Ecodan not modulating at negative outdoor temps?

[jonybat]

Yea, unfortunately I do not have monitoring on the secondary circuit. I have been checking temps with an infrared sensor, so those readings are only comparable between themselves.

I am considering buying the PAC-TH011E thermistors and connecting them to THW6/7 inputs and attaching them to the secondary pipes in/out of the buffer tank, possibly with a hose clamp and some thermal paste. At least the internal flow/return thermistors on the Ecodan unit seem to be done in similar way. Then I guess I would have them in HA, as Z1 Feed/Return Temp.

The next question is then what should the feed/return of both circuits be like? I read a lot about the dT5 out there, but I am yet to get the understanding, other than “heat pumps like it”. I have both seen people saying that the flow speed should be low to allow water to stay longer in the rads and allow them to dissipate heat, and others saying that it should be high. I have tried measuring, but there are too many variables to draw a conclusion.

About the modulation: It clearly can, it just doesn’t want to. At 16Hz/2kW, it runs for hours, but the issue here (as you confirmed previously), is that it only does that above a certain outdoor temp. In any case, I don’t think “my unit cycles a dozen times per day” will be enough to convince the installer to do anything about it, let alone replace the unit.

Regarding the 17th: it happened overnight, so I am 100% certain there were no open windows. The buffer temp cannot be the reason either, because it did 2 long cycles. Also, the tank also gets up to temperature in about 30m or less (100l)

Thanks for your input!

[gekkekoe]

It’s handy to know the delta T, but don’t spend too much money on it. The reason why I asked for delta T for the secondary circuit is to get an estimation of the capacity at that moment. And verify if its indeed not taking enough heat.

Regarding flow and delta T in general. Its different for radiators and UFH.

UFH:

• high flow
• low delta T

Radiators

• lower flow
• higher delta T

In general a heatpump is happiest when the output is ‘low’. So if we recall the formula
output = flow / 60 * (delta T) * 4.2, we can see that we have 2 parameters to make the heatpump happy (flow and feed temp (implicit via delta T)).

But since you have radiators, you need to maintain a higher delta T. I’m guessing that the radiators have a smaller surface to exchange the heat (higher delta T), and we would like the heat to be kept longer in the radiators (lower flow).

For UFH this is the opposite. So flow and feed temp are always 2 things that should be taken into account together. You just need to find the right balance for your system.

@jonybat

About the modulation: It clearly can, it just doesn’t want to. At 16Hz/2kW, it runs for hours, but the issue here (as you confirmed previously), is that it only does that above a certain outdoor temp. In any case, I don’t think “my unit cycles a dozen times per day” will be enough to convince the installer to do anything about it, let alone replace the unit.

Mitsi probably has a valid reason to only unlock those frequencies at certain temps. But I do know folks running fake NTC sensors to unlock lower frequencies.