Posted by: @sunandair

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So to summarise:- Of the 365 days of the year our heat pumps will try to operate for 160 days at less than 20% capacity. (If they can cope with the modulation, that is.)

On the other side of the coin - although not the subject of this topic - the heat pump will operate at greater than 80% capacity on only 12 days. Although an installer can be expected to design a system capable of meeting the heating requirements on these days, other influences are likely to lead to over-specification.  Without over-specification, the customer may be prepared to accept a handful of days when heat pump is struggling (e.g. to avoid a planning application, avoid a double-fan unit, avoid radiator replacement, etc). This is a variant of the '80/20 rule' - 80% of what you want for 80% fewer issues.

Posted by: @sunandair

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Posted by: @robs

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What was the total output capacity of your radiators at a flow temperature of 30C? Is it less than the minimum output of your heat pump at 30C flow temperature? 

At a 21kw combined rad load 

30c LWT = 1.785kw total output 

35c LWT = 3.47kw  total output

The first issue I recall, was to notice that the manufacturers correction charts do not go down low enough or fine enough to take account of 5c circuit DT (temp drop) and 30c LWT. 

... 

So after a bit of trial and error I was left thinking the 30c outlet temperature at under 2 kw output it was the reason for the cycling we encountered. 

Yes, your figures correlate well with your heat pump having a minimum output of about 3kW at these outside temperatures. Have you tried your heat pump at a 34C flow temperature? 

Posted by: @robs

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your figures correlate well with your heat pump having a minimum output of about 3kW at these outside temperatures. Have you tried your heat pump at a 34C flow temperature? 

At first I tried 32c for about 10days. It was ok but still cycling a bit too much. So I took it straight to 35c.

But you make a valid point. A small reduced flow temp, by 1 or 2 degrees, may give improved efficiency without increased cycling.

I will do this and let you know how I get on. Thanks for your useful observations.

 Behaviour of compressor is subtly different as flow temperature is dropped to 34 and 33c. The original lowest flow temp of 35c as set on weather compensation which was the set flow temp for all outdoor Ambients 10c and above.

At 34c the cycling increases to 4 per hour with short 5 minute pauses.

At 33c the cycling drops to 3 per hour but the temperature drop appears to be done by creating longer pauses of 10 minutes.

In all cases the flow temperature still registers 35 or 36c. It appears to use variable pauses in operation to reduce the average energy input to 33c.

This operation seems to still be within acceptable Mitsubishi operating tolerances.

To clarify, these graphs are from a radiator only system with a total RAD DT50 capacity of 21kw. Which is greatly reduced to 3.4kw when operating at flow temp 35c. These graphs are specific to our setup and not necessarily typical.