Why are my flow and return temperatures reading the same?

Bob77 · 2023-11-21T08:18:52Z

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?

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iancalderbank

(@iancalderbank)

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22/11/2023 2:20 pm

so the most likely explanation is that there is another circulating pump in your system, and you also have a buffer tank or low loss header (LLH) in your system.

the other pump will either be in the outdoor unit or near to the buffer tank / LLH, on the heat pump side of the piping. that's what's called the primary pump - its responsible for the flow rate you are seeing. the temperature sensors you are seeing are those of the primary flow and return, in / out of the heat pump.

when your system is in CH mode, the HP supplies warm water to top the buffer tank(or LLH), that water being moved by the primary side circulation pump. and it takes cool(er) water from the bottom of the buffer tank. The UFH takes warmer water fromt he top, uses it to heat the house, and puts it back into the bottom. That called the secondary side. Thus there are two separate circulating loops that join in the middle.

when your system goes into HW mode it'll switch a valve for the water flow so that instead of going from the HP into the LLH, it goes direct to the cylinder. So in that case, the flow rate, and flow/return temps, that you see , are those of water flow directly between cylinder and HP. So the HP is seeing the true DT and flow rate of the cylinder.

But in CH mode, your HP doesn't "see" the heating system. It simply "sees" a buffer/LLH that keeps on losing some heat, and keeps topping it up.

Note in CH mode via the buffer/LLH, that the flow rates of the two loops are completely separate, and if the flow rates differ significantly then the DT will also. Read this about how a system with LLH/buffers works (HeatGeek is always a good place to start) .

edit - I missed your info in previous posts. seems like you have an indoor unit with a PHE (Plate Heat Exchanger) and a separate primary side pump. The PHE is doing similar job to LLH as I mentioned above, separating the two sides. your temperature readings and flow rate that your system reports will still be on the primary side, you will never see the UFH side DT or flow rate without additional sensors.

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Bob77

(@bob77)

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Topic starter 28/11/2023 12:09 pm

@iancalderbank Thanks Ian for that explanation.

I have noticed that in recent days with colder weather and a higher LWT temperature, there is more of a difference between leaving and returning water temperatures. Anyway the house is warm and the electricity consumption has been reasonable, so that's really all that matters!

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