ASHP Short Cycling

nxdmsandkaskdjaqd

Below is my temperature run from Saturday.  Outside was a steady 7/8C and the flow was set to 29C.  I noticed during the start up the delta between flow and return was 3C (11 to 1pm) the HP cycled every hour (approx).  However, as the day march on the cycling became more frequent.  You will see from the chart 4pm onwards the delta is only 2C.  So cycling is more when the heating stabilises, which I can understand because everything is up to temperature.

I think my next course of action is to speak with the manufacturer on how I increase the hysteresis across the heat exchanger.

 

Reed_Richards

nxdmsandkaskdjaqd said:
.... it looks like a faster flow over the heat exchanger is better for the system.

This depends on what control method your heat pump is using.  If it were running at a constant power output then a faster flow would give you cooler water.  But if your heat pump is trying to maintain a constant water temperature then it will have to work harder if you increase the flow rate.  But, I think, if it is trying to  maintain a constant differential between flow and return temperatures then it has to work less hard. 

nxdmsandkaskdjaqd

Reed_Richards said:

nxdmsandkaskdjaqd said:
.... it looks like a faster flow over the heat exchanger is better for the system.

This depends on what control method your heat pump is using.  If it were running at a constant power output then a faster flow would give you cooler water.  But if your heat pump is trying to maintain a constant water temperature then it will have to work harder if you increase the flow rate.  But, I think, if it is trying to  maintain a constant differential between flow and return temperatures then it has to work less hard. 

I'm not sure what the system is trying to do.  I know the HP modulates.  

michaels

So the heat pump has a minimum 'modulation' power output, it seems like even running at minimum that is more heat energy than you need to maintain the house temp.  Suppose you need half the minimum output then the pump needs to run for 50% of the time.  The 'hysteresis' setting will determine whether than is 10 mins on, 10 mins off or 20 mins on, 20 mins off.  Obviously the longer the on and off periods the more likely you will see a fluctuation in the room temp.  Whether the shorter on and off periods are more or less efficient?  Who knows?

Reed_Richards

But it might not be running at minimum output.  If it's trying to reach some particular water temperature and that temperature is higher than necessary then that could easily cause cycling.  But then again if that were true then increasing the pump speed should make it worse, not better.  That's why it's important to understand what your heat pump is trying to do.

nxdmsandkaskdjaqd

michaels said:

So the heat pump has a minimum 'modulation' power output, it seems like even running at minimum that is more heat energy than you need to maintain the house temp.  Suppose you need half the minimum output then the pump needs to run for 50% of the time.  The 'hysteresis' setting will determine whether than is 10 mins on, 10 mins off or 20 mins on, 20 mins off.  Obviously the longer the on and off periods the more likely you will see a fluctuation in the room temp.  Whether the shorter on and off periods are more or less efficient?  Who knows?

I believe it running close to minimum.  Its a 3kw HP and last night it was running at around 1kw.  The house achieve a stable temperature of 22C and the HP was turned off at 10pm.  This morning the rooms were at 21C.  Outside it was 6/7C overnight.  The bungalow is well insulated and just does not loose much heat.

Nick_Dr1

Doesn't sound like too much of a problem to me!

1) If the house is well insulated then the pump doesn't need to put much heat in. You could check by opening a few windows to reduce the insulation and see if it has to go continuously then.

2) The alternative is that the emitters can't get rid of enough heat. I guess this would be a problem if the house wasn't warm enough. Implies an oversized heat pump I suppose as the house appears to be warm. Efficiency might take a hit, but can't see it having too much impact on absolute cost.