Using dHW on ASHP pre-plumbed tank with external plate heat exchanger

[GoodDIYjob]

Anyone have any experience of such set ups?

I have some good friends who have such a set up but a very strange anomaly exists.

The tank is 150l and the water is lifted to 50c by the ashp. (house is only usually 2 people).

blending to 38c gives a theoretical no losses duration of about 19 minutes based on an incoming temperature of 10c.

In reality, if the heatpump dhW is timed to be OFF when showing they get about 17-18 minutes, so far so good.

However, if the dHW is put to constant (as recommended by the manufacturer or ASHP and Cylinder) or the dHW is set to a timed cycle that is ON when showering, the availability of 38C hotwater fails after 11 to 12 minutes, a 33% reduction in available showering duration. At that point the ending tank temp will have fallen below 36C....

What seems to be happening is that the tank plate heat exchanger and pumped cct seem to be blending the whole tank to a blended temp as it passes through the plates and therefore not leaving the pocket of heated water on the top of the tank. And the only solution would be to wait for the entire tank to re-heat before being able to shower again....

Seems a strange outcome using what is marketed as a superior solution but actually is quite frustrating in use as you have to make sure the dHW is not active whilst drawing larger volumes of hot water....

Questions:
Is this what experienced users / installers of dhW tanks with plate heat exchangers would expect (especially those using ashps with lower flow temps).

Is there any solution apart from ditching the preplumbed expensive tank with plate heat exchanger for a coil based dhw tank?

Thanks in advance

 

[MeldrewsMate]

A diagram, photos, and perhaps manufacturer and model would help to clear the crystal ball, however at a guess you could be describing a heat store being used with the ASHP, in which case there would be an external pump delivering stored heat transfer fluid from the cylinder to the plate heat exchanger whenever the DHW is ON; this will have the effect of cooling the stored contents over time unless the plate hex is very well insulated.
This is of course guesswork until the facts are known.

In any case, it's a warning to the user that long showers are wasteful. 3 mins max, less time for baldies.

 

[GoodDIYjob]

Thanks for the reply and your questions.

The ASHP is a Mitsubishi ecodan - PUHZ-W85VAA
The cylinder is a pre-plumbed mitsubishi packaged items complete with pumps (primary and secondary) and outboard plate heat exchanger. FTC5.

There is no heatstore / thermal store employed in the install. Only the heat pump and the cylinder.

 

[GoodDIYjob]

No takers? The Mitsubishi pre-plumbed cylinder schematic is above.

Some extra info.

So if using the dhw whilst dhw on constant or timed on whilst showering..:

Start temp, Duration, end temp
55C, 12min, 27C

If using the the dhw so it is off or timed off.

Start temp, Duration, end temp

53C, 18min, 10C


I have a theory and it comes down to thermodynamics and the heat up time required for the cylinder and the dhw pump throughput. But I don't want to totally suggest an answer before some educated and experienced comments?

Any feedback appreciated...

 

[MeldrewsMate]

Your data seems to show that a longer shower without the heat pump operating causes the stored water to be cooler - exactly as one would expect, though I wouldn't be hanging around in a shower at 10C.

Do you have any data that supports your claim that something isn't right?

 

[GoodDIYjob]

Your data seems to show that a longer shower without the heat pump operating causes the stored water to be cooler - exactly as one would expect, though I wouldn't be hanging around in a shower at 10C.

Do you have any data that supports your claim that something isn't right?

Thanks for the reply.

Lots of data. But is 12min with dhw active vs 18 min with it deactive. And a mfr recommendation to run dhw on constant not enough?

 

[MeldrewsMate]

...And a mfr recommendation to run dhw on constant not enough?

Firstly may I make a request for you to make your points clear and unambiguous. That statement posed as a question is hardly clear.

Secondly, you will find that an ASHP will struggle to achieve 55C flow temp, so your hot water is not going to be any hotter than this without the immersion heater supplementing it.

Thirdly, your data shows that the hot water runs hotter for longer when the ASHP is ON. This is entirely as expected.

 

[GoodDIYjob]

Firstly may I make a request for you to make your points clear and unambiguous. That statement posed as a question is hardly clear.

Secondly, you will find that an ASHP will struggle to achieve 55C flow temp, so your hot water is not going to be any hotter than this without the immersion heater supplementing it.

Thirdly, your data shows that the hot water runs hotter for longer when the ASHP is ON. This is entirely as expected.

Mitsubishi, in the owners manual suggest using their ftc5 control panel to use dhw to constant. What is not clear?

2. I'm not asking for any flow temperature greater than 50 or 55c. They shower at 38c.

3. The duration. Is shower duration before you cannot shower any longer at 38c. No it doesn't. The opposite is true.

 

[Heatpump DIY]

@GoodDIYjob Hi, did you ever get to the bottom of this or resolve it? I have the same query.

 

[GoodDIYjob]

I forgot I posted this! Yes. Sort of. Mitsubishi were completely unhelpful in explaining anything in terms of the operation anomaly. But, if the owners change to a timed setting, so that the dhw is not set to heat whilst they are in the showering window, for example 7am to 8am, the full tank can be used and they can have decent length showers.

What seems to happen with dhw on constant is the heating cycle and plate heat exchange operating churns the tank. Rather than a tank with nice hot at top, as you'd get with a coil, the rapid pump speed circulating the hotwater tank through the plate, blends hot and cold, so although you don't loose Hest energy, rather than half a tank with 55c hot water and rest of tank at 10c. You end up with a tank at nearer say 30c, which is no use to anyone until it completes heating phase. Seems like a big design compromise with the "high performance plate heat exchanger.

Hope that makes some sense. If not. Come back and ill try to explain differently.

The owners of the system were all set with getting lawyers at the ready until I told them to try timing the system so it is NOT , heating dhw for their showering window in morning. They've never really complained since

 

[Heatpump DIY]

@GoodDIYjob thanks for the reply. Its an interesting one and i agree, i dont think its right to have to time schedule off the plant. I wonder how the Mitsi tank works, does the pump suck from the bottom and push through the PHX to the top of the tank? If its in reverse that probably would just mix the tank up into a blended state. Also it could be the DHW circulation pump flow rate is too great. Maybe it needs to be reduced to give the plate a chance to heat the passing water?

 

[GoodDIYjob]

I agree, it's not their recommended setting - they "mitsubishi" could not explain the operation, or probably, more correctly refused to acknowledge it, even when the operation was explained to be nonsensical by 1) the installing engineer 2) the visiting mitsubishi warranty engineer and 3) the owners. They never addressed it, they'd come back with all operating as expected in all official written conplaints and would not explain the operation!!

but unless the friends I refer to who have this system, put up with a 40% reduction in shower time, it is a solution that almost totally rights the anomaly without any system alteration or extra spend, or legal battles against the manufacturer or installer.

Personally, i am convinced its a design anomaly now of a plate heat exchanger for dhw from ashp.

There are 2 x pumps. One for primary heat flow (from ASHP) to plate exchanger. One for dhw tank supply through plate heat exchanger.

They have to closely matched. They are pre set up based on mitsubishi design and engineering to be about 30lpm I believe. Which, in a 150l tank for example, will churn through the whole tank contents in 5 minutes flat. Clearly there will be insufficient heat passed to the tank water to lift to required temp in one pass (5 minutes). So whilst it continues to run at a rapid flow throughput it mixes the tank hot and cold contemts to a blended temp. Once the blended temp falls below 37c, shower appears to be going cold from a shower users perspective!