Shower Water Getting Colder During Shower (linked to boiler?)

[mtaylor39]

I've got a situation where a Mira Thermostatic shower starts off at the right temperature but during the course of it being on it gets cooler. Not cold, but gradually gets colder. If you turn the shower temperature up it gets hotter, but then the temperature starts to reduce again. I get probably 2 minutes before it gets cooler and then 1 minute every time I turn it up.

The thermostatic shower has been replaced and it's still doing it, and probably getting a bit worse. The taps in the bathroom, kitchen and utility room seem fine, but I think the bath tap is also doing something similar. I think this is because the flow of the water is greater for the shower and bath compared to the taps.

The boiler is a Viessmann Vitodens 111-w 32kw storage combi and was installed in November 2022, and the problem has only been in the last month or so. It's under warranty, but Viessmann are saying it's nothing to do with the boiler, but the system itself. As the shower has been replaced I find it hard to believe it would be the pipes. I feel the issue could be a diverter value, heat exchanger or something with the storage element (thermometer?). But wondered if any of the people on this forum could suggest what it could be? Are Viessmann just trying to get out of the warranty obligations?

Happy to provide any more info if needed.

Thanks!!

 

[Johntheo5]

Maybe do a few HW only tap tests for a start.
The (my) mains temp is 18C just now, suggest setting the DHW temp to 60C and open one or more HW taps only to give a measured flowrate of exactly 10.5LPM, this requires a boiler output of 30.77kW which your boiler should output continuously without any fall off in temperature.

Then maybe set the flowrate to exactly 12.0LPM, the (stored) boiler should give this flowrate at 60C for a few minutes until the store is exhausted,
I'm not sure what happens then, whether the boiler will supply 12.0LPM at 56.2C at 32kW or whether it replenishes the store and only gives lukewarm water until this occurs and then reverts to 60C for a limited time again.

 

[mtaylor39]

Thank you for replying. I appreciate your suggestion, but what I don't understand is why I would need to do this, when it's been fine on the flow rate for the las 2 1/2 years. The change in the Thermostatic shower was only because it was leaking and it was replaced like for like. So the temperature and flow has been the same for the last 2 1/2 years.

 

[Johntheo5]

I think you should try and rule the boiler in or out of the problem.
One relatively easy way to determine if the boiler is producing its full output is to open all HW taps fully, after say 5 minutes and assuming the boiler is firing, take a photo of the gas meter and exactly 3 minutes later, to the very second, take another photo, gas consumption, kWh/hr is diff in readings X 11.0 X 20, would expect ~ 35/36 kWh.
You might also, separately, measure the actual shower flowrate in LPM.

 

[mtaylor39]

Thanks - I will give this a go.

Something else to add:
I should have said that both the shower and boiler have been in place since 2023 and this is a new issue that is happening. So it's been fine until recently.

I've also notice that after myself and my wife has showers one after each other and then I used the taps on the sink the hot water from the taps was lukewarm too. I left it a few minutes and the water was hot again. It's like I've drained the tank and it can't keep up, but this hasn't been an issue previously. Could it be the heat exchanger being less effective?

Does this change any of your thoughts?

 

[Johntheo5]

No, hasn't changed my thoughts.
A heat exchanger won't become fouled/"less efficient" overnight but could be fouled now which that test above should tell something.
This boiler has a 46L DHW storage cylinder.
A 46L (any) HW cylinder if heated to 60C from a present mains temp of 18C should give almost 88L at 40C before running out of hot water so two showers, say 44L/shower, is only 4.4mins/shower at a flowrate of 10LPM but it doesn't work that way IMO, as soon as you start drawing off HW then the boiler should fire up and keep replinishing the storage cylinder, as long as the flow demand is < 20.85LPM (water from 18C to 40C) then you should get a endless supply of HW, exactly the same as if you had a non storage 32kW combi, if the demand is > than this then eventually the storage cylinder will becone exhausted and has to be recharged. As I said above, will the HW temp return to 40C if the flowrate is/was now reduced to < 20.85LPM?, your experience would say not because opening a hot tap is scarcely going to flow 20LPM or so.
If you then turn off all HW demand and assuming the boiler/Phex is producing/delivering 32kW then the 46L storage cylinder will require 2.25kWh and will be fully recharged to 60C in 4.2 minutes, the recharging temperature may be 65C or even higher but still wouldn't expect longer than 5 to 7 minutes to recharge.

So, as well as the suggested tests above, listen or get someone to listen for the boiler starting up very soon after you start drawing off HW, also when finished drawing off a lot of HW after say a shower or two, ensure the boiler continues to fire for another 5 minutes or so before shutting down.

 

[mcmoby69]

Just one bathroom or two?
Possibility of a non return valve stuck open in the other bathroom. Causing cold water to mix with the hot water

 

[Johntheo5]

Viessmann state that a flowrate of 18.1LPM is possible at a dT of 35C. I think the 32kW Viessmann Storage combi has a output of 33.9kW to DHW but assume 32kW to be on the safe side. I can’t find a schematic of a Viessmann or any other storage combi but I imagine it must operate like I think it does, below.
A conventional combi on DHW just circulates primary “boiler” water around one side of a PHEX and cold mains flows through the other, secondary side of the PHEX and goes off to the user. A (Viessmann) storage combi still circulates primary water around one side of the PHEX but the cold water enters the bottom of the 46L storage cylinder and HW exits from or close to the top of the cylinder, another pump, called the cylinder loading pump takes cold water from the bottom of the cylinder and pumps it through the secondary side of the PHEX and delivers it to the top of the cylinder, a cylinder temperature sensor, probably located low down on the cylinder, starts and stops this pump at a setpoint cylinder temp of 60C or maybe doesn’t stop it until 65C to avoid the cylinder loading pump & boiler starting & stopping each time the cylinder reaches 60C. When the 46L of stored water is (if) used up the combi should then, IMO, operate just like a conventional combi, when HW usage ceases then the cylinder loading pump will just circulate water until the cylinder reaches 60 or 65C and then the boiler will cut out after ~ 5/6 minutes if the mains is as low as winter 5C. If the HW flowrate is 18.1LPM with a dT of 35C then the total power required is, 18.1*60*35/860, 44.2kW, 32.0kW from the boiler so the cylinder must and will supply 12.2kW until the stored HW runs out, 46L of stored water at 60C (from cold at 5C) will give up, 46*(60-5)/860, 2.94kWH and theoretically should last for, 2.94/12.2*60, 14.5 minutes after which the HW temp will fall to 5+(32*860/60/18.1), 30.3C and the flowrate will have to be reduced to, 32*860/60/35, 13.1LPM to allow the HW to again reach 40C.

In the Ops case, with a present mains temp of 18C then the flowrate can be as high as, 32*860/60/(40-18), 20.85LPM at 40C even with a exhausted cylinder. So, because the HW is delivered to the top of the cylinder and he states that a hot tap, (which will scarely run at 20.85LPM) only runs lukewarm after the showers have possibly exhausted the cylinder, then, IMO, points to some boiler fault, maybe a cylinder temperature probe or the like, a few simple tests should narrow it down?.

 

[mtaylor39]

Viessmann state that a flowrate of 18.1LPM is possible at a dT of 35C. I think the 32kW Viessmann Storage combi has a output of 33.9kW to DHW but assume 32kW to be on the safe side. I can’t find a schematic of a Viessmann or any other storage combi but I imagine it must operate like I think it does, below.
A conventional combi on DHW just circulates primary “boiler” water around one side of a PHEX and cold mains flows through the other, secondary side of the PHEX and goes off to the user. A (Viessmann) storage combi still circulates primary water around one side of the PHEX but the cold water enters the bottom of the 46L storage cylinder and HW exits from or close to the top of the cylinder, another pump, called the cylinder loading pump takes cold water from the bottom of the cylinder and pumps it through the secondary side of the PHEX and delivers it to the top of the cylinder, a cylinder temperature sensor, probably located low down on the cylinder, starts and stops this pump at a setpoint cylinder temp of 60C or maybe doesn’t stop it until 65C to avoid the cylinder loading pump & boiler starting & stopping each time the cylinder reaches 60C. When the 46L of stored water is (if) used up the combi should then, IMO, operate just like a conventional combi, when HW usage ceases then the cylinder loading pump will just circulate water until the cylinder reaches 60 or 65C and then the boiler will cut out after ~ 5/6 minutes if the mains is as low as winter 5C. If the HW flowrate is 18.1LPM with a dT of 35C then the total power required is, 18.1*60*35/860, 44.2kW, 32.0kW from the boiler so the cylinder must and will supply 12.2kW until the stored HW runs out, 46L of stored water at 60C (from cold at 5C) will give up, 46*(60-5)/860, 2.94kWH and theoretically should last for, 2.94/12.2*60, 14.5 minutes after which the HW temp will fall to 5+(32*860/60/18.1), 30.3C and the flowrate will have to be reduced to, 32*860/60/35, 13.1LPM to allow the HW to again reach 40C.

In the Ops case, with a present mains temp of 18C then the flowrate can be as high as, 32*860/60/(40-18), 20.85LPM at 40C even with a exhausted cylinder. So, because the HW is delivered to the top of the cylinder and he states that a hot tap, (which will scarely run at 20.85LPM) only runs lukewarm after the showers have possibly exhausted the cylinder, then, IMO, points to some boiler fault, maybe a cylinder temperature probe or the like, a few simple tests should narrow it down?.

Thanks for your help on this. However, it turns out it was a blocked heat exchanger after all. Once that was cleared out everything is working like normal again.

 

[dilalio]

Thanks for your help on this. However, it turns out it was a blocked heat exchanger after all. Once that was cleared out everything is working like normal again.

However, it's important to note that testing is used to indicate boiler or mixer fault when it comes to mains systems.