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Daikin Altherma LT R32 - poor SCOP and breaking heat exchangers!

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(@rgledhill)
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Hello everyone,

Last March, we had a Daikin Altherma LT R32 8kW system fitted last year, with a 160 litre DHW tank, running CHW for a 3 bedroom recently built house with good insulation.  Radiators are all properly specified, flow temps set at about 42C in current 5C outside temps.  The house gets warm with no problems, with 18C set-back temperature, and occasional rooms up to 21C (e.g. bathrooms in the morning, lounge in the evening).  It's configured to use weather compensation, so dropping to 30-35C flow temperature as the outside temps warm up.  Hopefully this is pretty much an optimal system.

The only thing is, I get poor SCOP - see below:

image

Looking at the CHW, this seems really low, and has since dropped to around 1.5!  Back in April, we started hearing fizzing sounds, and basically the heat exchanger had sprung a leak and coolant gas was fizzing into the water loop. As there was less and less coolant going round, the system had to work harder and so the SCOP dropped.  The heat exchanger was replaced at the end of April and the SCOP went back up from 2.58 to 2.93, and on DHW 1.78 to 2.34.

Well, back in October, it started fizzing again and the SCOP's on the way down too.

So...  any ideas firstly what's causing the poor SCOP?  This system is supposed to be about 3.5 to 5.5 SCOP, so 2.5-2.9 is pretty rubbish.

Secondly, any idea why the heater exchanger has failed again (so that's two brand new heat exchangers now that have gone)? I'm thinking the only reason they could fail is over-pressure or over-temperature. I do notice the hot gas inlet from the external unit is really hot, probably 55-60C - is that normal?

Any advice or experience with these Daikin units would be really appreciated.  The original dimensioning seemed very sensible, with about a 5kW heat input required in really cold temperatures, so 8kW seems well-dimensioned.  As I say, the house is warm, it's just really inefficient (and breaks heat exchangers!).


   
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(@heacol)
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The heat exchanger failure is a manufacturing fault, nothing to do with operation.

If you have a buffer tank or system separation, that is causing your poor COP, if not, it may be a third party thermostat, if you do not have one there is a serious problem with the machine.

Professional heat pump installer: Technical Director Ultimate Renewables Director at Heacol Ltd


   
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(@kev-m)
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@rgledhill,

what temperature is your DHW set at?  Your DHW COP is a bit low but not disastrously so and could be explained by a high-ish DHW temp. If the DHW is working OK there's probably nothing fundamentally wrong with the ASHP itself.

 


   
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(@oswiu)
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Question for those who better know how these machines work: why would the inlet refrigerant temperature to the indoor unit be 60C when flow temp is 42C (assuming OP didn't happen to check then the DHW was being heated)? Mine being a monobloc I don't know where the sensor is, but my reported refrigerant temperature seems to normally be just a little bit higher than the water flow temperature.

Assuming this is not right, could it be that OP's machine is working double time to get refrigerant temps too hot, perhaps because the heat exchanger still isn't performing?


   
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(@derek-m)
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Posted by: @rgledhill

Hello everyone,

Last March, we had a Daikin Altherma LT R32 8kW system fitted last year, with a 160 litre DHW tank, running CHW for a 3 bedroom recently built house with good insulation.  Radiators are all properly specified, flow temps set at about 42C in current 5C outside temps.  The house gets warm with no problems, with 18C set-back temperature, and occasional rooms up to 21C (e.g. bathrooms in the morning, lounge in the evening).  It's configured to use weather compensation, so dropping to 30-35C flow temperature as the outside temps warm up.  Hopefully this is pretty much an optimal system.

The only thing is, I get poor SCOP - see below:

image

Looking at the CHW, this seems really low, and has since dropped to around 1.5!  Back in April, we started hearing fizzing sounds, and basically the heat exchanger had sprung a leak and coolant gas was fizzing into the water loop. As there was less and less coolant going round, the system had to work harder and so the SCOP dropped.  The heat exchanger was replaced at the end of April and the SCOP went back up from 2.58 to 2.93, and on DHW 1.78 to 2.34.

Well, back in October, it started fizzing again and the SCOP's on the way down too.

So...  any ideas firstly what's causing the poor SCOP?  This system is supposed to be about 3.5 to 5.5 SCOP, so 2.5-2.9 is pretty rubbish.

Secondly, any idea why the heater exchanger has failed again (so that's two brand new heat exchangers now that have gone)? I'm thinking the only reason they could fail is over-pressure or over-temperature. I do notice the hot gas inlet from the external unit is really hot, probably 55-60C - is that normal?

 

Any advice or experience with these Daikin units would be really appreciated.  The original dimensioning seemed very sensible, with about a 5kW heat input required in really cold temperatures, so 8kW seems well-dimensioned.  As I say, the house is warm, it's just really inefficient (and breaks heat exchangers!).

 

Thanks

Richard

To have one heat exchanger fail would be unfortunate, and would probably indicate a manufacturing defect, but to have two heat exchangers fail on the same unit I would find rather suspicious. If possible, I think that you should request a full written report of the reason for failure of the heat exchangers, I think that you should also request Daikin to perform a full diagnostics on your unit to ensure everything is functioning correctly.

The SCOP on your system seems rather on the low side, which also may be indicative of underlying problems.

 


   
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(@rgledhill)
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Thanks everyone... some extra info below:

- The system seems to have three loops, which feels like at least one more than is really needed.  There's the R32 refrigerant loop from the outside unit to the inside heat exchanger; then a loop from the other side of the heat exchanger inside the unit, through a second heat exchanger which transfers heat to the central heating circuit.  There's a 3-way valve which can take this final heat exchanger out of the loop and instead divert it through the heat exchanger coils inside the hot water cylinder.  If the external unit can heat to 55-60C, why do we need any other heat exchangers at all...

- DHW is set to 56C, which I realise is somewhat high, but it's because I pay an extremely low rate overnight (5.5p/kWh) so it's cheaper to overheat the cylinder overnight, and then my top-up temperature is set to 43C if later in the day it needs it.  So I realise it would be better if I set the target temperature lower overnight, but it would actually cost more.

- @oswiu That was what I was wondering too.  Shouldn't the outside unit be sending compressed, heated gas at somewhere nearer to 40-45C, which the internal heat exchanger then condenses and extracts heat out of to get up to the required flow temperature of (e.g. today) 37C?  DHW heating is a bit different but I notice that the pipe is almost too hot to touch, even when doing CHW.

- Daikin did a full investigation on my system and showed nothing untoward, but I didn't see the results of their investigation into the failed heat exchanger.

This is all really helpful, thank you.


   
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(@derek-m)
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Posted by: @rgledhill

Thanks everyone... some extra info below:

- The system seems to have three loops, which feels like at least one more than is really needed.  There's the R32 refrigerant loop from the outside unit to the inside heat exchanger; then a loop from the other side of the heat exchanger inside the unit, through a second heat exchanger which transfers heat to the central heating circuit.  There's a 3-way valve which can take this final heat exchanger out of the loop and instead divert it through the heat exchanger coils inside the hot water cylinder.  If the external unit can heat to 55-60C, why do we need any other heat exchangers at all...

- DHW is set to 56C, which I realise is somewhat high, but it's because I pay an extremely low rate overnight (5.5p/kWh) so it's cheaper to overheat the cylinder overnight, and then my top-up temperature is set to 43C if later in the day it needs it.  So I realise it would be better if I set the target temperature lower overnight, but it would actually cost more.

- @oswiu That was what I was wondering too.  Shouldn't the outside unit be sending compressed, heated gas at somewhere nearer to 40-45C, which the internal heat exchanger then condenses and extracts heat out of to get up to the required flow temperature of (e.g. today) 37C?  DHW heating is a bit different but I notice that the pipe is almost too hot to touch, even when doing CHW.

- Daikin did a full investigation on my system and showed nothing untoward, but I didn't see the results of their investigation into the failed heat exchanger.

This is all really helpful, thank you.

Your ASHP is obviously a split system, hence the indoor heat exchanger.

If I understand your description correctly, you have a heat exchanger located within the indoor unit, which transfers the heat energy from the refrigerant gas to the primary water circuit, but you also state that you have a second heat exchanger which transfers heat energy from a primary water circuit to a secondary water circuit. (Some photo's would be useful).

If this is true then it seems a rather weird and inefficient type of system, though I suspect what you think is a second heat exchanger may actually be a buffer tank. How many water pumps do you have and where are they located?

Heat exchangers and buffer tanks can adversely affect the overall efficiency, since to achieve adequate energy transfer there must be a temperature difference between the inlet and the outlet, which in the case of a heat pump means it has to work harder to produce hotter refrigerant gas.

The pipework to your hot water cylinder will remain warm for a period of time after the 3-way valve has switched back to CH, particularly if it is well insulated. If it remains quite warm then check that the 3-way valve is operating correctly. Whilst producing hot water during the cheaper nighttime period may benefit your wallet, it will lower the actual COP measurement.

 


   
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(@rgledhill)
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Yes, it's an LT split.  Photo below, with atrociously poor mark-up:

image

There's one pump inside the Daikin internal unit, which circulates through the heat exchanger inside the same unit, through the 3-way diverter valve, then through the second heat exchanger and back again.  There's then a standard Danfoss CH pump which circulates heat through the other side of the second heat exchanger and internal the existing central heating pipes.

The 3-way valve seems to be working fine, in that the pipes to the DHW tank remain warm from conduction of heat but only get hot when it's switched to DHW, as you'd hope.


   
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(@derek-m)
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Posted by: @rgledhill

Yes, it's an LT split.  Photo below, with atrociously poor mark-up:

image

There's one pump inside the Daikin internal unit, which circulates through the heat exchanger inside the same unit, through the 3-way diverter valve, then through the second heat exchanger and back again.  There's then a standard Danfoss CH pump which circulates heat through the other side of the second heat exchanger and internal the existing central heating pipes.

The 3-way valve seems to be working fine, in that the pipes to the DHW tank remain warm from conduction of heat but only get hot when it's switched to DHW, as you'd hope.

The black vertical cylinder would appear to be a buffer tank rather than a heat exchanger. It is the same water on both sides, there is no physical separation. One of the problems with buffer tanks is balancing the water flow rate on the primary side and the secondary side, which can lead to much lower temperature water coming out than that going in. I would suggest that you measure the temperature on the two primary side pipes and compare that with the temperature of the two secondary pipes. It could be that the buffer tank is not required and could be removed or just bypassed.

For best efficiency the two upper pipes in and out of the buffer tank should be at approximately the same temperature and the two lower pipes should both be at a temperature approximately 5C lower than the upper pipes. If the outlet temperature is lower than the inlet temperature then the heat pump is working harder to produce warmer water at the inlet and hence is not operating in an efficient manner.

Fully open any TRV's that you may have fitted and then see what happens to the temperature in each room. If you find large differences then your radiators will probably need to be balanced. Also adjust any thermostats to a setpoint 2C above the desired room temperature and then modify the weather compensation curve to achieve the desired temperature.

 


   
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(@rgledhill)
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@derek-m Well, what do you know... I found a sticker! The black tank/exchanger is actually a Sprzeglo hydrauliczne!  (Thankfully the label also says that it's a low loss header (hydraulic balancer)).

2022 12 28 18.27.49

I've checked the temperatures on either side of this and the pipe temperatures left-to-right match very closely, both on the outlet (top) and return (bottom), so it seems to be working well.  The system's reporting 34C leaving the unit, 31C returning.

Ok, so now that I know what a low-loss header is (thank you, Google), I can see that the whole system from the internal heat exchanger through the heating pump and radiators is at the same pressure (+- a bit for the pumps).  The installer has set this loop to be about 1.7 bar - does that sound about right?

Although I have TRVs on most rooms, we also have two towel rails that are permanently slightly open so that the system never short-cycles on the radiator side and that act as a kind of buffer to smooth out changes.  I'll have a think about basically setting all the radiators to "on" and using the flow restrictor on each to balance one room against the next.  At that point it's a balancing act of chasing every tiny bit of COP vs having a higher overall heat input to (and therefore loss from) the house - sometimes a higher COP can lead to a higher electricity bill because of this.

This post was modified 1 year ago by rgledhill

   
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(@derek-m)
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@rgledhill

So you have a low loss header, which operates like a thin buffer tank, same inputs and outputs, but without any volume.

The system pressure on the water circuit will normally be set somewhere between 1 and 3 bar, if the pressure falls below 1 bar then this would probably indicate that you have a leak somewhere.

With a heat pump system it is necessary to have the radiator valves as open as possible, so that you achieve the maximum water flow rate around the system. TRV's should only be used in rooms where you require a lower room temperature, in the other rooms the temperature should be balanced by adjusting the lockshield valves on the radiators. If the indoor temperature gets too high then lower the weather compensation curve, which in turn should improve the efficiency of the heat pump. Also set any thermostats to 2C above the desired temperature to ensure that the system is operating in weather compensation mode.


   
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(@rgledhill)
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Thanks, so 1.7 bar seems fine then, and it hasn't dropped at all since recommissioning after the new heat exchanger was fitted.

I see what you're saying about setting the temperatures using balancing and the weather compensation curve, but this would effectively mean the ASHP would be running 24/7?  That sounds like a great way to wear it out, plus it's annoying having the background hiss of water running through pipes everywhere.  Feels like some kind of compromise would result in lower overall energy usage, at the expense of a slightly worse COP.

Anyhow, it's now running at 32C (weather compensation curve has been pushed right down), and I'll be lagging much of the pipework tomorrow, as well as the large white thing that looks like an expansion vessel except the CHW/DHW feed from the heat exchanger goes straight through it...   It gets pretty warm and is basically just wasting energy, so a bit of lagging seems sensible.

Looking at this white object, it appears to be an Ibaiondo 18-VI, an intermediate vessel intended to be used in line with an expansion vessel to protect it from high/low temperatures.  Can anyone explain to me what it's doing in the main loop pipework? I inadvertently omitted it earlier in my diagram.

ASHP internal 3

 


   
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