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What impact does compressor not running at low LWT have?

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Active Member Member
30 kWhs
Joined: 1 year ago
Posts: 1
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Helllo everyone, I'm new to the forum as we've just had a 6kW Daikin Altherma 3 Monobloc R32 ASHP installed by Octopus Energy in the last fortnight, replacing a gas boiler for DHW and CHW to radiators (all our radiators were replaced with doubles during the install).

It's been an interesting couple of weeks finding more and more information about ASHP on this forum and places like the Heat Geek YT channel. I've followed the advice to use weather dependent control, and to set an appropriate WD curve in our system, and it is keeping the house at a consistent 20C, which is perfect for us. Though clearly it is relatively mild right now, so I haven't been able to see if the curve is good for colder temps (ASHP geekery is making me want temperatures to go back to sub-zero so I can experiment some more, guess I'll just have to wait for next winter now!)

Early on we found that evening temperatures were a bit too warm for us, even with quite extreme setback values (which I understand is not best practice). The system had been commissioned with minimum LWT of 25C, I 've adjusted this setting to allow minimum LWT of 20C and since then our evenings have been more pleasent, with a setback of just -3C. In this video from Heat Geek (in what looks to be the Daikin training centre), he's going through the settings of the Altherma monobloc system and notes that the ASHP will disable the compressor at LWT below 25C and I've confirmed that is the case from the controller itself when our LWTs are below 25C.

My question to you knowledgeable folk is this; what is the impact of the ASHP disabling the compressor at the low LWTs that I've set?

This topic was modified 1 year ago by mortstar

Illustrious Member Admin
17001 kWhs
Joined: 3 years ago
Posts: 2339

Hello and welcome to the forum! It's great to hear that you've recently had a Daikin Altherma 3 Monobloc R32 ASHP installed and that you've been finding the information on this forum.

Regarding your question about the impact of the ASHP disabling the compressor at low LWTs, it's important to note that the compressor is the component of the ASHP that consumes the most energy. When the LWT falls below a certain threshold, the ASHP will disable the compressor to prevent it from running inefficiently and consuming more energy than necessary. In your case, since you've adjusted the minimum LWT setting to allow for a lower temperature, the ASHP will disable the compressor when the LWT falls below 20C instead of the default setting of 25C.

The impact of the ASHP disabling the compressor at low LWTs will depend on the temperature outside and how well-insulated your home is. If the outside temperature is mild and your home is well-insulated, the ASHP may be able to maintain a comfortable indoor temperature without the compressor running at all. However, if the outside temperature is very cold and/or your home is poorly insulated, the ASHP may struggle to maintain a comfortable indoor temperature without the compressor running, and you may experience reduced heating performance or even cold spots.

It's great that you've been experimenting with different settings to find what works best for your home and your preferences. If you do experience any issues with heating performance or comfort, you may want to consider adjusting the minimum LWT setting back to the default value of 25C.

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Noble Member Contributor
3119 kWhs
Joined: 2 years ago
Posts: 379

Welcome @mortstar. 🙂 Compressors are very well protected in ASHPs, and user and installer configurable setting adjustments to the water side controls won't have adverse impact. But if you think about the physics of it, setting the minimum leaving temperature to be 20degC when you want the house at 20degC isn't logical. You're consuming energy via the circulating pump motor to pump water round a system that's not conveying any energy for either heating or cooling. That's probably why the OEM default setting is 25degC, a few degrees above the comfort temperature most people want their homes. The compressor might de-energise on minimum turn down anyway. Compressors tend to modulate down to about 25% minimum output. For heating loads lower than minimum turn down, they'll start to cycle the compressor on and off. That's why oversizing ASHPs isn't a good idea. Most ASHPs will run at part or low output in a typical heating season, only getting near full 'nameplate' output when heating a house from cold, or providing hot water generation.

The input power consumed by a compressor is the energy required to compress refrigerant gas from a low to a high pressure. The bigger the pressure increase, the more power is required. The evaporating pressure and temperature determines the low side pressure, and the condensing pressure and temperature determines the high side pressure. The evaporator temperature has to be lower than the outdoor air temperature in order to capture heat energy from outside, and the condensing temperature has to be higher than the leaving water temperature in order to transfer that heat energy into the heating system water. The greater the difference, the greater the power consumed. ASHP power consumption increases significantly in colder weather, but that's just physics - outside it's cold, but the house needs warmer water to provide sufficient heating. Running costs go up massively in the colder October-March months, which alarms people new to ASHP who have not run their system through a winter season. That said, you get very good efficiencies at other times & seasons so, over a year, you realise a net saving in energy bills. You're consuming massively less energy as a household too 🙂



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