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Why are my flow and return temperatures reading the same?

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(@bob77)
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In trying to improve efficiency I have steadily been reducing the flow temperature on my Daikin Altherma 8kW ASHP, with the goal of being able to leave the thermostat set high so it is effectively "always on" without overheating the house.

 

However I've noticed that my delta T always seems to be at or close to zero - or at least the heat pump thinks it is, even though it can't be as heat is clearly being delivered to the house.

 

I have two UFH zones downstairs (with a mixing valve limiting the temperature to 35C) and radiators in the bedrooms upstairs. As far as the heat pump is concerned it is a single heating zone, so it should be outputting water at a temperature set by the WD curve, whenever either the UFH or the radiators, or both, are calling for heat. (The upstairs thermostat controlling the rads is currently set at comfort temperatures of 20C morning and evening, 18C otherwise, while the downstairs stats are currently set to 25C in the day, 18C setback at night, while I try to get the WD curve dialled in - so effectively on all day, off during setback.)

 

As it is currently quite mild my flow temperature is 29C this morning. When I check the sensor information screen I see that the leaving water temperature and inlet water temperature are almost always identical.

The compressor runs for a few minutes, bringing the LWT up to say 35C, and the inlet temperature rises at almost exactly the same rate. Both figures then slowly drop down to 25C or 26C, again staying within 1C of each other, and then the cycle repeats.

 

Surely this is not how it is meant to work? I thought the LWT should be kept relatively constant, with the return water temperature also fairly constant and a few degrees lower than the LWT?

When I put my hand on the flow and return pipes going into the floor I can certainly feel the difference in temperature, so something is not right. It seems to me that the pump is "short cycling", although I am not quite sure what that means - presumably when the flow temperature required is fairly low, in mild weather, then the compressor only needs to run for short periods anyway?

 

Example photos of the screen showing what I mean, taken five minutes apart - these are about the lowest and highest temperatures I am seeing, with an LWT of 29C set via weather compensation.

 

One thing I do notice looking at the settings is that the "minimum heating temperature" is set to 25C. Could that be why it is heating the LWT to 35C, to try to keep a delta T of 10 degrees? Or is it normal to overshoot the weather compensation flow temp by this amount?

IMG 1988
IMG 1992

 


   
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(@derek-m)
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Posted by: @bob77

In trying to improve efficiency I have steadily been reducing the flow temperature on my Daikin Altherma 8kW ASHP, with the goal of being able to leave the thermostat set high so it is effectively "always on" without overheating the house.

 

However I've noticed that my delta T always seems to be at or close to zero - or at least the heat pump thinks it is, even though it can't be as heat is clearly being delivered to the house.

 

I have two UFH zones downstairs (with a mixing valve limiting the temperature to 35C) and radiators in the bedrooms upstairs. As far as the heat pump is concerned it is a single heating zone, so it should be outputting water at a temperature set by the WD curve, whenever either the UFH or the radiators, or both, are calling for heat. (The upstairs thermostat controlling the rads is currently set at comfort temperatures of 20C morning and evening, 18C otherwise, while the downstairs stats are currently set to 25C in the day, 18C setback at night, while I try to get the WD curve dialled in - so effectively on all day, off during setback.)

 

As it is currently quite mild my flow temperature is 29C this morning. When I check the sensor information screen I see that the leaving water temperature and inlet water temperature are almost always identical.

The compressor runs for a few minutes, bringing the LWT up to say 35C, and the inlet temperature rises at almost exactly the same rate. Both figures then slowly drop down to 25C or 26C, again staying within 1C of each other, and then the cycle repeats.

 

Surely this is not how it is meant to work? I thought the LWT should be kept relatively constant, with the return water temperature also fairly constant and a few degrees lower than the LWT?

When I put my hand on the flow and return pipes going into the floor I can certainly feel the difference in temperature, so something is not right. It seems to me that the pump is "short cycling", although I am not quite sure what that means - presumably when the flow temperature required is fairly low, in mild weather, then the compressor only needs to run for short periods anyway?

 

Example photos of the screen showing what I mean, taken five minutes apart - these are about the lowest and highest temperatures I am seeing, with an LWT of 29C set via weather compensation.

 

One thing I do notice looking at the settings is that the "minimum heating temperature" is set to 25C. Could that be why it is heating the LWT to 35C, to try to keep a delta T of 10 degrees? Or is it normal to overshoot the weather compensation flow temp by this amount?

IMG 1988
IMG 1992

 

I would suggest that you check the speed of the water pump, it may be too high.

 


   
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(@bob77)
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Posted by: @derek-m

 

I would suggest that you check the speed of the water pump, it may be too high.

 

Do you mean the external pump? There is a Grundfos pump attached to the UFH manifold which has three lights, indicating that it is running at the highest speed. 

The ASHP shows a flow rate of 33.6 litres per minute at the moment. 

Should I try reducing the speed of the UFH pump?

 


   
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(@derek-m)
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Posted by: @bob77

Posted by: @derek-m

 

I would suggest that you check the speed of the water pump, it may be too high.

 

Do you mean the external pump? There is a Grundfos pump attached to the UFH manifold which has three lights, indicating that it is running at the highest speed. 

The ASHP shows a flow rate of 33.6 litres per minute at the moment. 

Should I try reducing the speed of the UFH pump?

 

Where is the primary water pump located? Is it within the heat pump or eternal?

You could try lowering the speed of the UFH water pump, if nothing else it will probably use slightly less electricity.

 


   
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(@bob77)
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Posted by: @derek-m

Where is the primary water pump located? Is it within the heat pump or eternal?

You could try lowering the speed of the UFH water pump, if nothing else it will probably use slightly less electricity.

 

To the best of my knowledge the main pump is inside the indoor unit - it's one of these, incorporating the DHW tank. There's also a pump that supplies the radiator circuit upstairs but AFAIK that only operates when the upstairs thermostat is calling for heat.

This post was modified 6 months ago by Bob77

   
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(@derek-m)
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Posted by: @bob77

Posted by: @derek-m

Where is the primary water pump located? Is it within the heat pump or eternal?

You could try lowering the speed of the UFH water pump, if nothing else it will probably use slightly less electricity.

 

To the best of my knowledge the main pump is inside the indoor unit - it's one of these, incorporating the DHW tank. There's also a pump that supplies the radiator circuit upstairs but AFAIK that only operates when the upstairs thermostat is calling for heat.

 

I would suggest that you have a read through the manual and see if the water pump speed or the desired Delta T can be adjusted.

 


   
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(@bob77)
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Posted by: @derek-m

 

I would suggest that you have a read through the manual and see if the water pump speed or the desired Delta T can be adjusted.

 

 

Only Delta T, from what I have seen. The "main zone" (which is the only zone, as both UFH and rads are run off the single leaving water zone as far as the heat pump is concerned) can be set up either as UFH or as radiators. If you choose radiator, it has a fixed delta T of 10C. If you choose UFH, it is variable from 3C to 10C.

Regardless of which is selected, the delta never seems to be more than about 1 degree. They both heat up together when the compressor is running, and then they both fall together when it is off.

 


   
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(@derek-m)
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Posted by: @bob77

Posted by: @derek-m

 

I would suggest that you have a read through the manual and see if the water pump speed or the desired Delta T can be adjusted.

 

 

 

Only Delta T, from what I have seen. The "main zone" (which is the only zone, as both UFH and rads are run off the single leaving water zone as far as the heat pump is concerned) can be set up either as UFH or as radiators. If you choose radiator, it has a fixed delta T of 10C. If you choose UFH, it is variable from 3C to 10C.

Regardless of which is selected, the delta never seems to be more than about 1 degree. They both heat up together when the compressor is running, and then they both fall together when it is off.

 

As far as I am aware, there are two ways in which a heat pump can regulate the quantity of thermal energy it is supplying to the heat emitters, one is by varying the water pump speed, and hence the flowrate, to maintain a fixed DT, the other is a fixed water pump speed and allowing the DT to vary.

If you have a low DT, that would seem to indicate that the pump speed is too high, but could also be due to poorly installed temperature sensors.

 


   
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(@iancalderbank)
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for the same heat power, if flowrate is high, DT is low, and vice versa, they multiply to make the heat power. you have an 8kw hp and its mild : I'm guessing your heat load is around 2kw. at DT of 1, flow 33l/min, the maths say  2.3 kw, which is what you've got. for the same power you can run at DT2 + 17l/min, or DT3 + 11l/min. 

If you keep the flow rate the same, the HP can't change the DT, without changing the the power its trying to deliver.

So if you want it to run at a higher DT you need to reduce the pump speed. To be honest, uf you are getting 33 l / min, that is a really good flow rate, implying you have a well engineered circuit that isn't too large, with a relatively good (low) resistance to flow. meaning you should easily be able to get away with less power from the circulation pump

your 8kw heat pump will have a minimum flow rate of somewhere around 10-12l/min (check the mfr spec) so you can reduce the UFH pump speed without hitting this.

My octopus signup link https://share.octopus.energy/ebony-deer-230
210m2 house, Samsung 16kw Gen6 ASHP Self installed: Single circulation loop , PWM modulating pump.
My public ASHP stats: https://heatpumpmonitor.org/system/view?id=45
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(@bob77)
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@iancalderbank The manual states minimum flow rate is 12 litres per minute. So it sounds like I don't need the UFH pump on max speed.


   
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(@iancalderbank)
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@bob77 2 more thoughts. It is possible with some heat pumps to wire a PWM output to the circulating pump. This allows the heat pump to tell the circulating pump to slow down, and thus gives the heat pump completely dynamic control of the flow rate and allows it to balance this against its desired DT and output power. I don't know if yours can do this, finding and reading the installer manuals may tell you.

short cycling in mild weather is an issue we all have to deal with , unless the system has super-sized emitters. the RWT very quickly approaches the LWT, at which point the compressor says "nothing to do here any more" and shuts down. 

one option is to run at a slightly higher LWT than that which gives a perfect in=out heat balance. This then means the HP will run a bit longer, your aim is that instead of it being the RWT is too close to LWT as the cause for the compressor stop, the cause is because there's no longer a call for heat - the thermostat becomes satisfied. then house cools down, then come on again. giving longer cycle times. this is an approach several of us use.

but dropping your flow rate should increase DT so it'll be interesting to see if you get more stable runs by just changing that. golden rule of tweaking - only change one thing at once!

My octopus signup link https://share.octopus.energy/ebony-deer-230
210m2 house, Samsung 16kw Gen6 ASHP Self installed: Single circulation loop , PWM modulating pump.
My public ASHP stats: https://heatpumpmonitor.org/system/view?id=45
11.9kWp of PV
41kWh of Battery storage (3x Powerwall 2)
2x BEVs


   
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(@bob77)
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Topic starter  

Well I have dropped the pump from "PP3" to "PP2" mode - proportional pressure, intermediate curve.

 

I am sure that the flow and return temperatures are incorrect though. Here is a photo of the UFH manifold. The large copper pipes going off the bottom are flow (left) and return (right). Placing a hand on the flow pipe it feels pleasantly warm to the touch. The return pipe is cold to the touch. Similarly there is a noticeable temperature difference between the flow (rear) and return (front) UFH pipes coming out of the manifold.

And yet the heat pump tells me that flow and return temperatures are virtually identical. Unfortunately I have no idea where the sensors are located. I assumed they would be built-in on the indoor unit.

 

However, when the system is in DHW mode, the return temperature shows as 5-6C lower than the flow temperature. (It's just heated up and the flow temp was at 49C with the inlet PHE temp at 43C when I checked.)

IMG 1998
This post was modified 6 months ago by Bob77

   
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