Posted by: @fjj

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Knowledgable forum members,

I have a Grant Aerona 3 10Kw ASHP set to provide hot water at 55C and a SolarIboost connected to the immersion heater set to heat to 58C. I have a question about the ASHP operation in hot water mode. I have set the hot water to come on for 1 hour using the thermostat, with set flow temperature of 55C in line with the standard Grant settings.

I can see from my Iboost that the water in the cylinder is at at least 58C today (as the Iboost records the tank as hot, ie does not differ solar to heat the water) but it seems the Grant does not record/measure the temperature of the water in the cylinder but heats the water/raises the flow temperature, regardless, for the set period of time. Is my observation correct? If so, is there a more intelligent way to ensure the water is only heated by the heatpump when required, other than manually turning DHW off and on? Or is there a setting I have missed that can link DHW production to cylinder temperature? 

Many thanks in advance. 

 

 

Try lowering the heat pump DHW setting to 50C, and raising the iBoost setting to 60C.

@FJJ, I have the same set up. Grant 10kW Aerona, pre-plumb 250l cylinder, iBoost. The iBoost just runs on the 3kW immersion heater, which I don’t think has an adjustable thermostat. Mine tends to cut out at an indicated 53-54degC on the Grant stat display, with the iBoost display saying ‘HOT’ indicating immersion thermostat demand is satisfied. I have the Grant HW flow temp set to 55degC, tank cylinder set to 50degC. I don’t think either the immersion stat or the Grant digital display stat are very accurate. That said, I’ve never run out of hot water yet, even when the tank cylinder is showing as low as 12degC useable hot water at 40degC is delivered. 

@allyfish thanks for that. The immersion thermostat can be adjusted but has wiring in thats live so that’s only for the confident (ie not me). I had not realised there is an additional thermostat in one of the pockets of the cylinder that the heatpump uses. The position on different parts of the tank and inaccuracy in measurement might explain things. I have - as always - followed instructions by Derek and reduced pump dhw temperature to 52 this will hopefully create the necessary gap between the two while keeping my water nice and warm.

@fjj 52degC is a good thermostat setting, the flow from the ASHP needs to be 3-5degC higher than the tank cylinder setting thermostat or you're just burning electricity for no benefit. For that reason I have the cylinder stat at 50degC and the ASHP HW flow at 55degC. The Grant will go up to 60degC flow temperature, but the CoP will be very low, and the refrigerant compressor discharge  pressures & temperatures very high. Above 50degC the iBoost takes over to raise the tank temperature 'for free'.

One annoying thing is Grant using dumb cylinder timers and thermostats which don't communicate with the Chofu ASHP controller unit. In winter the water just about gets to design flow temperature or the cylinder almost to thermostat set point and then the ASHP goes into a defrost cycle. All the while the 1hr boost timer is running down on the HW charge time period. On occasion that has caused my HW not to charge up properly. It's very poor control. I don't know if the Chofu controller would have better intelligence, to delay the defrost cycle if the hot water boost is nearing completion. The Chofu controller has some very advanced features for HW control, but Grant doesn't use them, preferring to use a simple external cylinder thermostat and an external digital timer. Fired boilers don't go into defrost however. They need to seriously rethink how they control their ASHPs for better efficiency of heating and hot water.

Another weakness is, if the HW is charging from quite warm, the HW cylinder can actually lose heat into the ASHP water heating coil, as the ASHP is still warming up the volume of water in the HW charging loop. Wasteful of energy as the HW tank initially cools a few degrees when it is supposed to be heating. The loop should go into bypass until the supply temperature from the ASHP is at least 5degC higher than then cylinder thermostat temperature, to ensure heat energy is added into the HW tank, and not removed from it. Again, this is another shortfall of Grant using a Y plan system circuit design designed and intended for fired boilers, where the delivery temperature is high very quickly, and using it for ASHP systems where the heat source takes much longer to get to a temperature higher than the HW temperature in the cylinder.

Grant's pre-plumb HW cylinders and system designs are old school 1980/90s Danfoss Y plan system boiler designs intended for fired boilers. They've put larger coils in the HW cylinders to adapt them for solar thermal and ASHP lower capacity heat sources [compared to fired boilers] but totally disregarded changing their system design from designs intended for fired boilers. They are far from optimal for ASHPs and renewable heat sources.

@allyfish you are absolutely right. I had the same thought, that during its HW cycle it seems to lower the cylinder temperature before raising it. In mild weather it takes the ASHP 45 minutes to reach 55 flow from around a 34 flow starting point so it then only uses 15 minutes to add hot water to the tank. (Still in combination with the Iboost this is sufficient) 

So I think I need to increase my flow temperature back up to 55 and get onto the cylinder thermostat to have a play with that and check this is set to 50. Glad I am not the only one, still learning every day…the cylinder thermostat should be in a pocket on my cylinder I assume? Back to my loft I go. 

It is possible to install a thermistor (10K, beta=3435, 1%)  in the DHW cylinder so the tank temperature can be read by the ASHP.  However this doesn't help unless you're prepared to completely remove the Grant controls and 'go native' with the ASHP running its own room thermostat and DHW controls.  The reason is that the ASHP's inputs from the Grant relay box are 'on/off' and 'DHW mode now please'.  In this setup the ASHP is entirely at the mercy of the external programmer, ie your traditional on/off boiler controller and the tank stat.  This overrides the ASHP's internal DHW temperature controls.  The result is your external programmer is telling your ASHP to heat water, but the tank stat is inhibiting it from running as it's up to temp.  Result is the house gets cold because the ASHP is in hot water mode and not doing heating. Or the timer runs out before the water is fully hot.

If you do the divorce there are three modes. which work slightly unusually. There are three configurable temperature settings - 'eco', 'comfort' and 'boost'.  First of all there's the legionella cycle - go up to the boost temp (55C usually) and then kick in the immersion (which the ASHP will control - does it all on a weekly cycle).  It's also possible to force boost mode manually.  Or you can select, 'off', 'eco' or 'comfort'.  In these modes the settings decide whether hot water or heating has priority.  Hot water is the default, in which case it'll heat up to the current eco or comfort temp and then straightaway switch back to heating.  Effectively it'll opportunistically get a bit of hot water heating only when it's needed.

The wall unit timer looks like a digital version of a 24h/7d mechanical timer with on/off periods in 15 minutes, except there's a surprise.  In timer mode the hot water is never off.  When the timer is 'on', the hot water is in comfort mode.  When it's off, the hot water is still enabled but in 'eco' mode. This is actually sensible because it's only opportunistically heating, so it's no big deal to kick in for a few minutes to keep it at temp.

However, inititally I had eco=40C and comfort=44C and what would happen is it would heat up to 44C on timer, but after the timer period finished it would fall back to keeping the water at 40C, resulting in it turning on randomly during the day and night (lost about 6C overnight which was enough to trigger it, and I didn't want it running at night).  Now I have eco=15C and comfort=44C, with the result that it only fires according to the timer periods because the water never gets down to 15C.  44C is good enough for a warm shower and more efficient than a higher temp.

Doing 'the divorce' is not hard: I fitted 4 relays and 2 tank thermistors (another for the buffer, although it turns out I'm not using it), plus some MCBs, a contactor and kWh meter for the immersion. However I wanted to do a proper job so there was a bit of work packaging it all up nicely (I also needed it to be ultra quiet so parts were chosen carefully).  I'm gradually documenting this, which is another task in itself - encouragement and motivation to do it is always appreciated! 

I will say it's transformed the ASHP into a quiet and competently purring unit, from a noisy and awkward thing that it was installed according to the Grant instructions.

@prunus thanks very much. I remember your extensive description of how you managed to optimise the native chofu controls in an earlier post. I am glad you managed to optimise the DHW as well! A bridge too far for me at the moment but I will keep this in mind for the future. For now I will monitor how this goes.

@prunus - I have just had a new Grant aerona system installed and very disappointed with it for this same reason.  Your fix sounds exactly what I am looking for. Would you be able to share more details please?