off.
You’re probably looking at that “desired” hot water temp of 41 °C and pondering the Legionella thing. The Ecodan is pretty blunt how it does that so we’ve overridden it and the computer watches for a time when the temp was high for a long time which would kill the Legionella. If it’s been too long since that happened it bumps up the target temp to zap 'em.
Now the fun bit. The last month has cost us 5.54GBP in hot water for a family of four including two teenagers, so quite a bit of water. The trick of course is a solar diverter. You can see the behaviour with this run:
To start with we’re getting dinged 7p for that day because the infrastructure around the heat pump uses electricity in monitoring and interacting with the internet. The hardware didn’t do anything at all - it was purposefully switched off by my program the whole time.
However, all the hot water came from the solar diverter. You can see around 15:00 that someone took a shower and it recovered pretty quickly. We’ve actually had to limit the immersion thermostat to 55 °C because it’s at the bottom of the tank and the top was getting so hot when the thermostat was at 60 °C that we couldn’t mix in enough cold to make a sensible shower. Luckily the local humans are quite good at taking showers in the middle of the day to absorb the solar.
You might also note the “flow” from the hot water was getting up to 30 °C even though the water pump wasn’t running. That’s due to the heat building up in the tank leaking out along the piping enough to warm it up where the Ecodan’s sensor is. Ho hum.
Here’s another run over a couple of days where there were five showers powered purely by the solar PV diverter:
In fact at one point we didn’t notice the circuit to the heat pump was off for a week because the solar kept it warm enough for all our showers. (there had been a power-cut which tripped it off)
I do sometimes manually nudge the heat pump on if there’s just a few hours of sunshine and I think it’s worth using the extra from the grid to multiply up by the heat pump CoP. Of course that should be automated but sadly there’s where my story is bad because something happened in March 2020 to distract me from working on the heat pump and in January 2021 I started a more serious job which is consuming all my time.
So right now it’s all humming along nicely and doing roughly the right thing.
My 16 year-old daughter said yesterday “when I move to my own house can I use your heat pump controller?” which is about the best review I can expect from someone who lives with the consequences of how it works.