Is this normal: return temp higher than flow?

KuniD · 2024-10-15T14:06:51.655Z

Hi all (apologies if I have tagged this to the wrong forum)

I’ve recently installed an L3 heat pump monitoring setup with separate DHW sensing. I had some issues with my heat pumps heat diverter valve (among other issues over the last year), so am only now getting to a stable and baseline setup, so I can start to optimise from here.

One of the things I’ve noticed is that the return temp is showing higher than the flow temp after each cycle, which doesn’t seem right and not something I have noticed when looking at other setups on heatpumpmonitor.org. Is a scenario where this is expected behaviour, any ideas what may be causing this?

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Few bits of background for context;

I’m using an Ochsner Air 18 C11A heat pump with Unifresh 1000 tank, which I’m convinced is oversized, but not much I can do about that now.
Home is 215sqm, with a further 45sqm outbuiding coming soon, that will share the same heating source.
The heat pump and tank are at the rear of the garden, next to the out building mentioned above, but 40m from the main house, connected via underground mibec duo insulated pipes for dhw fllow and return, and heating circuit flow and return, into my utility room, then on to mostly UFH and 3 additional radiators. Clearly there will be a level of heat loss through the long run between the plant room and house.
Heat curve is set to flow of 40c at -2c outdoor temp, 21c at 20c outdoor temp, no heat provided above 16c. This runs 24/7 on same settings except for a 3c set back between 4-7pm daily (reduce demand to optimise for Octopus Agile tariff)
DHW comes on daily at 2-4pm to warm tank to 48c (not sure if it matters how long this window is set for, most the time it achieves this much faster). I have a circulation pump in the plant room that only operates 7-9am, 7-9pm, to reduce heat loss during the day when DHW is not being used.

Andre_K · 2024-10-15T14:33:28.901Z

Welcome Kunal! Can you also show the flow rate in your screenshot? It looks to me as if there was no flow between cycles, otherwise the heat power would have to be nonzero when flow and return temperatures are not the same. If there’s no flow then the temperature readings are meaningless and just due to environmental influence at the sensor location.

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Timbones · 2024-10-15T16:28:39.699Z

This could be due to thermal syphoning happening when the pump is off, as the hot water flows to the upper portions of the circuit and cold sinks to the bottom. If the return temperature sensor is higher than the flow temperature sensor, then this could make sense.

I see a similar effect on my system after the DHW cycle as the temperature equalises:

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KuniD · 2024-10-15T18:14:48.241Z

Thanks Andre and Tim for coming back to me

Sure here’s a screenshot including flow rate

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Looks like no flow so guessing I’m ok to ignore the flow temp then during those segments

Strangely in this screenshot there’s a spike in the return flow to the right for some reason

So much more to learn !

Not sure whether this level of cycling is ok, I’m going to test reducing the flow temp slowly as the temp drops , but guessing due to oversize it won’t be possible to have it run more continuously at lower power

Timbones · 2024-10-15T19:04:37.120Z

KuniD:

Not sure whether this level of cycling is ok

Yes, this is well within manufacturer’s limits, quite common in the shoulder months.

It should cycle less often with higher flow temperatures and/or colder weather. Open some windows to let some heat out.