DHW not coming to temperature on new boiler - cycles down every 45 seconds

[misscookie]

Hello!

I have a Viessmann Vitodens 050-W 25kw boiler which was installed in the summer and have had the same trouble with it ever since. Aside from one of the radiators no longer working, which the plumber suggests is unrelated, the central heating is otherwise fine. But the upstairs hot water has not functioned correctly since installation.

The hot water runs fine downstairs - it reaches temperature quickly and maintains a steady temperature throughout the duration of running the hot tap.

Upstairs, all of our hot water fixtures - mixer taps, non-mixer taps and showers - have the same issue. When running the hot water it never reaches the correct temperature. It gets to around 10 degrees lower than the set temperature, then the boiler sounds like it powers down, the temperature drops a further 5-10 degrees, then the boiler powers up again as though it is cycling round. This happens around every 45 seconds. It does this constantly, no matter how long you run the shower or taps. I don’t think the gas ignition is turning off, it’s more of a slowing down sound than powering off completely.

In the summer it wasn't so bad, but now that the air is colder and, I guess because the water is colder in the pipes, it is very frustrating, especially in the shower as it's very uncomfortable.

The boiler is set to ECO mode for the hot water and the temperature is set at 50. Raising the temperature has no effect. Changing to COMFORT mode has no effect other than the water being warmer initially.

The water pressure upstairs has been noticeably lower since the new boiler installation but the pressure readouts on the boiler seem normal - between 1-2.

We can see the exact temperature of the water output going up and down in the Vicare app. I've made videos of this during the running of a shower/bath, as well as videos of the boiler itself to show the noise it makes while cycling up & down, and have sent them to both the plumber and Viessmann but they haven't provided any answers. The plumber is coming back at the end of the week.

We have 11 radiators, two showers and one bath.

Could this have anything to do with the flow rate of the boiler and if so, can this be adjusted? Is this something I could try to adjust myself?

Any ideas would be helpful, thank you!

 

[needyahelp]

One of the experts on here may be able to give a better view but it does sound like you may have a water pressure issue meaning theres not enough water pressure to push water out of the upstairs taps. As you say its only happened since install (and assuming youve not changed taps etc upstairs at the same time) your engineer should be able to confirm causes and what solutions are possible. Worth being aware that the boiler pressure visible in the vicare app (and the little dial under the boiler) is the pressure inside the central heating (ie the water going round and round in your radiators) not the pressure of water for your taps. Hth

 

[Johntheo5]

Can you measure the flowrate from a upstairs HW, non mixer tap only, run the water into a container for exactly 30 secs then measure this into a 1 liter container, X2 to give you the flowrate in LPM. If its more than 2.5 to 3LPM then that will keep the boiler flow switch satisfied and you should get continuous HW, if the HW temperature is set to 60C then 25kw will only give around a flowrate of 6.7LPM if higher then the flowtemp will start falling.
You may also have a problem with a mixer causing cold water to x over to the hot side preventing even the minimum flow from the boiler.

 

[misscookie]

Thank you both very much. I should have mentioned yesterday I followed a tutorial on measuring the flow rate out of some of the upstairs taps and came out with a whopping 15 LPM! It is fairly fast… but was even faster before the new boiler. None of the taps have been changed - only the boiler. The up & down in temp happens on all taps including non-mixer taps.

 

[Madrab]

I presume this is the combi variant of this boiler?

If you are getting consistent HW downstairs and 15L/Min (cold?) upstairs then the boiler sounds like it's running ok. If so then the HW water system upstairs needs worked backwards to see where the problems starts. You need an experienced plumber in to work backwards through the HW system starting at the outlets, it could get messy though if they need to get into the pipework runs, depending on where they are.

 

[Johntheo5]

Looks like something with the u/stairs plumbing or mixers alright.
a hot flow of 15LPM though will only result in HW temp of ~ 31C but it shouldn't be running hot/cold, just run the hot tap only at a reduced flowrate and see does it stay consistently hot.
Boiler should be under warranty? but don't thinkthe problem is here.

 

[misscookie]

Yes it’s a combi boiler and still under warranty - it’s only been in a few months and we never had this issue with the previous boiler. It started immediately with the new one.

From the DHW sensor readouts on the app, the water never actually leaves the boiler at temperature - it only ever gets to around 10 degrees below the set temperature and then drops down from there. Then kicks up again, then drops down again. So how could this be a fault in the upstairs plumbing when the water isn’t being sent up there hot enough to begin with?

 

[Johntheo5]

That certainly looks like the burner is dropping out the refiring again.
You might do that suggested test above, ie reduced upstairs HW flow and monitor the boiler flow temperature (this not the HW temperature) on your app or at the boiler.
and also run it as it is now and again monitor the burner flow temperature.

 

[misscookie]

I’ve just gone round all the upstairs hot taps running each very slowly for a couple of minutes. At this very slow rate the boiler does actually reach temperature and the water stays consistently hot. On our main shower turned down to a very low flow it also stays more or less consistently warm (to the touch it feels cooler than the bath/sink taps). But turn it up to a flow rate you might actually want to shower under and it starts fluctuating again - the boiler drops right down and starts cycling up & down. What does this indicate?

 

[Johntheo5]

It could mean a few things, one is that the plate heat exchanger is partially fouled (unlikely in a new boiler?) if fouled then the boiler flow temperature (the one you should try and monitor) will exceed 85/90C and trip the burner, of course the same thing should happen downstairs IF the flowrate reaches the same rate as upstairs.

 

[misscookie]

That does not appear to be happening as the flow temp never gets above 50. It just seems that the new boiler can’t cope with the speed of the flow rate upstairs when the taps are turned up. Is there no way of adjusting the speed of the flow rate on the boiler so that it better copes with the speed of the flow rate of the water in my upstairs taps?

 

[Madrab]

So how could this be a fault in the upstairs plumbing when the water isn’t being sent up there hot enough to begin with?

Because the boilers fires without issue when delivering the HW downstairs. If it didn't then yes, look to the boiler.

It just seems that the new boiler can’t cope with the speed of the flow rate upstairs when the taps are turned up

The boiler has a flow limiter that restricts the CW flow through the plate heat exchanger so flow shouldn't be an issue and the boiler should keep firing at high rate. If the flow temp isn't getting too high but it's still cycling/modulating then it isn't an overheating problem therefore we're back to a demand issue and that brings us back to the outlets where the issue is being seen.

 

[Johntheo5]

I think it would be prudent to rule out that its NOT the boiler by running, even if it takes a few downstairs outlets to get that "high" flowrate similiar to the upstairs ones, if the boiler doesnt then give erratic HW temperatures then it must be something wrong upstairs?.

 

[Madrab]

I think it would be prudent to rule out that its NOT the boiler by running, even if it takes a few downstairs outlets to get that "high" flowrate similiar to the upstairs ones

Whist I agree that the approach of re-creating downstairs, what happens upstairs, I would be very surprised if the flow upstairs would be any greater than it is downstairs and if it is then something funny is going on with the plumbing. The flow should be similar, no matter where is the house its run, especially if it's all mains.