Posted by: @hydros

The heat pump often runs for 10mins or so, then stops for a few minutes and then restarts, this behaviour happens most of the time, and many times per hour, and it often fails to achieve the desired flow rate. Is this normal heat pump behaviour?

Welcome to the forums @hydros. This is definitely not correct heat pump behaviour. The pumps are supposed to run smoothly for long stints. Our ASHP sometimes (very rarely) does this in the late summer when the house is warm, but not quite warm enough. But it's very rare.

At this time of year, your heat pump should not be doing this. Not only is this incredibly inefficient, I have heard that it's not great for the compressor either.

In terms of diagnosis, the only thing I can think of straight off the bat is that your flow through the pipe work and heat exchanger is too low and the heat pump is shutting off prematurely. Our ASHP needs water to be moving through the pipework at 28-30 litres per minute. When it drops to around 20-22 litres or less, the heat pump shuts off to protect itself. In our case, it was a blocked filter that was filling with little bits of corrosion. This took a while to get cleaned up, but for now, touch wood, we're running well. You may want to check your filters. The control panel for your heat pump may also have a metre that measures this. 

I don't think this is caused by an oversized heat pump. That's typically not an issue as heat pumps like to operate in the 25-75% range. Not too slow and not too fast.

It'll be interesting to see what others think.

It's funny when you read someone's post and you say that's me and you realise that your pump hasn't been running properly.

My pump cycled every 6 minutes and I was assured this was the correct operation by the installer.

I also wasn't getting sensible energy generation figures but hadn't even considered a link between the two.

The owner manual gave no information on the two points. But simply stated the controller was not for the owner's use. That didn't stop me.

Over time I did try and get to know the controller and one option I noticed for energy usage and generation, was the term "initialisation". The term itself suggested that it initialised energy monitoring. Eventually, I bit the bullet and pressed the initialised button hoping the energy usage started to make sense and since then I have had sensible energy usage and generation. I have also noticed a change in how the pump works. The pump now comes on for longer periods and appears more efficient. 

I am not suggesting that this is HydroS problem but.....

Posted by: @hydros

Hi

First proper post on the forum. Hope it's OK to dive straight in.

I have a Daikin Altherma 11kw monobloc, installed in Jan this year. First attempt by the an installer to put the pump in was shocking, after lots of to and fro, another installer was bought in and the system installed well this summer, at least on the surface in terms of quality of workmanship, correctly sized fittings, quality insulation, etc. but the system still isn't performing as I would expect it too. SCOP since corrected is about 2.8 but the system cycles on/off a lot, 6-7 times per hour. The heat pump often runs for 10mins or so, then stops for a few minutes and then restarts, this behaviour happens most of the time, and many times per hour, and it often fails to achieve the desired flow rate. Is this normal heat pump behaviour? I thought it was supposed to get up to it's desired temperature and then maintain that temperature, in the case of my setup, until the thermostat tells it to turn off.

I can give much more detail but this will probably do as an introduction. My initial thought is the heat pump is oversized and can't modulate to a low enough output to maintain a steady rate. The heat loss report from the installers suggested a heat demand of 20000kwh but my gas usage before was never more than 13000kwh. 

Hi HydroS,

Welcome to the forum.

My first question would be what is the operating mode set on your controller? Is it operating in weather compensation mode or water flow temperature mode, and if the later, what is the water flow temperature setpoint? If the water flow temperature is set at 50C, but the required water flow temperature is say 35C, then it would be like driving a car up to 50mph and then having to stop for a while because you need an average speed of 35mph.

Alternatively, as Mars suggested it could be reduced water flow through the heat pump.

It could also be different elements of your control system fighting with each other for control of the system.

Please provide more information about your system.

Type and size of property.

Type and size of heat emitters.

Heat loss calculations.

Type and size of hot water cylinder.

Type and size of buffer tank, if fitted.

Type of control equipment, Daikin controller, thermostats, TRV's etc.

"The heat pump often runs for 10mins or so, then stops for a few minutes and then restarts, this behaviour happens most of the time, and many times per hour, and it often fails to achieve the desired flow rate. Is this normal heat pump behaviour? I thought it was supposed to get up to it's desired temperature and then maintain that temperature, in the case of my setup, until the thermostat tells it to turn off."

@Hydros,

What do you mean here? Do you mean the ASHP stops before it reaches its flow temperature? Or the thermostat room temperature? Or do you really mean flow rate, although I'm not sure how you would know that. If it's flow temperature then there's something wrong becasue it shouldn't do that. If it's room temperature and it's your thermostats causing it to cycle then that can be fixed by changing settings. 

As @editor says, ASHPs can work at a range of outputs.  My ASHP can output between 4 and 16kW.  It cycles at about 3 times per hour when it's 8C or warmer outside. It may be a little inefficient compared to a smaller ASHP at these temperatures but it's using so little power then that it's not an issue.

This is what cycling looks like.  As the temperature outside drops the flow temp goes up and the cycling stops.  BTW my flow temps are actually a bit higher than that.

If you answer Derek's questions I'm sure he will be able to help.

Hi @derek-m and others for the feedback.

Type and size of property. Detached chalet-style bungalow, extended in 2014. 175sqm

Type and size of heat emitters. Rads, a mix of P+ (smaller rooms) and K2 (bigger rooms)

Heat loss calculations. How much detail do you want here? My EPC states 68 kWh/m2, 12000watts. The ASHP designers had it at 22,000 which I think is complete nonsense, I only spotted this detail recently. 

Type and size of hot water cylinder. Daikin unvented cylinder 300l

Type and size of buffer tank, if fitted. No buffer tank. a 3kw backup heater and a small inline 18ltr tank of the return to the HP, I don't think this is a buffer, I'm pretty sure it's something to do with water volume

Type of control equipment, Daikin controller, thermostats, TRV's etc. Daikin Controller (EBLQ011CAV3), Honeywell Home T3 thermostat and TRV's throughout, except towel rail in bathroom (now all TRV's fully opened)

It is setup with a weather compensation curve to adjust the heat pump flow temperature 50C@-3C, 35C@18C but pump operation is controlled on/off by the thermostat. Unfortunately I can't get historic data to graph out of the system, only the information through instantaneous views on the Daikin controller. I can also see the flow rate, this usually starts around 19-20 l/min and drops down a bit after it's been running for a while. I did rebalance the radiators in October which improved the cycling from 9times/hour to 6times/hour now.

@hydros 

Hi HydroS,

Where is your Daikin controller located? Is it in a room or the airing cupboard?

Dependent upon where your controller is located, the most efficient way to control your heat pump would be to use Leaving Water Temperature Modulation.

The next best way is to use weather compensation, but this will require the curve to be optimised. Do you know how to change the weather compensation parameters?

Does the controller show the leaving water temperature when the heat pump is running? What is the value?

Try increasing the thermostat setpoint by 1C and see if the room temperatures increase by 1C. If this is the case then the weather compensation curve will need to be adjusted.

Hi Derek-M

Thanks for taking an interest. The controller is in the utility room and easy to access. I'm not sure I know what the difference is between Leaving Water Temperature Modulation and weather compensation, but I do know how to change the weather compensation parameters and do know my way around the controller but haven't touched any settings other than DHW schedule.

The controller does display the desired leaving water temperature, currently 40C, whilst outside is 8C outside. It falls back to 25C when the thermostat isn't demanding heat, but only does this when outside temperature is below 10C.

The thermostat is currently set at 20C through the day and its been on and off a few times today but nowhere near as frequently as the stopping/starting of the pump, I'm not 100% sure but I don't think the two are related. If I raise the thermostat by a degree the pump will run for longer and the room temperature will gradually increase. 

Posted by: @hydros

Hi Derek-M

Thanks for taking an interest. The controller is in the utility room and easy to access. I'm not sure I know what the difference is between Leaving Water Temperature Modulation and weather compensation, but I do know how to change the weather compensation parameters and do know my way around the controller but haven't touched any settings other than DHW schedule.

The controller does display the desired leaving water temperature, currently 40C, whilst outside is 8C outside. It falls back to 25C when the thermostat isn't demanding heat, but only does this when outside temperature is below 10C.

The thermostat is currently set at 20C through the day and its been on and off a few times today but nowhere near as frequently as the stopping/starting of the pump, I'm not 100% sure but I don't think the two are related. If I raise the thermostat by a degree the pump will run for longer and the room temperature will gradually increase. 

Hi HydroS,

Weather compensation is where the controller measures the outside air temperature, and then sets the required water flow temperature in relation to the slope of the weather compensation curve. As the outside air temperature reduces, the water flow temperature is increased, and when correctly optimised the indoor air temperature remains fairly constant.

Leaving water temperature modulation is where the controller also measures the indoor air temperature, so if the weather compensation isn't fully optimised, or other effects such as solar gain or human activity cause the indoor air temperature to deviate from the desired, the controller will vary the required water flow temperature to bring the indoor temperature back to the desired value. It therefore provides more accurate control of the indoor air temperature, provided that the controller is receiving a temperature measurement from the room whose temperature is to be controlled.

If, when you increase the setting of your thermostat, the indoor air temperature increases, then the weather compensation curve is not correctly adjusted. The thermostat should not switch the heating on and off, but should be set at a value 1C or 2C above the desired indoor air temperature, to limit any temperature increase caused by solar gain or human activity.

When you refer to pump, I assume you mean a water pump inside your home. There is also a water pump inside the ASHP. Normally both water pumps would be run at the same time, but may be wired differently on your system.

I would suggest that you read through the manual to identify which parameters may need to be adjusted. You could initially change the 35C @ 18C to 25C @ 18C, and then try increasing the thermostat again. When the weather compensation is correctly adjusted, there should be no increase in indoor temperature when the thermostat setting is increased.

Any TRV's on downstairs radiators should be fully opened, to allow weather compensation to control the indoor air temperature.

Hi HydroS,

I agree, it could be the weather compensation is set too high.  My Vaillant has been operational for a couple of weeks and it was far too hot, 23/24 Deg C throughout the house.  I couldn't get the temperature down with the room temp controller and was advised to set the controller to switch the pump off when above a set point, like the operation of your thermostat.

After laying awake in the early hours considering this it just didn't seem right that I had to use the big off switch to stop the pump - after all the compensation is supposed to be the most efficient way to run ASHP's open the valves and leave it too it 24/7.

It turns out the compensation was set to a gas boiler at 1.2.  ASHP should be 0.6.  Each decrease lowers the flow for a given outdoor temp.  I set it to 0.4 to see if it copes with a low flow temp.  Following that it now controls lovely, no overshooting and no need for the big off switch.  

It may simply be running too hot and needs winding down to a lower flow temp.

Good luck.

@hydros 

Hi HydroS,

Did you manage to resolve your problem? Feedback is always useful.

Hi @derek-m

I've been out most of the day but did make adjustments to the system as you suggested. Leaving water temperature is now 39C with outside temperature of 5C so it's running cooler, house is still warm enough but the thermostat has still tripped off, though not as regularly as before I think, but as I mentioned I've been out a lot of the day. I'll keep an eye on it through the rest of the afternoon/evening. 

I did notice this morning that it was still cycling on/off a bit but maybe less. I'm also going to measure all the rad temperatures again to check the balancing isn't off.