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

Joined
8 Jan 2014
Messages
297
Reaction score
23
Location
Surrey
Country
United Kingdom
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
 
Sponsored Links
Joined
27 Jan 2010
Messages
1,210
Reaction score
305
Location
Buckinghamshire, Darn Sarf
Country
United Kingdom
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.
 
Joined
8 Jan 2014
Messages
297
Reaction score
23
Location
Surrey
Country
United Kingdom
Cylinder diagram.jpg

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.

 
Last edited:
Joined
8 Jan 2014
Messages
297
Reaction score
23
Location
Surrey
Country
United Kingdom
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...
 
Sponsored Links
Joined
27 Jan 2010
Messages
1,210
Reaction score
305
Location
Buckinghamshire, Darn Sarf
Country
United Kingdom
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?
 
Joined
8 Jan 2014
Messages
297
Reaction score
23
Location
Surrey
Country
United Kingdom
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?
 
Joined
27 Jan 2010
Messages
1,210
Reaction score
305
Location
Buckinghamshire, Darn Sarf
Country
United Kingdom
...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.
 
Joined
8 Jan 2014
Messages
297
Reaction score
23
Location
Surrey
Country
United Kingdom
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.
 
Sponsored Links
Top