Old ASHP cycling - lack of modulation?

QrizB

Not a great solution, but adding a buffer tank would increase the system volume and should lengthen the cycle periods.

michaels

QrizB said:

Not a great solution, but adding a buffer tank would increase the system volume and should lengthen the cycle periods.

It isn't supposed to be a great solution, just something cobbled together from some kit we got given free after the first user upgraded. We have v2h so can potentially run everything on 8p per unit electricity - our gas boiler has stopped working and we don't want to spend a fortune on it as we are planning to go to a 'proper' heat pump solution if and when Octopus can ever get their act together.

Netexporter

So the 13kw is based on a 2.59kw compressor and a cop of 4.8(!).  Our gas modulating gas boiler is 19kw

My old boiler (just taken away by the scrap man) was 19kW. I've been specified an 8kW ASHP to replace it, which suggests that your heat pump is grossly oversized, which would seem to be born out by the frequent cycling.

Strummer22

Netexporter said:

So the 13kw is based on a 2.59kw compressor and a cop of 4.8(!).  Our gas modulating gas boiler is 19kw

My old boiler (just taken away by the scrap man) was 19kW. I've been specified an 8kW ASHP to replace it, which suggests that your heat pump is grossly oversized, which would seem to be born out by the frequent cycling.

The heat pump needs to be big enough to heat the house when it's well below freezing outside. If it can't modulate down low enough in milder weather that's when you get the frequent cycling. 

@michaels is the trigger for the heat pump turning off that the flow (out or return) reaches a certain temperature? You may have the option in the heat pump controls to increase the temperature difference (dT) between flow temp and when the compressor starts.

It might be something like flow set point: 40°C, dT 3°C. The compressor would restart when the flow temp drops to 37°C. Increase this to 5°C and the heat pump will turn off for longer, and then run for longer to get the flow temp back up to the set point. 

matelodave

You appear to have two problems. Probably an oversized heat pump and a heating system that cannot dissipate the heat it produces especially if you've just hung the heatpump on in place of the old boiler. 

If the rads aren't big enough and the pipework isn't optimised there may not be enough water in the system to allow the heatpump to operate efficiently.  You need a decent flow rate and TBH TRVs don't really work everso well with a heatpump as they restrict the flow rate - you need a bypass valve or radiator to ensure sufficient flow for when they shut off.

It's hard to tell whether its inverter driven - if not then its hard to see how it can modulate. Looking at the circuit diagram and internal piccy with contactors and big capacitors it looks like a capacitor start which usually run at a single speed unless its got some clever controls.

Inverter driven compressors can control the compressor speed for modulation.

Have you got weather compensation enabled to get it to track the outside temperature

Using an external (non heatpump controller) is probably just shutting it on and off which causes the unit to go full blast every time it cycles on.

Mine cycles a lot more when it's being controlled by the room stat and is more efficient and cycle less when the weather compensation is active.  Likewise the fans and compressor are inverter driven and seldom go full blast except when the room stat turns the unit on and off. , they then slow down and cruise or even idle as the unit modulates

Reed_Richards

michaels said:

Problem seems to be that current temps we need about 4kw heat but the heat pump when on produces probably 7+kw so only needs to run about half the time but rather than say running 15 min on, 15 min off, is instead running 3-4 min on, 3-4 min off.

So get your "smart things hub" to stop that!  If it cannot limit the number of cycles per hour then it's not really very smart at all.  Because you can get a very cheap wireless room thermostat that can do that, this one for example: https://www.amazon.co.uk/EPH-Digital-programmable-room-Thermostat/dp/B07BB1SDTF?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&psc=1&smid=A11Z7MQ0ORIANE

michaels

Typed a long response that seems to have been lost.

Will try and think how I could be cleverer using the smart hub routines. it should be noted that the heat pump itself won't talk to anything else.  on the smart hub there is also an option of doing something in the cloud with more formal programming languages but a steep learning curve to use this functionality.

First thing to work out though is why the unit is drawing 4.5kw ramping to 5.5kw rather than the rated 2.6kw.  I think the defrost heater may be running full time, possibly a sensor or programming issue.  Any suggestions based on the manual P31 and 33?  Thanks
Grant Products HPAW85 user manual : Free Download, Borrow, and Streaming : Internet Archive

Reed_Richards

Sorry, it's a long manual and I haven't read it.  But some heat pumps start at low power and ramp themselves up as necessary, some start at high power and ramp themselves down as necessary and probably some start at medium power then decide what to do.  Your heat pump is on for such a short time it probably hasn't made up its mind about what output power it should be aiming for before it overheats the water and turns itself off.        

michaels

Reed_Richards said:

Sorry, it's a long manual and I haven't read it.  But some heat pumps start at low power and ramp themselves up as necessary, some start at high power and ramp themselves down as necessary and probably some start at medium power then decide what to do.  Your heat pump is on for such a short time it probably hasn't made up its mind about what output power it should be aiming for before it overheats the water and turns itself off.        

No problem.  Initial run is about 30 mins starting at 4.5kw and ramping up to 5.3 I guess as it heats all the water in the circuit, it is after that that it starts to cycle.

Reed_Richards

The only feedback your heat pump has to go on is the return water temperature.  It will need to draw more power as that heats up and the efficiency drops as a result.  But if it were capable of backing off that ought to happen as the return water temperature gets too high or the leaving water temperature starts to exceed whatever the target value is.  On the other hand your heat pump has no direct knowledge of the room temperature so that could take it by surprise and if the allowed hysteresis is too low could cause the short cycles.

I don't know if this would work but once it is cycling you could increase the set room temperature by a few degrees.  As the room warms up more that will reduce the efficiency of your radiators so could cause the heat pump to cycle even faster,  But if it stops cycling for a bit then the problem is your room thermostat.  I think this makes sense but there could be issues I'm not thinking of - in which case hopefully someone else will chime in and tell me I'm wrong.