That doesn't necessarily mean their ASHPs are undersized, but if the unit is spending, say 25% of each hour in defrost or rewarming up after defrost, a 10kW unit is currently a 7.5kW unit.
Is this correct, I wonder? I'm getting at the old energy vs power thing again. Isn't the (max available) power (in kW or could for example if it was a car be HP) constant, but if the unit only runs for 75% of the time, then it uses/provides 75% less energy than it would if it ran 100% of the time (in car terms, if you only drive for 45 mins (still at top speed) out of every year, you use 25% less fuel and cover 25% less distance than if you drive for 60 mins of every hour). In the above example, the 10kW heat pump runs at 10kW for 45 mins and so uses/delivers 7.5kWh of heat/energy during that hour. The (potential) power remains the same, it just delivers less heat because it is slacking for 15 mins of the hour. I'm asking because I get so easily confused about all this, and suspect others do as well, and some definitive answers might help.
During the defrost cycle the compressor probably runs at full speed to complete the defrost as quickly as possible.
See the graph below for usage in Oct when the unit shouldn't really be defrosting given outdoor temperatures are 7c+. It's not clear to me why it would come on for a very period and then switch off.
The graph for October would appear to show that your heat pump was running for a period of time and producing more energy than that required to satisfy the heat loss. The heat pump was therefore switching off until the temperature reduced, and then heating back up when heat energy was once more required. It was therefore operating in a similar manner to how a gas boiler would operate. Your system therefore needs to be correctly configured, which we will deal with later, but for the time being it is necessary to ensure that your radiators are operating correctly.
If you have a suitable thermometer, measure and record the temperature of each radiator at the top and the bottom section, in the central area.
Try bleeding any cool radiators.
Each radiator should have a valve at each end, one of which could be a TRV. On the coldest radiator ensure that both valves are fully open. If you have to open the radiator valves then monitor the radiator temperature to see if it gets warmer. If the radiator gets warmer then it would appear that the system requires balancing. I will explain the procedure later or you can Google for a video.
@soniks - the Grant Aerona controller has some basic user level parameters you can check including power consumption in W, flow and return temperatures in degC. They are approximate and rounded.
Check page 51 here. Press and hold the return button for 3s, the display will change to a flashing 00. Use the up and down buttons on the right, scroll up or down to the number corresponding to the information you want then press the tick button. EG: 00 is return temperature, 03 is power consumption, 09 is flow temperature. Temperatures are to nearest 1degC and power is rounded to nearest 100W, which is a bit annoying as the sensors inside the ASHP will be more accurate than that. You'll get an approximation of what's going on. Press and hold return button for 3s to get back to the home screen. You can't mess anything up at this level in the controller, it's only information display level so don't worry about that.
From your last graph showing the unit short cycling - running for a short while then off. What is controlling the ASHP? You mentioned a main lounge thermostat? It is switching the ASHP on and like it's trying to control a boiler? That is how Grant seem to install these things. The master thermostat should be set to be constantly on and calling for heat. Control the room temperatures using:
* weather compensation
* properly balanced radiators (might need a plumber to visit and do this properly throughout the house)
* TRV and only if needed
TRVs should be set higher than the room temperature you want, and the room should control simply by flow temperature and radiator balance. TRVs are there to stop rooms from overheating really, but if the weather compensation is set up right, your flow to the radiators is sufficient, and the rads are properly sized & balanced, the TRVs are just a safeguard or a simple & quick means of turning off heat in unused rooms.
Make sure you're getting good flow right round the whole system, or it's hasn't got a chance in this weather.
@hughf do you know anyone knowledgeable who could do this and would it invalidate any Grant warranty? I think you are right though grant just treat it as a boiler and the controls outside decide when it comes on and off. There is a passiv metering + monitoring installed which also controls when the heat pump comes on.
Off grid on the isle of purbeck
2.4kW solar, 15kWh Seplos Mason, Outback power systems 3kW inverter/charger, solid fuel heating with air/air for shoulder months, 10 acres of heathland/woods.
My wife’s house: 1946 3 bed end of terrace in Somerset, ASHP with rads + UFH, triple glazed, retrofit IWI in troublesome rooms, small rear extension.
See the graph below for usage in Oct when the unit shouldn't really be defrosting given outdoor temperatures are 7c+. It's not clear to me why it would come on for a very period and then switch off.
That looks to me like two things are happening. The large drops in flow are the thermostats turning the ASHP off. This happens at about 7, 9 and 10pm. These drops in flow are too big to be ASHP cycling. The shorter ups and downs in between are cycling because the lowest ASHP output is too high for the target flow. It's cycling at about 3-4 times/hour, which isn't too bad. You might be able to improve by upping the flow temp and limiting the room temp with the thermostat.
There are few if any ASHPs that can achieve their design output (normally at below zero in the UK) without some sort of cycling when it's warm outside (e.g. 12-15 deg).
@soniks a low loss header is a form of buffer, but also hydraulically decouples the primary and secondary sides of the system. They increase the system volume like a buffer tank. In Grant's case they permit lazy installs where installers don't have to bother too much about getting the secondary flow right or balanced properly. The grant one has a 3kW back-up heater, which could be useful in current weather to boost output, help with defrost, or act as a backup heater. I don't use the immersion heater as its expensive with a CoP of 1:1. Low loss headers are questionable at best on simple single zone domestic heating systems and can rob some efficiency & performance out of the system, especially if the secondary side maximum flow rate is higher than the primary.
@allyfish - thanks for clarifying. I think the important point is that marginal units, and even some not so marginal units, will struggle in colder weather, partly because of the defrost cycles, but also because COP goes down and max power output falls as well. This is what happened in my system over the last few days. I posted some figures this morning on actual real world output over the recent cold days. My 'nominal' 14 kW unit actually produces, according to Midea's app, less that 10kW. The key paragraph (slightly edited for clarity):
3. For the last four days, the average ambient temp has been at design temps, ie -1 to -2 degrees. According to the heat loss calcs for my house, the heat loss is 12.4kW, or a total of 297.6kWh over 24 hours (1kW for 1 hour = 1kWh, 12.4kW x 24h = 297.6kWh). To meet this, the heat pump put out an average of 236.75kWh a day (according to the app) over the four day period, which gives a real world power output of 9.86kW (if kWh = kW x h, then kW = kWh / h = 236.75 / 24 = 9.86). If we assume the heat pump was working flat out (which it no doubt was, see next point), the real world power output was significantly lower than Midea's claimed 11.3kW, and far far far lower than Headroom Heat Pump's made up figure of 12.4kW. Conclusion: you can trust neither the sellers not the manufacturers of heat pumps to tell you what the real world power output of your heat pump will be. This fact should be in capital letters on page 1 of any guide etc etc
You can see why I didn't want to get my power (kW) and energy (kWh) units mixed up! Though there is a logic to the way the units are named, in other respects the similarity adds to the confusion.
@cathoderay Yes, understood. My home heat loss estimate for the current weather averaging -1degC outdoor air temperature is 6kW or 144kWh per day. Add on about 10kWh for domestic hot water. Add on 12kWh electricity consumption for everything other than the ASHP, so I will need about 166kWh
My 10kW ASHP is 'just about' coping to hold 18degC indoor temperature at 40degC WC central heating supply, even with the numerous defrost cycles. I've set it as low as I dare and it is a bit trial and error. But I would not be without a log burner as a secondary heat source. The log burner provides comfort boost downstairs mid-afternoon to evening and gets the lounge to a more cosy 21-22degC,
I'll burn about 9kg = 36kWh of hardwood today, about 70% of that, 32kWh, will find it's way into the heated space. The remaining 130kWh will be from the ASHP on 24/7. My ASHP electricity consumption will around 40kWh today. Combined CoP is a little over 3 by my very rough and non-scientific estimate.
The 52kWh electricity cost seems scary right now at 0.362p/kWh = nearly £19 a day. But I'm now WFH, and saving on a daily commute cost, so I'm heating most of the house 24/7 to 18degC, I would never have done that with the old oil boiler. It would have consumed around 60-80kWh (~6-8 litres) in kerosene on days like this, but I would have had a very cold house during the daytime and overnight.
The current very cold spell lingering for 2 weeks or so is exceptional. Now matter what heating system fuel you have you'll be paying high and handsome right now simply to keep warm.
@soniks if some are hot and some aren't, your rads aren't balanced as balancing means they should all be the same temp. Was the system installed during warm weather?
12kW Midea ASHP - 8.4kw solar - 29kWh batteries
262m2 house in Hampshire
Current weather compensation: 47@-2 and 31@17
My current performance can be found - HERE
Heat pump calculator spreadsheet - HERE
@soniks That's useful stuff. Is the 44deC supply typical? The 12degK between your supply and return water temperatures is revealing. That suggests to me that the flow rate isn't high enough through the system. Typically, 5-8degK is more normal. Do you have a visual in-line flow setter / gauge on the system? Grant usually install one on the return to the unit. It's not the most accurate, but will give you an idea. Check the installation manual P.49 showing how to read the flow and, using flow plus supply and return temperatures, estimate the kW performance. (It's not that accurate as all the information from the controller is rounded)
This post was modified 2 years ago 4 times by AllyFish
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