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Is my setback/preheat doing any good?

bontwoody · 2024-10-28T15:30:05Z

As my electricity tariff provides very cheap electricity overnight, I though I would experiment with running my heat pump at night (until 5.30am) as a sort of setback/preheat. My reasoning was that it should reduce the amount of energy needed when I start my heat pump as normal at 8am. I expected that when looking at my heat pump graph I would see that the area under the electricity curve would be smaller at 8am. However looking at the graphs (this one is typical), the area under the graph is pretty much the same at 3am as at 8am. To me this seems as though Im not achieving anything with the preheat? Am I missing something? Addendum: Actually I think ive just worked it out myself, All i am seeing is the electricity needed to bring the flow temperature up to speed. I really need to see a graph of room temperature, superimposed on the one above to see what effect the preheat is having. Please feel free to ignore this post!

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Air Source Heat Pumps (ASHPs)

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bontwoody

(@bontwoody)

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Topic starter 31/10/2024 6:54 pm

@cathoderay Here is a zoomed in image. The water loses 7 C in just under 3 hours. All my pipework runs through the loft of my bungalow and has standard pipe insulation over it. Would you say this is a quick heat loss?

The heat meter actually uses modbus to talk to the OEM unit, but it would probably be simpler just to get one of OEMs custom temperature units. I was intending to but funds etc. Still xmas is coming 🙂

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60

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cathodeRay

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31/10/2024 7:25 pm

Posted by: @bontwoody

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Here is a zoomed in image.

It's the initial precipitous fall in LWT that I'm curious about, blue arrow points to it here:

The rest of the decline over time is what I would expect to see. It may be the precipitous fall doesn't matter, given it happens over such a short period.

That said, another point is that during the setback gradual decline (which we both have), most/much of the time the LWT is above the IAT, meaning the system is still giving heat to the house, albeit at a reducing rate, yet the energy in and out are both zero. My monitoring does the same thing, see my earlier post with the chart from my system. <= @jamespa is this something we need to consider? Heat is still being delivered during the setback.

Posted by: @bontwoody

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Still xmas is coming

You'd better write to Father Christmas then! But if you already have modbus polling setup, you are already almost there. Some more details in this post of the unit I used (3. The Room Temp Sensor).

Midea 14kW (for now...) ASHP heating both building and DHW

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bontwoody

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Topic starter 31/10/2024 7:41 pm

@cathoderay I see what you mean. I am able to show flow rate on the graph and it seems the pump gives a burst (for 2 minutes) when the heat pump goes off which would account for the fast drop.

Ill have a look at the temperature probe post thanks

This post was modified 8 months ago 2 times by bontwoody

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cathodeRay

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31/10/2024 9:01 pm

Posted by: @bontwoody

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it seems the pump gives a burst (for 2 minutes) when the heat pump goes off which would account for the fast drop

I see what you mean, interesting. It is amazing what we can learn from detailed charts.

Midea 14kW (for now...) ASHP heating both building and DHW

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Lucia

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01/11/2024 10:57 am

Posted by: @judith

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@lucia is it possible that your DHW is on for a constant time rather than to reach a set temperature?Otherwise it doesn’t make sense.

I have the answer: I have the water tank set to heat 48º between 05.00 & 05.30 (with no reheat) so the Daikin reaches the water heating set point and stops. It only continues water heating if I happened to take a shower before the 05.30 deadline.

The answer came yesterday when my installers returned to fit the K3 'big beast' I had been waiting for. They told me that the 'best time' to heat water is about 14.00 when the OAT is highest - particularly for people like me whose pump gets a lot of sun.

But it's a trade off - most of us shower in the morning so the water temperature will have dropped a few degrees by then. I'll leave it on the early morning schedule. I like high pressure, hot showers.

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