Posted by: @errmm

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So is this the norm for heatpumps that can only heat DHW to 50.  I know others with heatpumps who heat DHW once, presumably that system can heat to 55C and so even after two showers the water is acceptable?

It is the norm that heatpumps are set to heat DHW only to 50C, or even less.  Depending on the refrigerant (R410 vs R32 vs R290) they may be capable of going higher (up to 70 in the case of R290) but for efficiency they are usually set lower.

It is not the norm, assuming that the water tank is correctly sized, that the DHW is exhausted after 2 showers unless the individuals take very long showers.

Another thing to consider is the location fo the sensor in the tank.  If its low down it will see the cold water at the bottom whilst there is still plenty of hot water at the top.

To get to the bottom of this more info is needed

• How long are the showers, roughly how much water is used in each one?
• How big is the tank? (you mention tanks).  Is one tank DHW and the other a buffer tank, photos may clear that up.

This is a matter of correct design not the fact its a heat pump.  

If you provide more info then some more precise comments can be made.

Fully agree with @jamespa.

Just for context @errmm, what brand is the heat pump?

Just to add some numbers to my comments above, a 6ft high tank sounds like 300l at least.  300l at 50C will yield 400l at 40C when diluted with incoming water at 10C.   40C is a typical showering temperature.  At 20l/min. which is a very generously fast shower, this will last 20mins, which is 2 very generously long showers.  At a more reasonable 10l/min and say a 5 min shower thats 8 showers.  Of course I do know that some people insist on showering for 10mins at 20l/min although I have no idea why!

Hopefully that gives some context, if you provide the additional info perhaps it can be rationalised.

@jamespa, I can add a bit of real-world context from our own setup, which might help frame this a bit. We have a single 300L cylinder and we only heat ours to 45C. After my wife and I have our morning showers (roughly 10 minutes each) the reported cylinder temperature will often drop to around 35C. With a 10C hysteresis, that’s enough to trigger a reheat, even though subjectively the showers themselves have been perfectly fine. We eventually set our hysteresis to 15C, because there was still plenty of hot water.

Our shower head is around 10 L/min, so nothing particularly excessive.

That experience broadly aligns with your maths, but it also highlights how much the reported 'tank temperature' depends on sensor position and stratification rather than usable hot water at the outlet. A sensor lower down the cylinder will see the cold slug very quickly, even when there’s still plenty of hotter water sitting above it. Ours is slap bang in the middle.

So I’d be very reluctant to accept 'that’s just how heat pumps are' as an explanation here. Limiting DHW to 45-50C for efficiency reasons is normal. Rapid apparent temperature collapse after modest use is not. This definitely feels much more like a system design, configuration or control problem than a fundamental limitation of the heat pump itself.

Hi 

Attached is picture of the tanks.  It's a heliotherm. 

I've spoken with the installer and they mentioned flow rates on showers as well. 

Mike 

@errmm, wow, since it's a Heliotherm there should be zero issues with hitting target temperatures.

Images didn't insert. Please try again.

@errmm Difficult to tell from the photo what's going on, but those are at least 300l possibly 400.  Need a photo where you can see the pipework and whole tank to be sure if one is dhw and the other a buffer.

The most likely, but by no means the only, explanations are

1 The individual really is taking 10 min showers at 20l/min - need to ask the question 

2 The sensor is half or even 2/3rds of the way down the tank so shows it cooling well before the hot water in the tank is exhausted.- need to ask if you can continue running off hot water even though it says it needs a reheat.

Before we look any further we need to rule these two obvious ones out imho.

Hi,

The left is the buffer tank, the right is DHW.  It's right is one of these https://www.heliotherm.com/en/fresh-hot-water-system/fresh-hot-water-system.html   It says it's fresh pure hot water on demand.  So I don't don't understand that description along with a well two showers and it's luke warm....

Regarding sensor position, the person themselves is the one who said the water was lukewarm after the showers.  It's not the case that they're looking at this data.  It's me looking at it and going ok, that seems to be correctly reflecting what they are saying.  I will ask about shower habits just in case though, just to rule it out.

Mike

@errmm is your friend’s house that you’re helping a large property?

Posted by: @errmm

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Hi,

The left is the buffer tank, the right is DHW.  It's right is one of these https://www.heliotherm.com/en/fresh-hot-water-system/fresh-hot-water-system.html   It says it's fresh pure hot water on demand.  So I don't don't understand that description along with a well two showers and it's luke warm....

Regarding sensor position, the person themselves is the one who said the water was lukewarm after the showers.  It's not the case that they're looking at this data.  It's me looking at it and going ok, that seems to be correctly reflecting what they are saying.  I will ask about shower habits just in case though, just to rule it out.

 

Mike

 

 

 

From the website I think it's a heat store not a conventional dhw tank.  Not a good idea with an ashp and could very likely explain what is happening.

PS. I read it a second time, it is a heat store.  Interesting though that they sell it to work with an ashp.  Would need to examine the specs and operating instructions to understand how they think it should work.