How to reduce frequency of defrosts on Daikin Altherma?

Hardware Heatpump
1

i have upped my DT from 5 to 8 to try and reduce the defrost cycles , this seems to have changed the electricity input at the start of the cycle meaning the heat pump is working less hard at this point in time reducing the chances of a defrost cycle to quickly
HP is a Daikin on fan coil setting so you can change the DT

anyone else trying settings to reduce the defrost cycles in this cold damp period
https://emoncms.org/app/view?name=My%20HeatPump&readkey=1da9e37af3d5ac9669792233ea85675c

you can see the change when the black internal temp line drops dramtically as the system restarts

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I also run at a high dT, in theory, but only to keep the flow rate steady. This is Daikin, of course.

The best way to limit defrosts is to lower the flow temperature as much as you can.

The other thing you can do is restrict the flow rate, it reduces the aggressive nature of the Daikin space heating cycle start up.

Limiting the flow rate limits the heat production and therefore the power consumption which equals less defrosting.

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@Chris_Hill

I have also always found that the radiator setting gives me best efficiency. It was the same with my 9kW as it was with this 8kw, maybe just my house?

It sets the dT at 10c and I thought it was fixed, but I discovered this week that you can actually increase it, should you want to.

I have actually got to a dT of 10c in the last few days and seen the flow rate increase from minimum to maintain the dT of 10c.

I never thought I would see that, but it was quite cold here, colder than my data says.

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That’s an interesting idea… presumably the return temperature does still creep upwards since it’s really, say, 5 degrees. Does this tend to push the LWT up into overshoot to try to maintain the 8 degree gap? And if the HP cycles, does it tend to stay off for longer in order to allow the water temperature to fall back to (target LWT - 8) ?

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But I think you also run at fixed LWT. Not quite sure how that would affect things.

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Fixed lwt doesn’t mean much if you think about it.

A wdc is a fixed lwt with no change in the outdoor temperature.

The lwt moves a bit, even for me at a fixed lwt, in fact this is the only time you would notice this.

I think the flow temperature fluctuates because the heat pump isn’t very good.

When it is stable, it does tend to sit pretty close to the set lwt but does move a bit.

The return does follow the flow more or less, and this is because then emitters are already giving out what they can with the flow rate at minimum.

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My mental model of a heatpump is:

  • HP puts power into circuit, adding dTÂą to return temp
  • emitters pull power out of the circuit, dropping flow temp by dT²

When dT is matched on both sides of the circuit, the heatpump can run at a stable temperature.

When the emitters can’t output the heat fast enough (dT²<dT¹), then the return temperature creeps up, causing the flow temperature to also increase. The heatpump can respond by modulating down, putting less power into the heating circuit. If it’s already at minimum power, then it’s unable to maintain flow temperature.

The only control the heatpump has is compressor frequency, which it modulates to keep flow temperature within limits. It cannot control flow temperature directly.

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I was pondering just using the immersion heater (“powerful mode”) rather than HP to heat water. I currently heat water overnight, during cheap rate, but of course that’s typically when it’s coldest. I am concerned that it disturbs any equilibrium the heating has managed to establish, and once frosting/defrosting has set in, it seems to struggle to break out of that state.
Maybe settling for only 100% efficiency in heating water will payback elsewhere.

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9

But in this case, it’s not so much that the emitters can’t output enough heat, but that the configured delta-T is larger than what the emitters actually drop. (eg asking for 8 degrees when dT² is really 5.)

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If my radiators were bigger then my heat pump would hit the requested dT at my set flow temperature, as long as the return temperature was equal to, or greater than the room temperature.

Right now, my flow temperature is set at 31c.

The water is actually entering the house at an average of 31.1c, so bang on.

The return temperature is 24.1c

So my dT is 7c and my flow rate is as low as it will go.

Here it is, as steady as it gets (pretty much)

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The flow temperature is not increasing to achieve the requested dT of 10c.

The dT will get bigger if the heat loss increases.

It is the only thing that can change my actual dT as the flow temperature and flow rate are fixed.

The electrical input varies as necessary to keep the flow temperature at 31c.

I have an overshoot of up to 4c set but the heat pump doesn’t use it as the dT is acceptable to allow the heat pump to run continuously.

If it gets colder outside (when!) the dT will increase because the return temperature will drop.

The flow temperature and flow rate will remain as they are now.

The electrical input will increase to maintain flow temperature of 31c and provide the extra heat I am now producing (same flow rate but bigger dT = more heat).

If I exceed the set dT of 10c then the flow rate will increase to hold it at 10c.

You can see this here

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This is what happens if I change to fan coils and a dT of 5c, a wild unstable mess, nothing is steady!

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image1113Ă—1465 213 KB

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I have always found these Daikin heat pumps to be most efficient and most comfortable when stable.

Ideally, I want to be able to run at a fixed flow rate between 10 and 15lpm, but I can’t work out any way to do it with the controls Daikin provide.

My house was better with the 9kW as the minimum flow rate was 10lpm. I restricted it in the same way, radiators at a dT of 8c. Just a shame the minimum heat output like this was over 4kW!

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One thing about defrosting.

The `damage’ to efficiency isn’t just the defrost cycle itself.

If you watch carefully, as the ice builds up the COP starts to drop as electrical input increases and heat output decreases, all as a result of ice on the evaporator.

The ice has a much bigger effect than just the defrost energy shown in our charts.

It is so pronounced sometimes, you can just look at the data and `feel’ the defrost coming.

Avoiding the ice as much as possible is key to the least impact.

I do this by running continuously at the lowest flow temperature I need to get enough heat in the house.

Running higher and then turning off may end up costing more than running lower for longer.

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(we seem to have taken over a generic thread with Daikin discussion…)

I gave that a try, but not sure it helped. If DT is 8, doesn’t that mean it has to create more heat, in order to raise the temperature by 3 additional degrees? So couldn’t that make the problem worse, rather than better ?

I tried using the radiator setting, but it didn’t seem very different.

I’m now trying DT of 4 - had been meaning to try this for a while, since that seems to be the actual temperature drop as the water runs through the pipework. I’m not sure why - none of the radiator returns seem to be as high as this. Perhaps there’s a leaking bypass somewhere ?

Anyway, immediate observation is that pump is sitting at around 14l/min rather than the minimum of 10 previously. Which makes sense - for fixed min heat input, need to increase the volume of water in order to reduce the temperature rise. But I wonder if this is a good thing - it means the HP now has a little scope to reduce flow temperature if it needs to, whereas it was already at the minimum previously.

With DT set to 5, it seemed to have to keep increasing the LWT to try to keep it 5 degrees above the return temperature, which kept following DT upwards. (Not sure why it couldn’t just increase the flow rate, to try to reduce the temperature rise.)

I’ve finally got round to sticking some old 12V PC fans under the hall radiator - that had the largest space to heat, but never seemed to get as hot as the other radiators. I’m pretty sure this is helping. Maybe I’ll be able to reduce flow temperature a degree.

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Increased flow rate will make the return temperature rise even faster. It creates more heat.

It increases the flow temperature because the return is rising, your emitters cannot get rid of the heat being produced.

A bigger dT doesn’t create more heat if the flow rate is lower.

I run at a requested dT of 10c that I rarely see, all it does is keep the flow rate constant.

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Increasing flow rate on its own doesn’t do that, it distributes the heat over a bigger volume of water. But since HP is trying to maintain a requested DT, it typically adjusts power and flow rate together.

I guess it will choose higher power when it’s trying to get both LWT and RWT to increase, but, after a defrost cycle, I’m seeing it run at high power/flow even when the LWT exceeds the requested value, ie when there’s already enough energy in the system. It could (should) switch to a lower-power lower-flow state (reducing both proportionally so that energy/litre, and therefore DT, remains the same).

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Increase the flow rate to reduce what temperature rise?

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If the LWT remained steady and the heat pump increased it’s flow rate, then the RWT would rise even quicker. How would increasing the flow rate combat a rising return temperature? You even say at the end about reducing flow rate.

The heat pump is not choosing higher power to get the LWT and RWT to increase.

Daikins do not want the RWT to increase.

If you have set a dT of 5c with a flow temperature of 35c, a Daikin heat pump will try to maintain an RWT of 30c, it is not trying to maintain a dT of 5c, but actually a return temperature based on flow temperature and dT set.

If the RWT starts to rise, flow rate reduces, and vice versa, keeping heat output constant (except they aren’t very good at it in my experience!)

If the RWT continues to increase when the flow rate is at the minimum, it means that the emitters are at their limit at that particular mean flow temperature.

Increasing the flow rate at this point will just make the RWT rise even faster.

So the only weapon the heat pump now has to control the rising RWT is to increase LWT within the limits of any set overshoot.

If it uses all the overshoot and RWT is still rising and approaching LWT, then it will shut down.

The heat pump is only ever trying to increase RWT if it is below set flow temperature minus the set dT. Otherwise it is trying to hold or reduce it. RWT only ever increases as a result of the emitters not being able to get rid of the heat produced.

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Ah, okay, I’m probably being a bit careless in describing what I mean. I was thinking about traversal through the heat pump itself, but as you say, changing the flow rate also affects how heat is subsequently given out to the radiators and the house.

On raising RWT, I was thinking specifically about after a defrost cycle, when it is trying to get both LWT and RWT back up to target, so clearly running at high power/high flow is sensible, to get energy into the system. But once it gets back up to temperature, it doesn’t seem to reduce power (and flow rate), and overshoots.

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Hi Dave,

Mine goes straight back to the lowest flow rate when restarting after a defrost. It only goes through the high flow rate period when commencing a new heating cycle.

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I have my heat pump set as `soft’ as I can make it, otherwise the aggressive starts at low flow temperatures are too much.

But, I think I can only do this because of the big radiators and a small heat loss compared to the size of the heat pump.

If I needed more heat I would have to run the heat pump harder, obviously,.and then suffer worse performance.

The benefit of an over sized heat pump I guess.

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Could you summarise these settings please?