Posted by: @newhouse87

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@transparent had one of them at warehouse job years ago. cop of 3.05 at moment i think, including dhw. I know im missing something, i know i have to find lowest lwp to get house to temp, but i dont get the continous running thing, When i reach temp of room stats cut it off or are ye suggesting i find lowest lwt that will give me 21deg in each room and have stats calling for 22 so pump is continually supplying just enough heat to maintain21. Pump quite loud so dont fancy it running all night.

A heat pump can be reasonably loud when it first starts running, but like a car, once it gets up to normal running the sound reduces quite a bit.

Let me try to explain why a heat pump needs to operate like a Tortoise rather than a Hare, or running a car continuously rather than starting and stopping.

Because an 11kW heat pump was selected, I assume that the heat loss of your home was calculated to be in the 9kW to 10kW range, with an outside air temperature of -3C and an indoor temperature of 21C. The heat loss is not constant, but varies according to the difference in temperature between inside and outside. So if the heat loss is say 10kW, at the specified temperature difference of 24C (21 -(-3)), then when the temperature difference is only 12C (Inside 21C and Outside 9C), the heat loss will be approximately 5kW, which is each hour, so 120kWh for a full day. That sounds quite a great deal, but fortunately heat pumps are very efficient when correctly installed, commissioned and operated, so instead of 120kWh, the electrical input should be in the range of 30kWh to 40kWh under those conditions. The remaining 90kWh to 80kWh of heat energy is absorbed from the outside air.

For best efficiency and reduced running costs, it is desirable to extract as much 'free' heat energy from the outside air, whilst using the minimum amount of 'costly' electrical energy in the process. The amount of heat energy that can be extracted from the outside air is dependent upon the air temperature, the flow rate of the refrigerant gas around the system, and the time period over which the heat energy extraction is taking place.

Using the above conditions as an example, if the heat loss is 5kW, then over a 2 hour period the heat pump will need to supply 10kWh of heat energy to keep your home at the desired temperature. Running continuously it may be able to do so with a LWT of 30C, at a COP of 4, which requires a total electricity input of 2.5kWh, or 1.25kW for each hour.

If instead, the heat pump runs for 1 hour and then stops for 1 hour, it still needs to supply 10kWh of heat energy, but only has 1 hour in which to do so. It is quite probable that the heat pump would therefore have to work much harder, say at a LWT of 35C and a COP of only 3. So during the 1 hour that it is operating, the heat pump could be using 3.33kWh of electrical energy, but then very little for the remaining hour of the 2 hour period.

Comparing the two situations, the on - off method uses 3.33kWh of electrical energy to produce 10kWh of heat energy, whilst the continuous running method uses 2.5kWh of electrical energy to produce the same 10kWh of heat energy.

The on - off method would therefore appear to use something like 33% more electrical energy than the continuous method.

I hope that this answers your question.

@derek-m Thanks man, i get what your saying in theory but my question is how do i get it to run continuously physically, do i need to have call for heat constantly? Yesterday i turned on heat pump to living areas for maybe 4-5 hours, think 10kwh electricity used, not 100% on that. Kitchen got up to 22 and sun room and sitting room adjoining kitchen both at 21. Woke up this morning, all had dropped only 1 degree and pump had been off since 9pm. Heat pump didn't come on since so happy with that. Im thinking if i just have all loops open for living areas it will run for hours like that as sitting room stat in particular with laminated floor and only 2 loops takes longer to heat up so will cause all loops to get flow for longer. All that said was mild last night so not surprising i didn't lose much heat. As an aside, my pump shut off because rwt went up near as high as lwt. Is that because floor had absorbed enough heat and delta t had been reduced too much?

Hi @newhouse87,

Apologies for jumping in here.  I have followed the advice of @Derek-M and @Transparent with great results.  "This is the Way" as they say in the Mandolorian.  

I have opened up the whole of the ground floor so that the underfloor heating is a single loop, this made a huge difference.  I am now using the built in Vaillant control system to control the temp of the house rather than the individual room stats, this also makes a difference to the efficiency.  When using the room stats we raised the temp higher than what we required, and then let the heat pump curve do the work, in theory it never reached the required temp so never shut down, and the curve was low enough so as to never reach the temp required.  

We are now running our system in fully automatic mode with the heat pump controller doing the managing of everything.  Our heating curve is also in automatic which is working well so far.  Fingers crossed.  

A massive thank you to @Derek-m, @Transparent, @Editor for the site and all the great advice received.  

This is the way!

@drew-pa Oh im not questioning their methods at all, in fact the opposite, i want to know exactly how its done. Think im understanding more that i have to have call for heat constantly in living areas and it will never actually get up tot hat heat but to do so i have to figure out curve to have lower lwt, im going to leave bed areas alone for time being as i like them cooler at 18 so won't have loops open on them.

@derek-m , is it possible to do this just for my living areas which is half the house essentially and how does solar gain affect this? As you can tell im slowly understanding what you and transparent are saying, slowly 😀 . Is it essentially alter weather curve to suggested settings, put my living,sun and sitting room at 23, 2 deg above desired temp and wait few hours, if it goes to 22,lower curve by a degree? Them rooms don't heat as quick as each other due to glazing materials etc but i just wait for all to reach 21 i presume. Apologies about the slow uptake. Best to do on cooler day?

Posted by: @newhouse87

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@derek-m Thanks man, i get what your saying in theory but my question is how do i get it to run continuously physically, do i need to have call for heat constantly? Yesterday i turned on heat pump to living areas for maybe 4-5 hours, think 10kwh electricity used, not 100% on that. Kitchen got up to 22 and sun room and sitting room adjoining kitchen both at 21. Woke up this morning, all had dropped only 1 degree and pump had been off since 9pm. Heat pump didn't come on since so happy with that. Im thinking if i just have all loops open for living areas it will run for hours like that as sitting room stat in particular with laminated floor and only 2 loops takes longer to heat up so will cause all loops to get flow for longer. All that said was mild last night so not surprising i didn't lose much heat. As an aside, my pump shut off because rwt went up near as high as lwt. Is that because floor had absorbed enough heat and delta t had been reduced too much?

Now that you are getting a level of understanding about heat pumps and the whole heating system we can move on to the next stage.

A heating system can be divided into two main parts, the heat energy generator and the heat energy distribution system.

Your heat pump is the heat energy generator, which in your situation consists of the outdoor unit, which heats a refrigerant gas, which is used to transport the heat energy to the indoor unit, in which the heat energy is then transferred from the refrigerant gas into the CH water. The temperature to which the water needs to be heated is set by the heat pump controller.

The heat energy distribution system is the water pump and the UFH manifolds and loops.

So how does, or should, the system operate to maintain reasonably constant indoor temperatures? I am pleased to see that you understand solar gain, but there is also 'human activity' (cooking, using electrical/electronic equipment, and even just human's in a room), that will also be a source of heat energy. Wind chill and the cooling effect of rain may also need to be considered, since they all have an affect on actual heat loss.

The documentation provided by your installer should have included heat loss calculations, but what do heat loss calculations actually mean.

It could be that when the indoor temperature is 21C, and the outside temperature is -3C, it has been assessed that your home may lose 10kW of heat energy. This is an assessment, based upon room sizes and insulation levels of the building materials used in the construction. It will therefore not be precise, and takes no account of the additional affects listed above. The main parameters defining actual heat loss are the indoor and outside temperatures. As the outside temperature reduces, and/or the indoor temperature is increased, the heat loss will increase. The heat loss for each individual room should have been calculated, with the total heat loss being all those values added together.

To keep the indoor temperature reasonably constant in any one room, requires putting in sufficient heat energy, to match the present heat loss of that room. Heat energy is put into the room via the UFH loops, with the quantity of heat energy being controlled by the water temperature and the flow rate. The higher the water temperature and flow rate, the more heat energy that will be transferred into the room, and if the heat energy supply exceeds the present heat loss, the room temperature will start to increase. At some point the room thermostat will shut off the water supply, or the increase in room temperature will increase the heat loss, to the point where it matches the heat energy supply, and the system will be balanced.

Rather than starting and stopping the water supply, because it is too warm, it is better to lower the water temperature, since this in turn reduces the amount of work that the heat pump needs to perform. Of course, lowering the LWT to control the temperature in the warmest room, may cause other rooms to then be too cold. The other method of varying the amount of heat energy being supplied is by varying the flow rate.

To optimise your system, it will therefore be necessary to reduce the flow rate slightly to warmer rooms, and increase the flow rate to colder rooms, with the objective of achieving the desired temperature in each room, whilst operating your heat pump at the lowest LWT that meets demand.

The above method works fine during colder weather conditions, but what is likely to happen as the weather gets milder? This is where the room thermostats may come into play. As the outside temperature increases, the WC curve should automatically reduce the LWT, and hence the amount of heat energy produced by the heat pump, but there may be rooms that get plenty of solar gain, which causes the room temperature to increase. This is why the room thermostats should be set to a temperature 1C above the desired level, since the increasing room temperature will cause the UFH loops to be shut off until the room temperature falls again. The net effect should be that the heat pump will not have to work so hard to supply the other rooms.

As I think you found, eventually the heat demand will reduce to the point where the RWT starts to increase in relation to the LWT, this reduces the DeltaT, and eventually the heat will stop operating for a period of time. This is perfectly normal provided that the heat pump does not start short cycling (switching on and off more than 3 times in a 1 hour period).

@derek-m Thanks, that must have taken a lot of time to write. Im getting it now tbh. My 3 biggest areas are sun/sitting and kitchen which also have utility and main bathroom on kitchen stats and im not too worried about heat in them as i lowered flow rates there and its nice heat now, was too hot in them. The kitchen and sun room have 3loops and sitting 2 loops hence why it takes bit longer to heat up, kitchen heats up quickest due to tiling actually went up to 22 and others were 21 and 20.5 so maybe could slow flow rate in kitchen, other then that its pretty ok for balancing im thinking and sitting room has stat to decimal point so 20.5 on that would do fine and not have to reach 21 like kitchen and sun room(family room on plan). So if i alter curve and have stats looking for 23 and 22.5 in sitting room(living room on plan) i can see what way it goes. Is this method still as efficient  from may-oct or would i just wait until winter months? Very little usage i have from may -sep and would heat pump running constantly be ok in long run like a car that has more mileage. Currently at 44 for -3 and 28 at 17degrees outside, what would be good starting point? Attached loop plans but plumber said over specced so didn't follow to a tee.

Is this method still as efficient  from may-oct or would i just wait until winter months? Very little usage i have from may -sep and would heat pump running constantly be ok in long run like a car that has more mileage.

@newhouse87

It can take quite some time to get the WC curve correctly adjusted, and it is important to not go on just one day's measurements, since you may get a lot of solar gain which skews the results.

As you say, you may have to wait for colder weather to fully adjust the WC curve. In the mean time, try dropping the WC curve by 1C at the warm end and 2C at the cold end. See how the system performs then repeat the exercise if necessary.

Ideally you want the lowest LWT, at a reasonable flow rate, with as many UFH loops in service as possible. As the weather gets milder it may be more cost effective to allow the room thermostats to stop the heat pump for lengthy periods of time.

@derek-m will do that man, ill keep eye out in advance for colder weather but may be good while again before that comes to trial it more but will report back intermittently, would have made lot of difference last winter i reckon. 1 DHW cycle for hour2-3pm instead of 2 cycles early morning and night time seems to be working fine for the 2 of us too. Thanks again for all the help.

Posted by: @newhouse87

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@derek-m will do that man, ill keep eye out in advance for colder weather but may be good while again before that comes to trial it more but will report back intermittently, would have made lot of difference last winter i reckon. 1 DHW cycle for hour2-3pm instead of 2 cycles early morning and night time seems to be working fine for the 2 of us too. Thanks again for all the help.

Thank you for your kind words, but it is all part of the service to reduce energy consumption.

By the way, the bill is in the post. 😋 

@derek-m Post very slow from England to Ireland, brexit and all that 🤣 .

@derek-m Thank you from me too - this thread has helped me. You are very right about needing to be 'ready' to hear the next piece of information. I read all the threads each day and the vast majority goes over my head but today the next little bit sank in 😊.