Hi, I have posted this elsewhere, but not had any help and wonder if anyone on this forum has any advice or ideas on how I should proceed.
My 12kW Gen 6 ASHP normally will heat my domestic hot water from 20 deg C to 53 deg C in about 30 minutes and uses around 1.5kWh. Recently, it has been intermittently struggling to do so with the compressor stopping twice during the process and using much more energy. The two figures from PassivLiving illustrate the change in behaviour. On 8th November, the flow temperature rises steadily to 63 deg C. The energy output which is recorded over 2 minute intervals runs at around 0.265 kWh (ie power of nearly 8kW). By contrast, on 9th November, the flow temperature rises more slowly and the energy output after a brief peak drops to the same level as consumption. Both then drop to zero as the compressor stops for a short period. By the end of the process consumption is greater than energy output. The flow rate is 19L/min, which is slightly lower than when space heating (22L/min). I have had no problems with space heating, so I wonder if it is something to do with the DHW cylinder which is a Telford Tempest Heat Pump 170 L tank.
Hi, I have posted this elsewhere, but not had any help and wonder if anyone on this forum has any advice or ideas on how I should proceed.
My 12kW Gen 6 ASHP normally will heat my domestic hot water from 20 deg C to 53 deg C in about 30 minutes and uses around 1.5kWh. Recently, it has been intermittently struggling to do so with the compressor stopping twice during the process and using much more energy. The two figures from PassivLiving illustrate the change in behaviour. On 8th November, the flow temperature rises steadily to 63 deg C. The energy output which is recorded over 2 minute intervals runs at around 0.265 kWh (ie power of nearly 8kW). By contrast, on 9th November, the flow temperature rises more slowly and the energy output after a brief peak drops to the same level as consumption. Both then drop to zero as the compressor stops for a short period. By the end of the process consumption is greater than energy output. The flow rate is 19L/min, which is slightly lower than when space heating (22L/min). I have had no problems with space heating, so I wonder if it is something to do with the DHW cylinder which is a Telford Tempest Heat Pump 170 L tank.
The fact that the heat pump is stopping would possibly indicate a problem with the heat pump itself. It is a pity you don't have the RWT on the chart.
The fact that the heat pump is stopping would possibly indicate a problem with the heat pump itself. It is a pity you don't have the RWT on the chart.
Do you have a similar chart for CH mode?
Thanks @derek-m. The reassuring thing is that space heating is working fine as can be seen from the figure below. It is just DHW heating that has been affected and initially this was intermittent. Here is the chart for space heating. Please ignore the fact that the flow temperature is labelled incorrectly as hot water. This is due to incorrect wiring by the installer (G L).
So your heat pump works fine at the moment when operating in CH mode, but has started to struggle when in DHW mode.
I would suggest that you start by checking all the settings within your controller, particularly the limits.
Is the external air temperature sensor showing correct readings, it seems to be indicating approximately the same temperature on all the charts?
The chart in question, appears to indicate that the compressor is being stopped for a period of time, because some limit may have been reached when the heat pump is working near maximum capacity. Since the weather is getting colder, the heat pump will need to work even harder when heating DHW.
Check that the water pumps are operating correctly and that there are no blockages in any filters.
My 12kW Gen 6 ASHP normally will heat my domestic hot water from 20 deg C to 53 deg C in about 30 minutes and uses around 1.5kWh. Recently, it has been intermittently struggling to do so with the compressor stopping twice during the process and using much more energy.
I have the 16kW version the Gen 6. I do see the same behavior, intermittently [the last occurrence I can find was on the 1st Nov]. I am still trying to get to the bottom of it. As you mention, the cut out normally follows a period of very low COP it also follows a very low deltaT and high compressor frequency.
you might have already been down this road but never hurts to ask the obvious: have you checked the FSV settings for the timers for DHW vs CH, which wins when and for how long?
I'm in the first winter, my DHW is on a run-overnight timer, I'm experimenting with a lower CH setback, and its getting colder, meaning that CH is now for the first time competing with DHW for the heat pump's attention at the same time during the night. I saw a behaviour similar to yours and traced it to FSV 3025 not being high enough. the default is 30 mins. which allows a CH demand to interrupt a DHW run after that time. Its now set to the maximum of 95 mins which is needed if my 300L cylinder is fully cold.
If the cylinder is already partially warmed, particularly at the bottom, energy input and output being the same for a substantial period is not inconceivable on a DHW run. It needs the return to be suitably cool for the ASHP to "see" a DT, and thus a meaningful energy output to occur. If DT is really low then energy output may even be technically negative. I suspect (I don't know, I'm not a HP designer!) that the HP may have logic that reacts to this lack of DT by shutting off the compressor as well. as derek says, graphing RWT as well is really useful.
I have the 16kW version the Gen 6. I do see the same behavior, intermittently [the last occurrence I can find was on the 1st Nov]. I am still trying to get to the bottom of it. As you mention, the cut out normally follows a period of very low COP it also follows a very low deltaT and high compressor frequency.
@william1066 Thanks for sharing this. It is somewhat reassuring to know that I am not alone in experiencing this. The main difference between it working normally and abnormally is the lower Delta T, as you point out. On the last occasion, which wasn't severe enough to cause the compressor to stop, the flow rate was normal, but Delta T was around 4, rather than 6. The rate of rise of LWT was noticeably slower (T2) compared with the normal behaviour (T1) and although I don't know why this should occur, it may explain the reduced Delta T.
I saw a behaviour similar to yours and traced it to FSV 3025 not being high enough. the default is 30 mins. which allows a CH demand to interrupt a DHW run after that time. Its now set to the maximum of 95 mins which is needed if my 300L cylinder is fully cold.
@iancalderbank Thanks for your input. I don't set the DHW to heat during space heating times, so I don't think this is the cause in my case.
It needs the return to be suitably cool for the ASHP to "see" a DT, and thus a meaningful energy output to occur. If DT is really low then energy output may even be technically negative.
This may well be part of the issue as it has certainly seemed worse when the Solar iBoost had already warmed the DHW.
This may well be part of the issue as it has certainly seemed worse when the Solar iBoost had already warmed the DHW.
agree. at this time of year when there will not be enough PV surplus to properly heat the cylinder, for exactly this reason, I avoid having the PV surplus go the cylinder at all. It either goes to the house's battery or the car. If its in the house battery, it's re-usable by the ASHP later of course. A cylinder with the lower part warmed to high 30's / low 40's by PV immersion, completely screws with the DT and COP when doing a reheat of that cylinder by the ASHP. It also isn't hot enough to be useful hot water, so I've come to the conclusion it's to be avoided.
If you can work out the timings to fit with your own usage , I am sure its better let the cylinder go as cold as you can get away with . then do a full reheat to your desired target temp (e.g. 50-55) using the ASHP. I've been able to achieve a COP over the full reheat cycle of 3 , with a target temp of 55 , at OAT of 7-9C , by doing this, from cylinder temps of 35Ctop, 15C middle and 15C bottom.
if you can make the timings work, you could then "top it up" after the ASHP run, using immersion/PV , to get even higher than the cylinder temp that you get out of the ASHP, and thus have more usable hot water.
