Mitsubishi cycling while on chilled water output

downfield · 2025-06-29T16:39:52Z

We have our Mitsi 14kW Zubadan on light cooling during hot weather. The flow temp is set to 15C on the controller to avoid condensation problems. Today I noticed that the unit is cycling every few minutes because it is chilling down to around 12.5C and then turning off. The return temp is around 19C and seems fairly consistent. Not sure if it makes a difference but I have a 100 litre 4 pipe buffer but very little other volume as all Jaga fancoil rads - perhaps this volume is too small for the ASHP? Anyway, is it possible to reduce the cycling?

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Air Source Heat Pumps (ASHPs)

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IaAck

(@iaack)

Trusted Member Member

Joined: 2 years ago

Posts: 43

03/07/2025 10:04 am

@downfield I think the cooling capacity ratings for you fan coils are misleading for your application (I am assuming the units are mounted just above skirting level) as in this position cooling becomes self limiting. You may well achieve the rated cooling output from them when you first switch on but after a short period of running the cooler (denser) air that they have previously generated will sink back to floor level only to sucked back into the fan coil thus now reducing effective delta t. Thus to be effective at cooling the room the fan coils need to be mounted higher up at head height or ideally at ceiling level. At skirting level you'll get cool ankles I guess but not a great deal more.

Perplexity AI summarises as follows: The height at which you position a cooling device significantly affects its efficiency in a room.

Higher placement (ceiling or high on the wall) generally improves efficiency because hot air rises, so the device can more effectively cool the warmest air at the top of the room. As this cooled air sinks, it helps distribute cool temperatures more evenly throughout the space.

Better air circulation is achieved when the cooling device is higher, allowing it to exchange and mix air more effectively across the room, which enhances overall cooling performance.

If a cooling device is placed low (near the floor), it mainly cools the already cooler, denser air at floor level, which is less efficient for whole-room cooling. The device may not be able to address the warmer air accumulating near the ceiling, leading to uneven temperatures and longer run times.

The energy cost to generate cool air does not change with height, but the ability to target and remove heat from the room is improved at higher positions, making the cooling process more effective.

In summary, placing a cooling device higher in the room increases its efficiency by targeting the warmest air and improving air circulation, resulting in more even and effective cooling.

So is the cooling capability for these fan coil units being overhyped by the marketing people?? Sorry to say but I think that's a good possibility.

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downfield

(@downfield)

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Joined: 10 months ago

Posts: 79

Topic starter 03/07/2025 2:35 pm

@iaack yes there's no denying that cold air sinks! I suppose that's why aircon units are mounted high up.

The Jaga units are able to produce quite a strong air flow which easily circulates around a normal domestic room so there's a good mixing effect. We don't notice it being cooler near the floor although there must be some temperature gradient.

The top of the units are around 80cm from the floor not at skirting height. If you want rads to heat and cool there must be a compromise on position and anyway it isn't practical to move them higher. I never really worried about cooling capacities - any cooling is a massive bonus and an advantage of the ASHP that just isn't possible with a conventional CH system.

Regarding the cooling data - I believe Jaga to be a serious company with a focus on commercial buildings rather than domestic, which are small beer to them. I suspect that they would quickly be found out by architects and heating engineers if their published performance figures didn't stack up in real world applications.

Mitsubishi Zubadan 14kW with Mixergy 210l DHW in 220m2 barn property. 24 solar panels = 9kWp with GivEnergy 5.0kW Hybrid inverter and 19kWh GivE batteries. Jaga Strada fan-assisted rads throughout. Landvac vacuum glazing/triple glazed windows.

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