Obviously it is dependent upon the size of your solar thermal system, the volume of your hot water cylinder and the quantity of hot water used, but I am surprised that you may still be using the immersion heater at this time of year. Remember, the water may still be hot in the top half of the cylinder when the bottom half is cold.

The first point I would make is to try to heat the water to 60C using the solar thermal, to both provide hot water, and perform the required disinfection.

If there is insufficient solar then heat the water to 48C to 50C using the heat pump.

Only use the immersion heater if absolutely necessary, or if you are on a cheap overnight tariff.

As of 2nd April I am now expecting to get all my family DHW and Legionella heating from the sun. That should continue until around mid October. We are already getting around 55c at some point in the week. 

Because the heat exchanger is placed low in the storage tank we are also getting around 40% more hot water than the HP alone can give us. We also have 2 additional thermistors mounted 250mm from the top and 250mm from the bottom of the hot water tank. This, on average, informs us that the top of the tank is usually about 8c hotter than the mid point of the tank and after a long shower, if the hot water runs out in the HP thermistor (located in the middle of the tank) we can anticipate 60 litres of hotter water still in the tank.

I’ve gone the relatively low-tech route with my Solar Thermal/ heatpump interaction. I am relying on the hw settings in the HP controller to AVOID the need for the HP to warm the DHW. In other words the solar system warms the water as best it can and if the temperature of the HP thermistor is above the activation temperature then the reheat cycle is not activated.

The solar system has a simple temperature difference controller which activates a pump. Switching on the pump at TD of 11c and off at TD of 3c. This feeds into the lowest section of a 250 litre hw tank via an internal heat exchanger

You can see the DHW temperature rise on the MELCloud “Hourly Temperatures readout”

Posted by: @alan-m

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Now I can begin to look at whether and how to schedule the DHW function of the heat pump to specific times to avoid wasting the opportunity for the solar thermal to heat the water for free. Who knows, one day we might get to the point where the local weather forecast helps decide which heating source should be used when.

Yes... I 'get it'.
And I think you're heading in the right direction.

Up until now, all heating and DHW control systems in the UK are operating on measurements made of the present conditions.
As we move across to renewable-energy sources, it's becoming more important to know what's about to happen over the next few hours.

So if I have a solar source, and there's full sunshine expected in an hour's time, then I probably won't want to waste electricity (or gas) to operate a heat-pump (or boiler) to bring DHW up to the set-temperature in the meantime.

There has to be a 'signal' sent to the thermostat/controller to say "hold off" from obeying your normal schedule.
And then you need an indicator (LED or display icon) to let you know that the DHW isn't being heated yet because there's a "hold off" operation in progress.

It's on my list of "things I need to design, but I haven't got around to it yet"!

Posted by: @sunandair

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The solar system has a simple temperature difference controller which activates a pump. Switching on the pump at TD of 11c and off at TD of 3c. This feeds into the lowest section of a 250 litre hw tank via an internal heat exchanger

 

Fascinated by the wider range in TD that you have compared to mine, do you know if those are the factory settings for your system or whether you chose those by trial and error/calculation?

@alan-m hi,

these TD settings are as per the factory settings.

Ive noticed that on an overcast day heat will be slower to build so if the TD is lower the pump might move the heat too quickly and cause cycling on and off perhaps too frequently. 

11c to 3c seems to give a reasonably wide operating range so the pump is on for longer periods. 

Having said that I’ve never had cause to even think about this before today.

@sunandair I think I will gather a week or so of data with my current parameters and then try the wider spread like yours. Hopefully I'll get a range of weather conditions in both samples so that comparisons of the data will be useful. I'll post the results on here.

Hi @alan-m 

It was 7 years ago when I set up the solar. It was a diy install. And last autumn the bigger tank was added along with the Ecodan heat pump using a recognised installer. We also didn’t get any guidelines on how to get the solar and heat pump to work together. But they seem to be working happily together.

On the solar installation I recall there was a flow rate mechanism and having to adjust the flow rate using the guidelines in the installation guide. So you may have some similar guidelines with your system. You may already know that small adjustments in flow rate will also optimise the heat transfer to the tank. I just set up the flow rate as per the installation guidelines.

THE BIG QUESTION I HAVE IS:

Can the Ecodan be programmed to cool down the tank when I’m on holiday?..... just like the antifreeze program will circulate the radiator water through the compressor to prevent freezing.

Can the FTC 6 be set up to open the DHW VALVE and circulate the water through the heat pump to lower the DHW TEMPERATURE?

We are going to have to come up with a solution for this one since temperatures in the tank can get pretty hot after 14 days of British summer sunshine.

Posted by: @sunandair

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Can the FTC 6 be set up to open the DHW VALVE and circulate the water through the heat pump to lower the DHW TEMPERATURE?

Can you please remind us of the layout of your cylinder?

Is it a DHW cylinder or a thermal store?

Posted by: @alan-m

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I will gather a week or so of data with my current parameters and then try the wider spread like yours.

There is a correlation between the width of the hysteresis you set on the controller and the curve it uses to define the pump speed.

Most Solar Thermal controllers operate the pump using a variable-speed algorithm.
As these are 240v AC pumps, the method used is similar to that for old-fashioned incandescent theatre lights. When it first starts, there's an imperceptibly small pulse to 'get it started'.

As more heat is extracted from the roof-top collector, the temperature of the water being drawn into the cylinder falls. The pump speed is lowered accordingly.

The better the controller manages the pump speed curve, the narrower you can set the temperature hysteresis.
But there comes a point when the electricity you're using on the pump to acquire the heat from the roof isn't worth it !

So if you want to spend less electricity running the pump, then widen the temperature hysteresis.

The pump will then 'sleep' longer until there's enough temperature on the rooftop collector to make it worthwhile running again.

Few controllers allow you to configure the slope of the pump-speed curve, or the minimum speed it will be run at.
But that's also probably indicative of the number of customers who would understand how to configure it anyway!

The point is... you can't just copy the temperature hysteresis being used by another site.
There are too many variables to assess.

But it definitely is worthwhile trying a different range of settings to see what works best for you.

Posted by: @sunandair

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Hi @alan-m 

It was 7 years ago when I set up the solar. It was a diy install. And last autumn the bigger tank was added along with the Ecodan heat pump using a recognised installer. We also didn’t get any guidelines on how to get the solar and heat pump to work together. But they seem to be working happily together.

On the solar installation I recall there was a flow rate mechanism and having to adjust the flow rate using the guidelines in the installation guide. So you may have some similar guidelines with your system. You may already know that small adjustments in flow rate will also optimise the heat transfer to the tank. I just set up the flow rate as per the installation guidelines.

 

THE BIG QUESTION I HAVE IS:

Can the Ecodan be programmed to cool down the tank when I’m on holiday?..... just like the antifreeze program will circulate the radiator water through the compressor to prevent freezing.

Can the FTC 6 be set up to open the DHW VALVE and circulate the water through the heat pump to lower the DHW TEMPERATURE?

We are going to have to come up with a solution for this one since temperatures in the tank can get pretty hot after 14 days of British summer sunshine.

 

My first question would be how high do temperatures get in your hot water cylinder, and is this actually a problem?

If the solar thermal system is switched off, can the panels cope with getting quite hot? What is the maximum permitted temperature?

Is there anyway in which the panels could be covered or shaded to reduce the heat output?

@transparent hi, it’s a DHW cylinder for kitchen, bath, shower etc. In the height of summer we have had the solar relief valve discharge into the container once. There is a recirculating option on the solar loop but the heat exchanger is at the bottom of the cylinder so I’m pretty sure that wouldn’t cool down the whole tank.

just wondered if the Ecodan could work in reverse to cool it while we were away.