Hi Cressie,

Welcome.

My experience with a similar Ecodan and solar setup is that there isn't one solution for summer and winter but it all depends on your solar panel capacity and inverter max output.

In the summer it is a no brainer, use the heat pump to heat the water at the sunniest time of the day plus a half hour top up just before the nighttime cheap rate ends.(It will probably not power up for the full half) Depending on the size of the Ecodan and the available solar and inverter, the big load from the ASHP can be handled and even if it drains the battery, there is time to recharge.

Winter is different because the heat pump demand is far more than the solar and the battery can deliver and so you end up drawing on the mains at the more expensive rate and a near empty battery that doesn't have enough time/ sun to recover and supply the evening/ night requirements.

Consequently I have reverted to 2 sessions of DHW at the off-peak rate.

Hope that helps  

Posted by: @cressie

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Hi, recently had a Ecodan ASHP fitted with solar panels. Has anyone had much experience with the FTC7 controller when it comes to scheduling the DHW. It asks you to prohibit times which I assume prohibits the use of the heating to allow the DHW to work? I’m looking to set DHW to come on for a couple of hours in the early morning and again early afternoon for about 90mins to take advantage of the solar panels (DHW temp 47 degrees).

Any advice to make sure I set the schedules properly?

We also have a FTC7 and a newer R290 Mitsubishi heat pump. There are prohibits for heating and DHW, to schedule the DHW you want to prohibit the DHW for times you don't want it to run. DHW takes priority over heating so you don't need to prohibit the heating. The other thing to make sure you have enabled is DHW schedule (the on/off toggle on the DHW screen).

What sized heat pump and DHW tank do you have? You might want to investigate if you can do just one DHW a day. We have solar and are on the Octopus Cosy tarrif (which has a cheap rate period 1-4pm), so do DHW at 1pm - if there's solar then it's powered by the sun or cheap rate electricity if no solar. 

@davidalgarve thanks for the advice, appreciated.

@robs Hi Rob, thanks for the reply. We have the R32 heat pump. The tank holds 142L and to be honest we don’t use a great amount of DHW so I was wondering whether I’d only need to heat the tank up once a day and obviously in the afternoon to take advantage of the solar panels.

Im on Octopus’s 15M fixed tariff at the moment but have looked at the cosy which has obviously been designed for ASHP users. Do you think it would be worth switching? Only thing putting me off was the Peak Rate charge.

The default setting is that the DHW is always 'on' but actually runs when the tank temperature drops below the max temp drop value. To force the DHW to run at a time of your choosing you insert a prohibition period, and a DHW run commences when the prohibit ends.

So, to make the most of the solar panels' generation you want a run in the middle of the day, or whenever your production is at its peak - it may vary depending on the orientation of the panels. When In the first few months I had the heap pump I thought I was being clever by manually starting a hot water boost run from the button on the controller or from MelCloud when it was a sunny morning to guarantee a solar powered DHW run. Unfortunately all that was happening was the second scheduled run was still occuring, the heat pump was using energy to get the flow temp up to the maximum temperature required for the HW, but this was not then needed to actually put any heat into the tank.

If you can last a full day with a single DHW run, frequently solar-powered, this will be more energy efficient than having two runs. To make this more likely you could consider raising the temperature you heat the tank to, up into the fifties, so your are simply storing more heat. You'll have to see how much hot water you regularly get through and whether it lasts into the morning - I have a 210l tank and the morning showers are still plenty hot enough.

The other way to avoid programming two runs is to make use of the reheat settings. If you insert a single short prohibit to force a middle-of-the-day run then after that is completed the DHW remains 'on' and a further DHW run is allowed should the tank temperature fall beyond the max temp drop value. The default for this is 10°, which means for you 10° below 47°. You might be surprised how low the measured tank temperature can get before you notice that you are running out of hot water - it depends on where the thermistors are placed in your particular tank. The tank will also lose temperature without any hot water draw of several degrees a day. So to avoid unwanted reheats you might choose to effectively restrict the DHW windows quite narrowly - as you are doing. The alternative is to increase the max drop value a lot so you are closer to being out of hot water before the reheat kicks in. It might be a strategy to allow for high use evenings not having a consequence of tepid water in the morning, without unnecessary non-solar DHW runs.

Posted by: @cressie

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@robs Hi Rob, thanks for the reply. We have the R32 heat pump. The tank holds 142L and to be honest we don’t use a great amount of DHW so I was wondering whether I’d only need to heat the tank up once a day and obviously in the afternoon to take advantage of the solar panels.

Im on Octopus’s 15M fixed tariff at the moment but have looked at the cosy which has obviously been designed for ASHP users. Do you think it would be worth switching? Only thing putting me off was the Peak Rate charge.

142L is not very big but if you don't use much DHW then it's worth seeing if only one DHW reheat a day works. 

Do you have a battery with your solar? As there is a cheap rate period just before the peak rate it is possible to use the battery and avoid the peak rate charges entirely. 

@harriup thanks for the advice and information. I’m leaning towards going one run in the afternoon but increasing DHW temp to low 50s as with the amount of water we use we would probably get away with it.

@robs unfortunately I don’t have a battery which is the reason I’m apprehensive about the cosy tariff!

@cressie I have noticed a trend in the last year or so; the attitude seems to be towards having a battery - even in preference to solar PV if the budget won’t run to both. I think it reflects the use of TOU tariffs as much as anything. Regards, Toodles.

Posted by: @cressie

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@robs unfortunately I don’t have a battery which is the reason I’m apprehensive about the cosy tariff!

Cosy has the 3 hours of peak rate and the day rate is appropriately 1.5p per hour more - which is appropriately equivalent to another hour and a half of peak rate. With 8 hours of cheap rate, even without a battery it should be cheaper. But not as much as if you had a battery. 

I wonder if I am missing something here, regarding heating DHW with the heat pump in the middle of the day.

I found that the heat pump would be be taking 10kW or more for a significant time and my solar can only deliver 4kW. Consequently I am using the remaining power from the grid at the time that it is most expensive.

The heat pump is on weather compensation for the radiators and obviously is consuming far less than that, so could the heat pump be modulated in some way when heating DHW so that it is within the capacity of the inverter?

I have to add that it is a big tank, (500l) and the thermostat is about 2/3 down.