Fixed Speed vs Inverter controlled ASHP

[Stones]

The appearance of a fixed speed ASHP on a well know auction site has got me thinking about this. 

The general thrust of opinion on the closed forum was that an inverter controlled heat pump is far better than a fixed speed for a number of reasons, but primarily;

Soft start so kinder to the grid and matches its output to actual requirement rather than short bursts on full power (short cycling)

A large buffer tank could of course be fitted to increase system volume and thus reduce short cycling of a fixed speed unit, but I think such a system would need to be very carefully designed to ensure the heat pump and buffer was sized such that it could run for a decent length of time.

Noise and being prone to icing up are the two other concerns I have in respect of a fixed speed heat pump.

I do wonder if we are overthinking the subject.  An oil boiler for example (AFAIK) is either on or off and therefore when teamed with UFH needs a buffer tank / thermal store to avoid short cycling.  It's not a massive issue for oil, so should it really be for an ASHP? True, an ASHP cannot deliver the high temps an oil boiler can, but even so, is running a heat pump at full power for 20 - 30 minutes every 2 hours really that bad?

Thoughts?

[ProDave]

I will throw self generated solar PV into the discussion.

A modulating unit running at low power will more likely be using up available self generated power and not importing much. A constant but low power load is much better suited to solar PV.

A non modulating unit when it's running at full chat for its bursts, will much more likely be using some imported power, followed by periods of not using any self generated power.

So the modulating unit is by far the best if you have solar PV.

Can I also buck the trend re oil boilers. I have been running an oil boiler directly feeding UFH (and DHW) for 13 years. I don't see this short cycling that people keep mentioning.  Unlike a gas boiler, oil boilers (or at least the one I have) tend to be quite large capacity big steel tanks with a burner down the middle, so in themselves have quite a hig heat store.  What I find is when running with just a low UFH demand, the boiler will fire up for a couple of minutes every 20 minutes or so. I don't see that sort of short cycling as a problem, certainly not one that needs to be "solved" by fitting a buffer tank.

[Stones]

It's a very good point you make about PV (and one that TBH I hadn't even considered as PV is very much a post build project).

A good analogy is the house we currently rent. It has a 5kW turbine and with a bit of thought could have a heating system tailored to maximise use of wind generation.  Instead it has a number of power hungry storage heaters which all switch on and off at the same time.  Running a lower but constant load, a fan heater for example) would keep the house warm but a virtually zero cost, whereas as it stands, for several hours a day, our import massively exceeds generation.

On the oil boiler (or indeed gas boiler) front, I'm guessing such short bursts don't represent any problem, but the collective wisdom does seem to suggest that it is best avoided with a heat pump compressor.

So other than being cheaper, a fixed speed heat pump has little going for it?

[Jeremy Harris]

Our inverter-controlled ASHP pretty much never runs at full power.  90% of the time it's running at its lowest power level, around 400 to 500W input.  It also starts so slowly that you can barely hear it ramping up, unlike the noisy full-power start of a non-inverter model.  If there is a need for high power, I think ours takes around 30 seconds or so to gradually increase speed, which makes the start far less noticeable.  It does much the same when turning off, too, and ramps down gently.  This seems to make quite a big subjective difference to the overall noise impact.

[Onoff]

9 hours ago, ProDave said:

Can I also buck the trend re oil boilers. I have been running an oil boiler directly feeding UFH (and DHW) for 13 years. I don't see this short cycling that people keep mentioning.  Unlike a gas boiler, oil boilers (or at least the one I have) tend to be quite large capacity big steel tanks with a burner down the middle, so in themselves have quite a hig heat store.  What I find is when running with just a low UFH demand, the boiler will fire up for a couple of minutes every 20 minutes or so. I don't see that sort of short cycling as a problem, certainly not one that needs to be "solved" by fitting a buffer tank.

Good to know. I can keep my Bluebird then! Only reason it short cycles at the moment is that everywhere in the house leaks heat like a sieve!