@porthy, do you think your buffer tank is in the HW cylinder?

An intermediate update.  A Grant U.K. technician has now spoken with the installer.  On Monday the installer is sending an electrician to alter the parameters and possibly some wiring.  Here’s hoping to hot water when needed and the heating is on as required?  It will mean that the weather compensation is switched off though, so we’ll need to keep a weathered eye on costs!

Posted by: @porthy

@editor Time for an update!  

Oh dear, still frustrated this end.  Relatively pleased with the warmth in house, but the DHW’s heat continues to disappoint.  I’ve been in touch with Grant in the last few days and they have been very helpful with advice, so I’ve been experimenting.  

Overnight I ensured the central heating did not trigger between 4.30 a.m. and 6 a.m. when I had timed the hot water generation to be on (Grant’s recommendation). THE DHW is also set for hourly boosts late morning and tea time.  

At 10.30 p.m. last evening I had the hall stat drop from 20 to 18 degrees and then at 4.20 a.m. the hall stat was set to reduce to 15 degrees to ensure the central heating pump did not kick in.  The DHW had 1.5 hours to heat up.

At 6 a.m. the hall stat was timed to call for heat at 20 degrees again.  Immediately the rads were really hot, but this changed to a warmth quite speedily.  By 6.30 a.m. the DHW was luke warm, so I needed to put the DHW stat on to heat up the DHW (around 7 a.m. my wife enjoyed a hot shower - hooray) and I switched the central heating stat off.

I understand that you can’t have the DWH and central heating working at the same time, but the DHW seems to be robbed of its heat when the CH is switched on.  It would seem that the CH pump circulates water through the cylinder, but consequently the temperature of the DHW rapidly depletes.

Apologies for being thick, but does the ASHP not build up its store of hot DHW and then when the CH is timed to call for the specific heat you require, pump this heat directly from the ASHP around the CH system, leaving the DWH to sit in the cylinder until required? Clearly, later in the day, should the temperature of the cylinder drop, the cylinder stat would call for heat and as it has priority the CH would be turned off for a while, whilst the heat of the DHW is replenished.

Any thoughts would be appreciated.  THANKS.

Hi Porthy,

I have just noticed your posts.

It sounds like you may have a faulty/sticking valve problem. A heat pump is not designed to heat DHW and your home at the same time for two reasons. The first being that in most instances the DHW needs to be heated to a higher temperature than that for CH, and since the heat pump itself cannot produce water at two different temperatures at the same time, it would require additional equipment to cool the water supplied for CH. The second reason is that it would be highly inefficient and cost an arm and a leg to run.

For those reasons you should find that a 3-way diverter valve has been installed, or conversely two 2-way valves. If a 3-way valve is installed, the warm water coming from your heat pump will be connected to the single inlet port, and one of the two outlet ports will feed the warm water through the heating coil within the DHW cylinder, whilst the other outlet port will feed the warm water to the heat emitters. The 3-way valve should divert the water flow one way or the other, but not in both directions at the same time. If the valve should get stuck in the half way position, then it would be possible for this to happen.

In a system with two 2-way valves installed, one valve would be open when the other valve is closed. When the system changes over from DHW to CH, the valves should also change over, the open one closing and the closed one opening. If one of the valves gets stuck in the open position the system could again be trying to supply DHW and CH at the same time.

If you trace the pipework back from your DHW cylinder heating coil, you should locate either the 3-way valve or one of the 2-way valves. On the side of the actuator, attached to the valve, there should be a lever for manual operation. Try moving this lever back and forth. If the lever moves freely, this would indicate that the valve is open, if resistance is felt when moving the lever, and it returns to the start position when the lever is released, then the valve is in the closed position, and is being manually opened by moving the lever.

@derek-m Thanks for your thoughts.  When the installer returns on Monday I’ll suggest this. Best wishes, Chris.

@editor Hi - an interim update.  We had the installer visit us twice last week and the chap spoke lots to Grant technical over the phone.  The upshot is parameters were changed, wires altered and the weather compensation switched off.  

The advice was to keep the DHW call on 24/7.  This resulted in hot (and noisier) rads and a happy wife who emerged from her morning shower like a lobster.  The call for space heating was set (on the hallway stat) to 19 degrees centigrade during the day (appears fine for our ageing selves) setting back to 18 degrees centigrade between 21.40 and 06.00 the next morning.

The result has been effective, but electricity use has increased.  Thankfully the Myenergi Eddi is boosting the heat in the thermal store, now the days are getting longer & Cornwall has enjoyed sunnier days of late.

I’m now experimenting with the DHW call on the ASHP being on from 04.30 until 09.00, an hour boost at lunchtime and then coming on for cosy evenings between 18.00 and then going off at 21.40.

When the call for DHW is switched off, via the timer, the call for space heating is set to 19 degrees and the in the loft CH pump is coming on, but the radiators are providing the lower warmth heat to the radiators as opposed to the hot touch heat when the thermal store is calling for heat from the ASHP.

My next experiment will involve not having the lunchtime hot water demand come on and see what difference that will make to electricity consumption and efficiency.

I’m not saying I’m ultra confident that I’ve got everything in order yet, but I’m feeling more satisfied. 

I guess the shock of increased electricity usage & costs (thank goodness I’m locked in to 21p per kilowatt until September 2023) even after offsetting the oil we would have used for space heating and DHW, is greatly more than we paid before the ASHP was installed.  Roll on the warm summer days!  Yesterday, which was a really sunny day, meant we consumed 37.2 kWh, with a grid import of 25.9 kWh - thanks Zappi!  

All the best, Chris.

Hi all,

Related issue here...

As soon as we run any hot water at all the tank temperature drops massively, which triggers the hot water heating function (even when the tank is clearly full of hot water).

Any ideas? Maybe the tank thermometer is right next to the cold water intake, but why would anyone design a tank like that?!

Cheers in advance!

Posted by: @edmj

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Hi all,

Related issue here...

As soon as we run any hot water at all the tank temperature drops massively, which triggers the hot water heating function (even when the tank is clearly full of hot water).

Any ideas? Maybe the tank thermometer is right next to the cold water intake, but why would anyone design a tank like that?!

Cheers in advance!

You could be correct that the temperature sensor is badly positioned. There may be pockets higher up the tank to which the temperature sensor could be moved, though it may not be best to move the sensor too high if you use lots of hot water.

@derek-m thank you for this. We hardly use any hot water at present, as our shower is electric and we have a dishwasher. All the more annoying when the tank reheats because someone washes their hands!

I have multiple options for DHW production with my Daikin HT system:

1. Automatic storage operation - This has a binary setting for either daytime or night-time automatic storage (or both), and the option to set the start time, and the target temp. Heating continues until the user-set DHW temperature is achieved;
2. Manual storage operation – User-set timings for the system to produce DHW to a specified temperature;
3. Reheat – this has settings to allow a minimum and maximum reheat DHW temp and kicks in if the DHW temp falls to the minimum temp and continues until the maximum reheat temp is achieved;
4. Weather dependent DHW – according to the manual ‘If enabled, the storage set point will be set weather dependent. In case of a higher ambient temperature (e.g. during summertime), temperature of supply water to the domestic hot water tank will be higher too, so that storage temperature set point can be set lower in order to keep the overall equivalent hot water amount the same during the whole year. Therefore it is recommended to use this function’. In this mode the ‘storage set point’ is over-ridden and the controller decides – presumably in a similar way to using the WCC for space heating.

At installation, we were recommended to use reheat and were told it was the most efficient setting. We ignored that advice and played around with various settings. We have solar panels producing lots of lovely free electricity (28 kwh on Saturday, although averaging 17 kwh for the last 30 days – we do not have a solar diverter. As such, we decided to use a combination of (1) and (2) – we have the automatic storage (1) kick in at 1pm to take advantage of the peak in solar production and it usually takes less than an hour to get to temp (50 C), then we also have a manual storage cycle (2) set to start at 10pm which runs until the DHW gets back to temp. We have a 500L thermal store, two people taking showers between 7 and 8am and our son taking a shower or occasional bath usually around 7pm; space heating is WCC controlled with a curve of 0.6 at the moment, although I’ve just switched this over from 0.8 as we have been having warmer weather of late. I estimate from various calcs (no monitors I'm afraid) that we use an average of around 5kwh/ d just for the DHW production.

Apologies for the long context, but my query would be if anybody else has experience of using (iv) which we’ve never utilised…but given the vast improvement in efficiency we’ve seen using the WCC for space heating, I’m wondering if my next tweak would be to try the weather dependent DHW, using an automatically set storage temp? Obviously it might be best just to give it a go – I’m intrigued to know what DHW temp it will use in winter v summer – but just wondering if other systems have this setting and how it works in practice as I haven’t as yet seen any reference to it as a DHW option in this thread or others.

@derek-m I was rather hoping you might be able to shed some light on the weather dependent DHW setting - is it common? As I say, I've not seen it mentioned much in conversations about ASHP DHW production.

Posted by: @marzipan71

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@derek-m I was rather hoping you might be able to shed some light on the weather dependent DHW setting - is it common? As I say, I've not seen it mentioned much in conversations about ASHP DHW production.

I had noticed that there are one or two manufacturers who offer this capability, but I must admit I have not looked at it in detail, and the reasoning behind doing so.

Probably the most likely reason would be to complete DHW heating as quickly as possible, since when heating the hot water, the heat pump is not providing central heating. I cannot remember the actual details, but I assume that the controller increases the LWT with falling outside temperature, as is the case when operating in WC for CH.

The only benefit that I can see, is that it may improve efficiency during milder weather conditions, when it is heating the hot water over a longer time period in a more efficient manner. This could be even more beneficial if there is also a solar PV system, which is providing the electrical energy to operate the heat pump.

@derek-m thanks - that all makes sense. I think I might experiment with it versus our current scheduled DHW production...the installation manual seems to suggest that the only effect is to replace a user-defined set point (e.g., 50 degrees in my case) with a weather dependent set point which will lead to some implied efficiencies, although they are not as explicit about this as they are for space heating. I may also try using reheat for a week and see how they stack up against each other in terms of consumption. Diagram below for reference.