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

[rgledhill]

(Apologies to anyone who's already seen my post on this on RenewableHeatingHub)

 

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:

 

 

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

Edited December 28, 2022 by rgledhill

[JohnMo]

Over temp un-likely, over pressure also unlikely unless caused by freezing. 

 

What pressures are you operating at?

 

What levels of Glycol are you running or are you using freeze valves?  We had some freezing nights in April but the days were warm so heating was basically off.  Could that of been an issue or are you just freezing the heat exchanger and have no glycol protection or not enough.  I think if you are running freeze valves you need additional protection for the heat exchangers via secondary heaters

 

SCOP /CoP is a result of flow temps and or short cycling.

 

How many zones?

Buffer or no buffer?

Low loss header?

 

Could you reduce the flow temperatures, run longer and at lower temps?  42 at 5 sounds high for a heat pump system.

 

[IanR]

52 minutes ago, rgledhill said:

I do notice the hot gas inlet from the external unit is really hot, probably 55-60C - is that normal?

 

What's the distance between the outside and inside units? (I'm assuming you have the split version). How's the pipework between the two insulated? Are you happy that the pipes are insulated to spec? Do you know the return temp, back to the outside unit?

 

Does the "back-up" heater come on in normal use? Can you block the back up heater and does it effect the system performance?

[rgledhill]

Thank you both and as I'm not the installer, just the user (albeit a fairly knowledgeable engineer one, but certainly no ASHP expert), I don't know the answer to many of those questions... but I'll try:

 

The water loop from the internal heat exchanger is running at 1.1bar it appears. No idea about the loop to the outside unit which is using R32 refrigerant, rather than water+glycol so there should be no issue with freezing.  The heat exchanger that's gone (twice) is in the one in the inside unit, not outside.  The system has both backup and booster heaters installed and configured correctly (as far as I can tell) and if the system goes from a relatively low cycle (e.g. overnight, when the whole house is at 18C target temperature) to suddenly needing a bit more (e.g. one bathroom is set to 21C) then the booster comes on to get the temperatures up more quickly.

There's a single heating zone but programmable TRVs everywhere, though none is set to be hugely different to 18C-20C, so it's not like they're completely off, then completely on.  Most of the time the system trickle heats the house at about 18C very effectively, it seems.

I've tried reducing the flow temperature and it has no noticeable effect; running around 37C and we're at a SCOP of less than 2 so far today.  I tried disabling both boost and back-up heater as they would obviously drag the SCOP down but it had no noticeable difference.

The system is a low-temperature split, so it has an outdoor unit with a short run of about 3m (well insulated, outside feels room temperature) to the inside unit, using R32. The inside unit has a heat exchanger to a relatively short 28mm pipe run, either going through another heat exchanger to pass heat to the CH circuit, or diverting through the coil in the hot water tank.  Return temperature appears to be around 30C which corresponds to the read-out from the sensors.

 

I will try dropping the flow temperature down to 30-35C and keeping the booster and back-up heaters disabled, because really this should result in a decent SCOP.  Outside temp today is around 9C and breezy so fairly good conditions really.  The outside unit has a good amount of space to breathe too.

 

Thanks again, very helpful!

[rgledhill]

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.

[IanR]

14 minutes ago, rgledhill said:

The system is a low-temperature split, so it has an outdoor unit with a short run of about 3m (well insulated, outside feels room temperature) to the inside unit, using R32.

 

I was looking for a reason for the low CoP. Since you have a short distance between inside and outside unit, the reason is not to be found here.

 

But:

16 minutes ago, rgledhill said:

TThe system has both backup and booster heaters installed and configured correctly (as far as I can tell) and if the system goes from a relatively low cycle (e.g. overnight, when the whole house is at 18C target temperature) to suddenly needing a bit more (e.g. one bathroom is set to 21C) then the booster comes on to get the temperatures up more quickly.

 

Suggests you may be relying on resistive heating (the back-up heater) to support short running of the Heat Pump. Using the Back-up heater will lower your overall CoP, since it has a CoP of around 1.

 

I assume the Boost is a short-term increase in flow temp, so ignoring that for the moment, I'd look to block the back-up heater and extend the heat pump run times (or give it longer to lift the temp from 18°C to 21°C), so that it can cover the heating and hot water requirements for the house without any resistive heating. Even if that means increasing the flow temp a little. This will provide better overall CoP.

 

Your hot gas temp is in the correct range, so this doesn't point to any issue, and the water circuit is running at a low pressure. Fingers crossed that it's a coincidence that two heat exchangers have gone, or perhaps Daikin had a quality issue with a batch.

[HughF]

Well there’s your scop problem. Inefficiency caused by the plate heat exchanger after the indoor unit. Get Brendon to re-plumb it and get it running on the daikin controller only (no external stats) and you should see a marked improvement in efficiency.

[JohnMo]

1 hour ago, rgledhill said:

then through the second heat exchanger and back again.

That's a low loss header not a heat exchanger, the auto vent on top is the give away.

 

I would be tempted to disable any immersion until you figure things out.

 

I would wind the trvs to max, and bring flow temp down 10 degrees.  If rooms are not getting up to temp add a few degrees.  When most the house is the correct temp, balance the rest, to get them where you need room temp wise.

 

Night setback should be on the HP flow temp not the trvs, otherwise you gain little or no benefit.  Treat the trvs as limit stops for room temp rather than controllers.

 

Reducing flow temp is key to good a CoP 

[markocosic]

Header is killing scop.

 

What water treatment and metals/materials for heat exchanger?