Using setback with heat pumps

[Jon Glanfield]

We are starting to collate some useful data on heat pumps that we have installed with meters to monitor their kwh consumption.

We generally go for the simplest set up, HP to buffer, air/floor stat to pump on house side of buffer, to try and avoid complications for the customer and errors in usage.

However I am intrigued by how we can improve efficiency further by using set back. whilst we would only sanction a setback of 1.5-2 degrees I would be interested in other peoples experiences in terms of how much difference it makes in reality.

Also on some of the legacy units we have installed (Daikins) we can choose to setback the air stats and the flow temp to the buffer, but I am not sure whether 1 or both should be employed.

Many thanks

Jon

[MikeHolm]

Good question, the prevailing wisdom from those who taught me floor heating (the Germans) said not to turn down thick slab systems (100mm+). We use a 40mm overpour (over wood floors) on suspended floors and these we can turn down 1-2C. I also think I can pump more heat into a really think slab over night when power is not at its peak cost. Raising the floor 1-2C overnight means good financial savings during the peak period.

I would think on air units that dropping the temp narrows the lift that the HP needs to overcome and that can't hurt efficiency.

[desA]

What is 'setback'?

[Jon Glanfield]

Reducing either the design air stat temperature or the flow temperature targeted by the heat source. Usually employed overnight when design temperatures can be lower.

[Bigfreeze]

You would save about 5%-10% if set back at night and if no one is in house during the day. No more than that. You should not run a HP at night as you are raising the house temp at a time when you don't need as much heat and when the outside temp is at its lowest. So in effect you maximise your heatloss at a time you needn't be running at all. Running air to water units on night rate exacerbates their deficiencies from that very reason also. Colder outside temp, lower COP

[Jon Glanfield]

But if the house loses a significant amount of its heat overnight with the HP off and then has to be recovered through hard work from the HP does this not offset the losses due to an impaired COP overnight?

[Bigfreeze]

If its going into setback at 10/11pm at night then it will usually take all night to drop back the 3C that a setback would entail unless its poorly insulated. One hours running would usually bring the house back to temp in the morning.

At night, if building up a heat store, you're dropping efficiency 2 fold. Firstly the outside temp is colder and secondly you would normally run to a higher temp to take advantage of the night rate. Both will drop your COP. I don't know about you but I couldn't sleep in a house thats 22/23 degrees at night. 18 would be more my preference. The duvet does the rest

[MikeHolm]

The systems i am talking about don't have sleeping areas in the basement where the extra heat would be stored and the cement is quite thick so it would take some time to show a lot of extra temp (yes, I know, the higher the difference the faster the heat will leave the slab).