Best thermostat hysteresis for ASHP

[smart51]

I've just had an ASHP fitted with under floor heating downstairs and radiators down stairs.  I also have Heatmiser neoStats for each zone.  The neoStat has a user selectable hysteresis, which can be set to 0.5°, 1°, 2° and either 3° or 4°.  The default is 1°.  What would be more efficient for the heat pump?

 

The heat pump has a weather compensation curve, so it turns down the flow temperature if it is warm outside.  Even so, the thermostats will turn on and off.

 

With the stat set to 21°, the heating will turn off when this is reached and will turn back on when the temperature drops by the 0.5° or 1° hysteresis setting.  A smaller hysteresis will have the zone switch on or off more often and for shorter durations.  A bigger hysteresis will have each zone on for longer, then off for longer.  Part of me thinks that a 0.5° hysteresis would allow the thermostats to be set to 20.5° instead of 21°, which would save some power.  I wonder which would give a better CoP.  What do you think?

 

 

[JohnMo]

If your running lots of zones on thermostats why bother with WC, you may as well simplify and run a set temperature.  Or do WC properly and set the thermostat as limit stops, not temp controllers.

 

I would, and did, dump all the zones and use a single 0.1 hysterisis thermostat for the UFH as a single zone.  I can still get a 0.5 Deg swing in temperature, which you can feel. 

[JoeBano]

I’m using heatmiser stats, Ufh I have it 0.5c as it take a while for the floor to charge up again. Upstairs rads 0.5c as my flow temperature is normally around 36-37c but if you have a high flow I would have it as 1c. My stats are really a temperature limiter, as my weather compensation is the lowest for my comfort level. With the neogen you can enable enhanced history and keep a tab on temperature levels 

[smart51]

20 hours ago, JoeBano said:

I’m using heatmiser stats, Ufh I have it 0.5c as it take a while for the floor to charge up again. Upstairs rads 0.5c as my flow temperature is normally around 36-37c but if you have a high flow I would have it as 1c. My stats are really a temperature limiter, as my weather compensation is the lowest for my comfort level. With the neogen you can enable enhanced history and keep a tab on temperature levels 

UFH heat up time is a good point.  I've been running tests with the WC curves to see what the flow temperature needs to be in different weathers.  I've found that it takes 1.5 to 2 hours to get from 18° overnight to 20° (depending on the outside temperature and the WC curve. It then crawls up at about 0.3° per hour.  Even with a 0.5° hysteresis, the cycle time would be several hours.

[smart51]

20 hours ago, JohnMo said:

If your running lots of zones on thermostats why bother with WC, you may as well simplify and run a set temperature.  Or do WC properly and set the thermostat as limit stops, not temp controllers.

 

I would, and did, dump all the zones and use a single 0.1 hysterisis thermostat for the UFH as a single zone.  I can still get a 0.5 Deg swing in temperature, which you can feel. 

I'm sure you know.  WC improves the CoP of the heat pump by reducing the flow temperature in mild weather.  I'm slowly setting up my WC curve.  Typically, the weather has warmed up.  I could do with a couple of cold days to do further tests.  I'm setting the WC curve to be a balance between the house sitting at the desired temperature by lowering the flow temp, and our wish for it to be cooler over night and then warm up in the morning.  I've found that setting the flow temperature that would give an equilibrium temperature of 22° allows the rooms to warm from 18° to 20° in a reasonable time.

 

The limitation is I can only set one WC curve.  The living room has huge solar gain and is always a degree or two warmer than everywhere else.  Even if I had the flow temperature lower, the stat in the living room would turn on and off.

[PhilT]

33 minutes ago, smart51 said:

The living room has huge solar gain and is always a degree or two warmer than everywhere else.  Even if I had the flow temperature lower, the stat in the living room would turn on and off.

That's a great example, one of many, where auto adaptive mode, for those lucky enough to have it, is a big step improvement over pure WC mode

[JamesPa]

56 minutes ago, PhilT said:

That's a great example, one of many, where auto adaptive mode, for those lucky enough to have it, is a big step improvement over pure WC mode

If one room is always warmer the starting point for a solution is a smaller emitter or throttling the flow to that room (turn down the lockshield).  Basically balance for equal room temp not equal dt.

Edited February 4 by JamesPa

[PhilT]

17 minutes ago, JamesPa said:

If one room is always warmer the starting point for a solution is a smaller emitter or throttling the flow to that room (turn down the lockshield).  Basically balance for equal room temp not equal dt.

Indeed but once balanced solar gain could potentially cause throttling in many or most rooms. Isn't it better to have some form of automatic modulation of heat pump output according to room temp?

[JamesPa]

9 minutes ago, PhilT said:

Indeed but once balanced solar gain could potentially cause throttling in many or most rooms. Isn't it better to have some form of automatic modulation of heat pump output according to room temp?

Only if the temperature of other rooms doesn't matter.  If you throttle the hp output to all rooms the one with the solar gain will still be hotter.  So the requirement if this room is always hotter (which is what we are told is the case) is to make this room cooler without affecting other rooms.

 

Obviously this depends on whether it's really the case that the room in question is always warmer, but that's what we have been told.

 

Logically it makes sense (as heat geek advocate) to balance for room temp not for equal dt.  Balancing for equal dt assumes that the emitters are correctly sized, which is not the case (ever).  But of course it's more difficult so installers are unlikely to do it and balancing for DT is often good enough.  But homeowners can!

Edited February 4 by JamesPa

[JoeBano]

13 hours ago, smart51 said:

UFH heat up time is a good point.  I've been running tests with the WC curves to see what the flow temperature needs to be in different weathers.  I've found that it takes 1.5 to 2 hours to get from 18° overnight to 20° (depending on the outside temperature and the WC curve. It then crawls up at about 0.3° per hour.  Even with a 0.5° hysteresis, the cycle time would be several hours.

Will all my zone on Ufh heating on, the heat pump will my cycle will last 1hour 30mins - 2 hours. My kitchen is south facing with a lot of glass. You can see the solar gain on the neogen it will raise 2c within a hour but my living room north facing temperature will not increase in temperature, I have solid walls in between. I do have a 3 port buffer tank which allows me to heat the living room still, 2 cycles per hour. 
I don’t look too much at COP I look how much it’s costing me, I’ve averaged £3 a day so far this winter for heating and hot water.