What is the size of your home and the calculated heat loss?

What are the present weather compensation settings?

When you say your system struggles to maintain a DeltaT of 5C, is that lower than 5C or greater?

@derek-m 3-bed semi detached, about 100 square metres. The MCS document says something like "Room specific heat loss 40-60W/m2" which I have no idea what it means. Do I multiply it by the number of rooms to get the total house's heat loss? That looks like quite a low number...

Present weather compensation settings are 42@-2, 35 @ 12 or something like this, as I played a lot with them and the manual offset.

When I say it struggles to maintain delta T, I mean the delta T is lower than 5C. This only when flow temp < 38 and outside temperatures are milder >5 degrees. When this happens, heat pumps cycles a lot more, and this can be reduced by manually changing the delta T to 3 instead of 5.

Posted by: @frappzy

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@derek-m 3-bed semi detached, about 100 square metres. The MCS document says something like "Room specific heat loss 40-60W.m2" which I have no idea what it means.

Present weather compensation settings are 42@-2, 35 @ 12 or something like this, as I played a lot with them and the manual offset.

When I say it struggles to maintain delta T, I mean the delta T is lower than 5C. This only when flow temp < 38 and outside temperatures are milder >5 degrees. When this happens, heat pumps cycles a lot more, and this can be reduced by manually changing the delta T to 3 instead of 5.

The room specific heat loss is a measure of how much heat energy is required to keep a room at the standard 21C indoor temperature, for each square metre of floor space, when the outside temperature is around -2C. So the overall heat loss for your home could be in the region of 4kW to 6kW.

The fact that you have a 14kW heat pump installed would indicate that it may be oversized.

As the outside air temperature increases, the heat loss reduces, and your heat pump controller will reduce the water flow rate, to reduce the amount of heat energy that it is supplying to your home. There is a minimum speed at which the water pump will operate, so you get to the point where the heat pump is supplying more energy than the heat loss of your home. This in turn causes the Return Water Temperature (RWT) to start to increase, which has the effect of lowering the DeltaT between LWT and RWT. The heat pump controller will sense this reducing DeltaT, and will eventually stop the heat pump to prevent the rooms from overheating too much. This I think is what is happening in your situation, particularly since your heat pump appears to be oversized for your home.

Whilst it is not possible to prevent cycling of your heat pump, there may be a way to reduce the frequency.

If you have an on - off type thermostat in your home which is connected to your heat pump controller, try lowering the temperature setting to the point where the thermostat overrides the WC mode in the controller. Because the hysteresis of thermostats is often 1C, it should hopefully switch off your heat pump when it reaches the temperature setting, but not switch it back on until the indoor temperature falls by 1C or more. This will hopefully reduce the cycling frequency and may even slightly improve efficiency.

I would suggest that you give this method a try and report back the results.

Posted by: @derek-m

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The room specific heat loss is a measure of how much heat energy is required to keep a room at the standard 21C indoor temperature, for each square metre of floor space, when the outside temperature is around -2C. So the overall heat loss for your home could be in the region of 4kW to 6kW.

The fact that you have a 14kW heat pump installed would indicate that it may be oversized.

As the outside air temperature increases, the heat loss reduces, and your heat pump controller will reduce the water flow rate, to reduce the amount of heat energy that it is supplying to your home. There is a minimum speed at which the water pump will operate, so you get to the point where the heat pump is supplying more energy than the heat loss of your home. This in turn causes the Return Water Temperature (RWT) to start to increase, which has the effect of lowering the DeltaT between LWT and RWT. The heat pump controller will sense this reducing DeltaT, and will eventually stop the heat pump to prevent the rooms from overheating too much. This I think is what is happening in your situation, particularly since your heat appears to be oversized for your home.

Whilst it is not possible to prevent cycling of your heat pump, there may be a way to reduce the frequency.

If you have an on - off type thermostat in your home which is connected to your heat pump controller, try lowering the temperature setting to the point where the thermostat overrides the WC mode in the controller. Because the hysteresis of thermostats is often 1C, it should hopefully switch off your heat pump when it reaches the temperature setting, but not switch it back on until the indoor temperature falls by 1C or more. This will hopefully reduce the cycling frequency and may even slightly improve efficiency.

I would suggest that you give this method a try and report back the results.

Ah, that's what I feared. Why did they have a clearly oversized heat pump installed? That sounds counterintuitive as more costly to run.

But what I don't understand is why can't the heat pump run the LWT at say 32 with a delta T of 5? Surely heat loss and heat demand decrease when it's warmer outside, but so does the LTW in WC mode. Why isn't the pump just able to reduce the heat output if the WC tells it to set the LWT to, say, 32 degrees? When I try to do that, cycling kicks in: pump and compressor start, LWT gets heated above setpoint, delta T is lower than 5 because the return water temperature doesn't drop (emitters not emitting??), pump and compressor switch off. Cycle start again. Clearly there is something technical I am not getting. I hyave seen people having LWT of 28 degrees at 12 outside for an indoor temp of 20-21.

I'm not completely sure I understood the method you are suggesting: I do have on-off thermostats with 1C histeresis, are you simply suggesting to use it set at constant temperature? If I understand correctly, this is not solving the cycling per se: sure it will make the heat pump work less hours, but at a higher LWT, am I wrong? My point was to lower the LWT so that I can match heat loss and leave the heat pump on 24/7 (though this is something I still haven't quite figured out in terms of cost-efficiency, as the house is left empty 8 hours a day, I'm still exploring this...). Using your method is a bit more complex because, for someone unclear reason, some rooms are so much colder than others even though they are near well-heated ones. I will have to understand why, but it's completely unclear to me (living room heats up well, other room next to it on the same side is near impossible to heat up...perhaps issues with flow rate for the loop?). But this means I cannot control the whole house with just one thermostat, unless I accept having much colder rooms.

Posted by: @frappzy

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Posted by: @derek-m

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The room specific heat loss is a measure of how much heat energy is required to keep a room at the standard 21C indoor temperature, for each square metre of floor space, when the outside temperature is around -2C. So the overall heat loss for your home could be in the region of 4kW to 6kW.

The fact that you have a 14kW heat pump installed would indicate that it may be oversized.

As the outside air temperature increases, the heat loss reduces, and your heat pump controller will reduce the water flow rate, to reduce the amount of heat energy that it is supplying to your home. There is a minimum speed at which the water pump will operate, so you get to the point where the heat pump is supplying more energy than the heat loss of your home. This in turn causes the Return Water Temperature (RWT) to start to increase, which has the effect of lowering the DeltaT between LWT and RWT. The heat pump controller will sense this reducing DeltaT, and will eventually stop the heat pump to prevent the rooms from overheating too much. This I think is what is happening in your situation, particularly since your heat appears to be oversized for your home.

Whilst it is not possible to prevent cycling of your heat pump, there may be a way to reduce the frequency.

If you have an on - off type thermostat in your home which is connected to your heat pump controller, try lowering the temperature setting to the point where the thermostat overrides the WC mode in the controller. Because the hysteresis of thermostats is often 1C, it should hopefully switch off your heat pump when it reaches the temperature setting, but not switch it back on until the indoor temperature falls by 1C or more. This will hopefully reduce the cycling frequency and may even slightly improve efficiency.

I would suggest that you give this method a try and report back the results.

Ah, that's what I feared. Why did they have a clearly oversized heat pump installed? That sounds counterintuitive as more costly to run.

 

But what I don't understand is why can't the heat pump run the LWT at say 32 with a delta T of 5? Surely heat loss and heat demand decrease when it's warmer outside, but so does the LTW in WC mode. Why isn't the pump just able to reduce the heat output if the WC tells it to set the LWT to, say, 32 degrees? When I try to do that, cycling kicks in: pump and compressor start, LWT gets heated above setpoint, delta T is lower than 5 because the return water temperature doesn't drop (emitters not emitting??), pump and compressor switch off. Cycle start again. Clearly there is something technical I am not getting. I hyave seen people having LWT of 28 degrees at 12 outside for an indoor temp of 20-21.

I'm not completely sure I understood the method you are suggesting: I do have on-off thermostats with 1C histeresis, are you simply suggesting to use it set at constant temperature? If I understand correctly, this is not solving the cycling per se: sure it will make the heat pump work less hours, but at a higher LWT, am I wrong? My point was to lower the LWT so that I can match heat loss and leave the heat pump on 24/7 (though this is something I still haven't quite figured out in terms of cost-efficiency, as the house is left empty 8 hours a day, I'm still exploring this...). Using your method is a bit more complex because, for someone unclear reason, some rooms are so much colder than others even though they are near well-heated ones. I will have to understand why, but it's completely unclear to me (living room heats up well, other room next to it on the same side is near impossible to heat up...perhaps issues with flow rate for the loop?). But this means I cannot control the whole house with just one thermostat, unless I accept having much colder rooms.

You are not unique, since most people struggle to understand the concept, surely water at 40C must contain more heat energy than water at 30C, and in fact that is the case. What people fail to consider is the reference point, since it does require more heat energy to increase the temperature of water from say 0C to 40C than from 0C to 30C, but it takes approximately the same amount of heat energy to increase the temperature of water from 30C to 35C as it does from 35C to 40C, provided that it is the same volume of water.

As you stated yourself, lowering the LWT should reduce the amount of heat energy being absorbed and emitted into your home, but it may not reduce the amount of heat energy being produced by your heat pump. As you then discovered, the RWT remains the same or starts to increase, which in turn reduces the DeltaT, to a point where the heat pump controller stops the heat pump for a period of time to allow the system to cool down slightly. The rate of cycling therefore increases.

The systems that can operate without cycling down to a LWT of 28C either have greater heat loss, in relation to the size of heat pump, because of lower levels of insulation, or possibly undersized heat pumps.

Most heat pumps are designed to operate with a DeltaT of 5C, since this allows an adequate quantity of heat energy to be transferred without requiring such a high LWT and water flow rate. If the water flow rate is increased from 10 litres per minute to 20 litres per minute, the amount of heat energy transported will be doubled. To reduce the amount of heat energy being transferred from the heat pump it is just a matter of lowering the water flow rate, but of course the water pump has a minimum operating speed, which limits how low the water flow rate can be taken. This is the point where the cycling will rear its ugly head, and the only way to counteract this would be to allow the DeltaT to reduce, but of course the heat pump controller is designed to vary the water pump speed to control the DeltaT at 5C. Maybe in the future, some heat pump manufacturer's will modify the programming to allow the DeltaT to be lowered further once the water pump has reached minimum speed, thereby reducing the potential heating capacity of the heat pump.

Your problem of warm and cold rooms is probably because your system is not correctly balanced. I provided quite a detailed explanation of how to balance UFH for Curlkatie elsewhere on the forum. I suggest that you read that.

Posted by: @derek-m

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As you stated yourself, lowering the LWT should reduce the amount of heat energy being absorbed and emitted into your home, but it may not reduce the amount of heat energy being produced by your heat pump. As you then discovered, the RWT remains the same or starts to increase, which in turn reduces the DeltaT, to a point where the heat pump controller stops the heat pump for a period of time to allow the system to cool down slightly. The rate of cycling therefore increases.

Thank you for your explanation, I will need to think about this. Because I am still struggling to understand this point. Why would an oversized heat pump incur the cycling issue when heating water to 32 degrees at delta T of 5: if the RWT cannot drop to 27 degrees, what does have this to do with the heat pump itself and not simply the emitters losing efficiency (unless, as you say, you could theoretically lower the flow rate to any level)? I am having such a hard time grasping this concept: wouldn't a correctly sized HP also have the same issue of maintaining delta T with LWT of 32 if you keep the house and UFH system the same? All the HP has to do is 1) heat LWT to 32, the rest is about controlling the flow rate (as it is down to the heat requirement of the house), and I don't understand how this is related to the power of the HP rather than simply the heat delivery component of the system. I am not sure if I am explaining my confusion clearly, I'm sorry 🙂

When it is warmer outside, wouldn't manually changing the deltaT settings solve the problem? Changing it to delta T of 3 instead of 5? This might mean the flow rate would be probably higher, but then I would need to understand whether the increase in pump consumption (as it needs to work harder) is offset by the fact the compressor would be cycling less often...

Concerning the balancing, I will definitely give it a look, thanks!!

Posted by: @frappzy

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Why would an oversized heat pump incur the cycling issue when heating water to 32 degrees at delta T of 5

I have been looking into this as well.  I think this may have something to do with the capabilities of the inverter on the pump.  From what I could find out there is a minimum output that can be delivered, either 1/5 or 1/10 of capacity (not an easy number to find).  If you need less than that, the heatpump will have to switch off and on.

The energy being put into the house will be flow and dT dependant.  If this is < min capacity of pump, it will have to switch off/on/off/on.

Heat transferred\

kW = l/s ⋅ 4.18 ⋅ ΔT

Posted by: @frappzy

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Posted by: @derek-m

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As you stated yourself, lowering the LWT should reduce the amount of heat energy being absorbed and emitted into your home, but it may not reduce the amount of heat energy being produced by your heat pump. As you then discovered, the RWT remains the same or starts to increase, which in turn reduces the DeltaT, to a point where the heat pump controller stops the heat pump for a period of time to allow the system to cool down slightly. The rate of cycling therefore increases.

Thank you for your explanation, I will need to think about this. Because I am still struggling to understand this point. Why would an oversized heat pump incur the cycling issue when heating water to 32 degrees at delta T of 5: if the RWT cannot drop to 27 degrees, what does have this to do with the heat pump itself and not simply the emitters losing efficiency (unless, as you say, you could theoretically lower the flow rate to any level)? I am having such a hard time grasping this concept: wouldn't a correctly sized HP also have the same issue of maintaining delta T with LWT of 32 if you keep the house and UFH system the same? All the HP has to do is 1) heat LWT to 32, the rest is about controlling the flow rate (as it is down to the heat requirement of the house), and I don't understand how this is related to the power of the HP rather than simply the heat delivery component of the system. I am not sure if I am explaining my confusion clearly, I'm sorry 🙂

When it is warmer outside, wouldn't manually changing the deltaT settings solve the problem? Changing it to delta T of 3 instead of 5? This might mean the flow rate would be probably higher, but then I would need to understand whether the increase in pump consumption (as it needs to work harder) is offset by the fact the compressor would be cycling less often...

 

Concerning the balancing, I will definitely give it a look, thanks!!

In a central heating systems there are two interconnected, though distinct parts, the heat generator and the heat emitters, but they both obey the laws of physics.

The amount of heat energy emitted by the heat emitters, is dependent upon their heating capacity (size), and the difference in temperature between the average water temperature and the temperature of the rooms being heated. So if the LWT from a heat pump is increased from 30C to 40C, with a constant water flow rate, the temperature of the heat emitters will increase, and they will transfer more heat energy into the home. As more heat energy is extracted from the water, RWT may increase, but not by the 10C increase in LWT, so the DeltaT may increase to 7C or 8C.

If the heat pump controller is programmed to maintain a DeltaT of 5C, then it needs to increase the speed of the water pump, to increase the water flow rate, so that more heat energy is being sent to the heat emitters. By sending more heat energy, the RWT coming from the heat emitters will increase, because the quantity of heat energy that they are absorbing has not changed, since the LWT has remained at 40C.

Now let's look at what happens at the heat pump end.

Increasing the LWT setting from 30C to 40C will cause the compressor to speed up, to increase the pressure and temperature of the refrigerant gas entering the Condenser, which in turn raises the LWT, until it achieves the desired 40C. Initially the RWT will still be at 25C, which may limit the rate at which the LWT increases, but eventually both the  LWT and RWT will increase. If the water flow rate remains the same, the system may stabilise with a LWT of 40C and a RWT of 32C, hence producing a DeltaT of 8C. But since the controller desires a DeltaT of 5C, the only way that this can be achieved is by increasing the RWT, since the LWT is already at the specified value. As described above, increasing the water flow rate increasing the quantity of heat energy being transported, and by doing so increases the RWT from the heat emitters.

In your system the reverse is occurring when you lower the LWT.

The reduction in LWT causes the heat emitters to absorb less energy from the water, so the DeltaT starts to increase, which in turn causes the speed of the water pump to be reduced. If the water pump reaches minimum speed before the required DeltaT of 5C is achieved, then the DeltaT remains too low. I don't know the exact algorithms inside the heat pump controller, but eventually it reaches the point that it decides to shutdown for a period of time whilst thing cool.

I appreciate it is a difficult process to understand, since there are quite a number of different parameters both involved, and changing, at the same time.

If a heat pump is rated at 14kW maximum output, and has a turndown capacity to say 25%, then its minimum continuous output capacity would be 3.5kW. It therefore cannot run continuously if the loading is less than 3.5kW.

Applying the same to a heat pump which is rated at 10kW, would give 2.5kW.

If the heat loss of your home is 6kW at -2C, then at +9.5C the heat loss will only be 3kW. So if the minimum output capacity is 3.5kW, your heat pump will likely cycle at outside air temperatures of 7.6C and above. Conversely, if the minimum output capacity was instead 2.5kW, then cycling would probably not occur until the outside air temperature had increased to approximately 11.4C.

I don't remember seeing the capability to change the required DeltaT from 5C to a lower value, maybe one of the Ecodan owner's can advise, or if you discovered this possibility then please post the details.

Having a buffer tank and/or low loss header can reduce the capability of the system to transfer the heat energy from the heat pump, to the heat emitters, and could possibly be a further cause of cycling.

Have any energy efficiency improvements been made since the original heat loss calculations were performed and the heat pump sizing carried out?

If you are out of the home for 8 hours each day, and you have a programmable thermostat, you could set a schedule to lower the indoor temperature using the thermostat. This would shutdown your heat pump unless it is very cold and require heating to maintain the lower indoor temperature. Obviously arrange for the heating to return to normal temperatures before you get back home.

A further cause of cycling could be the fact that you are not fully utilising the full capacity of your heat emitters since you have some cold rooms.

Posted by: @derek-m

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If a heat pump is rated at 14kW maximum output, and has a turndown capacity to say 25%, then its minimum continuous output capacity would be 3.5kW. It therefore cannot run continuously if the loading is less than 3.5kW.

Applying the same to a heat pump which is rated at 10kW, would give 2.5kW.

If the heat loss of your home is 6kW at -2C, then at +9.5C the heat loss will only be 3kW. So if the minimum output capacity is 3.5kW, your heat pump will likely cycle at outside air temperatures of 7.6C and above. Conversely, if the minimum output capacity was instead 2.5kW, then cycling would probably not occur until the outside air temperature had increased to approximately 11.4C.

Does this mean that a smaller, 9kW heat pump can avoid cycling simply because it is "allowed" to have lower flow rates? If mine cannot go, say, below 10l/min, at a delta T of 5 it means its minimum output is, indeed, 3.5kW as (10l/min x 4.2 x 5)/60. If I change the delta T to 3, that means 3kW can be provided with a flow rate of FR = 3/(4.2 x 3) = 0.24l/s = 14.4l/min. By changing the delta T to 3, am I not able to make the heat pump able to provide 3kw at an acceptable flow rate?

By the way: this is a Daikin Altherma. I can change the Delta T settings by going into Installer Mode.

Posted by: @derek-m

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Have any energy efficiency improvements been made since the original heat loss calculations were performed and the heat pump sizing carried out?

Not that I am aware of.

Posted by: @derek-m

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If you are out of the home for 8 hours each day, and you have a programmable thermostat, you could set a schedule to lower the indoor temperature using the thermostat. This would shutdown your heat pump unless it is very cold and require heating to maintain the lower indoor temperature. Obviously arrange for the heating to return to normal temperatures before you get back home.

 

Yes I think I am getting to the conclusion that when it's very cold, it's best I have the heat pump set to run 24/7 on lowered weather dependent curve, and with setbacks when it's milder.

Posted by: @derek-m

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A further cause of cycling could be the fact that you are not fully utilising the full capacity of your heat emitters since you have some cold rooms.

Hope the engineer who will come next week will help with that, as at least two rooms are near impossible to heat up like the others, with no visible reason as to why.

Posted by: @william1066

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I have been looking into this as well.  I think this may have something to do with the capabilities of the inverter on the pump.  From what I could find out there is a minimum output that can be delivered, either 1/5 or 1/10 of capacity (not an easy number to find).  If you need less than that, the heatpump will have to switch off and on.

The energy being put into the house will be flow and dT dependant.  If this is < min capacity of pump, it will have to switch off/on/off/on.

Heat transferred\

kW = l/s ⋅ 4.18 ⋅ ΔT

Yes it might be this asas I have never seen the pump go below 10l/min, which for a delta T of 5 that means 3.5kW. But then I do not understand why I can "force" the system to provide less heat by manually changing the delta T to 3, so that at 10l/min the heat output would be 2.1kW. Unless the pump is not allowed to do so by some other control I am not aware of (as, again, I am by far not an engineer or expert). Perhaps the minimum allowed output will still be 3.5kW, meaning the flow rate will have to be 17l/min instead of 10l/min. But when I changed the delta T to 3, it started cycling less.

Clearly I have a LOT to learn about how these systems work 😆 

Posted by: @frappzy

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By the way: this is a Daikin Altherma. I can change the Delta T settings by going into Installer Mode.

Posted by: @derek-m

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Sorry, I was thinking of another post when I mentioned Ecodan. If you can reduced the required DeltaT setting then this may allow the heat pump to run continuously at a lower output rating.

Hope the engineer who will come next week will help with that, as at least two rooms are near impossible to heat up like the others, with no visible reason as to why.

It sounds like the flow regulating valves for the UFH in your colder rooms may need adjusting, or the solenoid valves are not operating. Have you checked the UFH manifolds?