ASHP Short Cycling

nxdmsandkaskdjaqd

Reed_Richards said:

nxdmsandkaskdjaqd said:

Reed_Richards said:

How long does the heat pump run for when it is on?  30 minute cycles are long, not short.  It would only be short cycling if your heat pump was only on for a few minutes then off for the remainder of the 30 minutes.

The HP would be off for 3 minutes and then on for 27 minutes .  It would cycle this most of the time.  So this is not short cycling?

No, that is most definitely NOT short cycling.  I agree that it's very irritating but it means that your heat pump is supplying all the heat that your system needs per 30 minutes in 27 minutes.  The most obvious thing to do to remedy this is to reduce the leaving water flow temperature (as others have suggested).

I made a mistake in my energy monitoring app.  It does go off for 3 minutes and then on for 12 minutes.  `So it cycles 4 times every hour.  Does this still not mean short cycling?

Reed_Richards

nxdmsandkaskdjaqd said:

Reed_Richards said:

nxdmsandkaskdjaqd said:

Reed_Richards said:

How long does the heat pump run for when it is on?  30 minute cycles are long, not short.  It would only be short cycling if your heat pump was only on for a few minutes then off for the remainder of the 30 minutes.

The HP would be off for 3 minutes and then on for 27 minutes .  It would cycle this most of the time.  So this is not short cycling?

No, that is most definitely NOT short cycling.  I agree that it's very irritating but it means that your heat pump is supplying all the heat that your system needs per 30 minutes in 27 minutes.  The most obvious thing to do to remedy this is to reduce the leaving water flow temperature (as others have suggested).

I made a mistake in my energy monitoring app.  It does go off for 3 minutes and then on for 12 minutes.  `So it cycles 4 times every hour.  Does this still not mean short cycling?

Well manifestly 12 minutes is shorter than 27 minutes but I still don't think it is short in the context of short cycling.  If the on time were less than 5 minutes it might be more of a concern but the off time is also important.

The idea of short cycling as "a bad thing" arises from boilers.  I think the concern there is that because the insides of the boiler do not reach full temperature then the combustion process is not as complete as intended and so not as efficient.  You might also get soot forming that doesn't have time to burn off.  In this context a short on time followed by a long off time would be worse than a short on short off because if the off time is short there is less opportunity for cooling.

With heat pumps it's not nearly so clear why a short cycle might be bad but the most credible suggestion I have read is that too much of the cycle is spent warming up feed pipes that don't contribute heat to your house.  If so, then a short off time would again be much better than a long one.

shinytop

Exiled_Tyke said:

Agree with @Reed_Richards keep lowering the temperature until it struggles to get the room to temperature. 

I'm not so sure.  This sort of regular cycling happens because the water flow reaches its set temp then stops, cools and starts again, etc.  Lowering the LWT may not stop this; in fact it might make it worse.  At least that's what I've found but I have rads not ufh.   

Even 4 cycles per hour is OK and unlikely to have a noticeable effect on energy consumption. 

matelodave

Another option might be to increase the hysteresis (the temperature difference between when the heat pump stops and when it restarts) If the gap is too small then the heatpump will cycle, although as said above four times an hour doesn't sound too bad.

That's probably in the Engineering or Commissioning menu - mine is set to +2/-2 degrees around the set point.

In the end you are trying to extract roughly the same amount of heat out of the water as the heat pump is producing and I'd suggest that turning the pump speed up just returns hotter water to the heatpump before the emitters have had a chance to dissipate it.

As I pointed out, underfloor heating is very slow to respond, especially if its buried into the screed as it acts like a storage heater so it will be slow heating up and slow to give out heat, therefore speeding up the flow sounds counter productive.

TBH setting it up is probably more an art than a science but it needs patience and an idea of what is happening when you adjust a setting, so you need to measure, monitor, keep records and give enough time for a tweak to settle down and all this will be complicated by the varying setpoint whilst weather compensation is active.

nxdmsandkaskdjaqd

shinytop said:

Exiled_Tyke said:

Agree with @Reed_Richards keep lowering the temperature until it struggles to get the room to temperature. 

I'm not so sure.  This sort of regular cycling happens because the water flow reaches its set temp then stops, cools and starts again, etc.  Lowering the LWT may not stop this; in fact it might make it worse.  At least that's what I've found but I have rads not ufh.   

Even 4 cycles per hour is OK and unlikely to have a noticeable effect on energy consumption. 

I have found that a flow of 27C causes more cycling.  Currently running 29C today to see what the cycling is like once the system is up to temperature.  Bit cooler outside today at 7C, so is not a real comparison with yesterday at 10C.

nxdmsandkaskdjaqd

matelodave said:

Another option might be to increase the hysteresis (the temperature difference between when the heat pump stops and when it restarts) If the gap is too small then the heatpump will cycle, although as said above four times an hour doesn't sound too bad.

That's probably in the Engineering or Commissioning menu - mine is set to +2/-2 degrees around the set point.

In the end you are trying to extract roughly the same amount of heat out of the water as the heat pump is producing and I'd suggest that turning the pump speed up just returns hotter water to the heatpump before the emitters have had a chance to dissipate it.

Adjusting the hysteresis might be a good idea, just looked at the manual and the only modification is to do with the DHW.  I will contact the manufacturer for advice on where this setting is.

I am currently running the pump on 3.  Previous tests gave me the conclusion that it was better to get the max flow over the heat exchanger. 

matelodave

I'd still suggest that you need the incoming water cool enough to ensure that the leaving water doesn't quite achieve the set point which means that it should be flowing at a rate to allow the emitters to give up as much as their capacity allows.

A high velocity of water hurtling through the system will allow hotter water to return to the heatpump which will cause it to reach its set point and shut down.

There's always the possibility that either the heat pump is too big for your system and/or that the emitters are too small and incapable of dissipating the heat that's being produced. Have you got any short loops or even something like a towel rail which is "short circuiting" the system.

Try reducing the flow on some of the loops, especially if you've got short ones - most u/f systems have flow meters on each of the loops so you could slow the water down a bit allowing the floor to give up more of their heat.

Don't close them down too much or you might cause flow problems. Has your system got a bypass valve and if so is it activating

I found that if floor loops closed down under thermostat control then the bypass would operate and the boiler would short cycle.

nxdmsandkaskdjaqd

matelodave said:

I'd still suggest that you need the incoming water cool enough to ensure that the leaving water doesn't quite achieve the set point which means that it should be flowing at a rate to allow the emitters to give up as much as their capacity allows.

A high velocity of water hurtling through the system will allow hotter water to return to the heatpump which will cause it to reach its set point and shut down.

There's always the possibility that either the heat pump is too big for your system and/or that the emitters are too small and incapable of dissipating the heat that's being produced. Have you got any short loops or even something like a towel rail which is "short circuiting" the system.

Try reducing the flow on some of the loops, especially if you've got short ones - most u/f systems have flow meters on each of the loops so you could slow the water down a bit allowing the floor to give up more of their heat.

Don't close them down too much or you might cause flow problems. Has your system got a bypass valve and if so is it activating

I found that if floor loops closed down under thermostat control then the bypass would operate and the boiler would short cycle.

I did initially set the UF in accordance with the design specification of flow rates (months ago).  Over time I have modified these, reducing them down such that I can achieve a 24hr system (not there yet).  

My short loops (bathrooms, entrance lobby, utility) have all been reduced down more and now are set a flow rate of approx 1 on the flow tube.

I am at the moment checking flow temperatures of the loops across the manifold with a IR thermometer, to make sure that the temperatures are not too low or too high.

Today with a flow of 29C and outside 7C, the HP has only been turning off once or twice per hour.  But I have not reached the equilibrium temperature of the house, so as to flow and return become narrower later and I suspect I will see more HP turn off's.

Another day, I will test running the pump at setting 2 to see what happens.

A lot happier today. But more to do.

matelodave

perhaps a shufti at this will help - https://youtu.be/OX_ltEbSBgI

nxdmsandkaskdjaqd

matelodave said:

A high velocity of water hurtling through the system will allow hotter water to return to the heatpump which will cause it to reach its set point and shut down.

I tested slowing down the UFH pump to 2 today.  I let the house warm up and stabilise and then turned down the pump.  Unfortunately I saw more cycling, 6mins on 3 mins off.  So it looks like a faster flow over the heat exchanger is better for the system.