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Air Source Heat Pumps (ASHPs)
Last Post by HughF 2 years ago
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 HughF
(@hughf)
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Joined: 3 years ago
Posts: 489
Topic starter 23/01/2024 11:29 am  

Posted by: @derek-m
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Posted by: @hughf
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@derek-m single heat pump, just run on weather comp, no stats, no setback.

The Carel platform that my heat pump is built on allows full tuning of the PID settings, along with full control of the compressor ramp speeds. It’s close to maintaining setpoint, but I’ve still got some work to do. I’d like to deal with the initial overshoot next, I think I’ve got the oscillation damped quite well.

When it’s around freezing outside, it runs happily for a long period of time, it’s dealing with the overshoot when it’s cycling that’s the next area of tuning that needs my attention.

I suspect that it will not be possible to accurately control the LWT during milder weather conditions, simply due to the fact that the heat pump will be starting to operate outside its control envelope. i.e. it cannot turn-down any further.

As far as controlling IAT is concerned this should not be a major issue, though it could take some time to optimise for maximum efficiency.

Out of interest what are your PID settings?

 

It is my suspicion too, that in milder weather, we are simply oversized. I didn't take into consideration the possibility that the ACH value used in the heat loss spreadsheet (and by heating engineer) could be considerably higher than we actually achieve.

The fact that it was tracking the requested setpoint to within 0.1 degree during the last 2 weeks of freezing weather leads me to think my PID settings aren't a million miles off.

At the moment they are: P = 5 deg. C, I = 50 secs, D = 0 secs.

The controller also allows me to open up the overshoot window, which gives headroom for compressor rampdown - currently set to 5 degrees. I can't remember the rampdown settings though, it's specified in Hz/sec inside the Power+ inverter settings.

To comment on the original question in the thread title - I was away for a lot of the 2 week freeze, but the residents reported stable internal temperatures (21-22) 24/7 and there were no issues with defrosting, even with our limited system volume.

Whole house electric consumption was running around 50kWh/day - all electric house now.

This post was modified 2 years ago 2 times by HughF

Off grid on the isle of purbeck
2.4kW solar, 15kWh Seplos Mason, Outback power systems 3kW inverter/charger, solid fuel heating with air/air for shoulder months, 10 acres of heathland/woods.

My wife’s house: 1946 3 bed end of terrace in Somerset, ASHP with rads + UFH, triple glazed, retrofit IWI in troublesome rooms, small rear extension.


   
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