Choice of heatpump's flow/return dT setting

Hardware Heatpump
1

@matt-drummer
Hi Matt, I’m having trouble understanding the logic of this setting.

There is the physical dT that is happening at any given time when the leaving water has been around the emitters loop. You say that most of the time this dT is smaller than the dT you have requested in settings.

I thought that when this actual dT is smaller than the dT in the settings the heatpump reduces flow rate or shuts off and waits a while for the return to cool further (ie. cycle)

But it seems you have found that your heatpump continues chugging along.

Could you help me get a handle on this logic please.

Ta, David.

2

Hi David,

Yes, the flow rate reduces to the minimum in an attempt to achieve the requested dT.

It can’t get there if my flow is 30c and the house is 22c, the return will never be 20c whatever I did.

The heat pump still runs, just at as big a dT it can achieve.

This kind of self regulates the heat output with a fixed lwt. I don’t change the lwt much, it spends most of the time set at 31c, I sometimes turn it up or down by 1c or 2c.

As it get colder the dT increases, the house loses more heat so the return temperature falls, and more heat is produced.

As it gets warmer, the reverse happens.

Why do I do it?

Firstly, I have always found that my house works best on the radiator setting in terms of efficiency, it was the same with my 9kW as it was with this 8kW

Secondly, which is really connected to the first reason, the heat pump is more stable in the radiator setting and this results in a more predictable heat pump and better efficiency.

Really, in radiator setting, the only things that change are the flow temperature and electricity input. The flow rate is fixed on the floor and the return doesn’t change much, certainly not quickly.

In fan coils, with a smaller dT set, everything is on the move, constantly. The heat pump is less stable and the efficiency suffers quite a lot.

In radiators setting my heat output is pretty steady, in fan coils it’s up and down with the flow rate.

This heat pump cannot run at a fixed lwt, fixed return and fixed flow rate unless the flow rate is fixed to the floor.

I would love to run at 10 to 15lpm but I can’t.

3

Thanks Matt.

I get why you want to do this, but I don’t understand why the heatpump continues to run when its dT is smaller than your setting.

When my dT reduces below a threshold, the heatpump cycles.

4

Why wouldn’t it run at a dT of 6c or 7c?

Your heat pump doesn’t shut down because the dT is smaller than you set.

It shuts down because the return is too close to the flow to continue running.

It just looks the same as my situation if you just compare the return to the dT, but you need to to compare return to flow, once they are close, it will stop, miles apart like mine, it will run.

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