Bath fluctuating hot water

[Johntheo5]

Also I wanted to ask how you calculated the dT for primary/DHW? I am interested to know what the HEX temp should when the bath mixer is on. At the moment it is going up to 88C to heat DHW of 9.6LPM @ 55C which seems too high as the manual says 86C is the max possible temp and for DHW it should be capable of 13LPM @ 40C rise.

You supplied the readings, at 4.5LPM the hex temp is 67/68C and the DHW tem is 54/55C, avg dT, 67.5-54.5, 13C, (boiler output calculated 10.83kw)
at 8.2LPM the hex temp is 83/85C and the DHW temp is 54/55C, avg dT, 84.0-54.5, 29.5C, (boiler output calculated 19.74kw)
"it should be capable of 13LPM @ 40C rise." calculated boiler output, 13*60*40/860, 36.28KW which (36.0kw) is the rated output of the boiler when producing DHW.
As stated previously, since the primary water flow is constant then the boier flowtemp has to rise to give increased hex/DHW dT so the hex (and boiler) output increases but if the PHEX is fouled then you just won't get the performance unless the the primary flowtemp rises to unacceptably high temperature, but as stated its essential that you actually measure the DHW outlet temp to confirm the displayed reading.
The very max constant output is 19.74kw (54.8% of the rated outout), you will get a improvement if you drop the DHW setpoint temp but thats not a fix for the problem.

There may be some way in the boiler parameters that you can limit the primary flowrate temperature to say 82C (while on DHW), at least then the boiler will not cycle untill you address the problem.

 

[robbo83]

If the boiler is cycling then the plate heat exchanger(hex) may be restricted and therefore the flow through the shower doesn't allows enough heat to be transferred quickly enough and the boiler then overheats and cuts out. It then cycles on and off as the temp rises and falls. The flow through the other hot taps is high enough for the plate to lose enough heat and the system doesn't overheat so symptoms do not occur. It's a well known issue and symptom when plate HEX's get fouled up.

Are you in a hard water area and how is the system water health, is it clean?

Yes it does seem that it is the plate HEX, my only doubt was the CH flow pipe getting hot.
Yes we live in a hard water area. The system water is actually pretty clean now but it has been drained a few times since we moved in 2 years ago. The first time I drained it down it was pretty dirty. Thinking back I now suspect the issue has been brewing for a while because when we had the bathroom done a year ago we had the exact same issue with the bath mixer going hot and cold. The plumber who did bathroom said the mains pressure was too high for the boiler and it wasn't capable enough to heat the demand from the bath which is why we had a PRV fitted on the mains. Once that was installed it was working fine but it was probably because the flow was lower so the plate HEX could cope? Now it seems to have got worse as it is struggling with 8.6LPM now.

You supplied the readings, at 4.5LPM the hex temp is 67/68C and the DHW tem is 54/55C, avg dT, 67.5-54.5, 13C, (boiler output calculated 10.83kw)
at 8.2LPM the hex temp is 83/85C and the DHW temp is 54/55C, avg dT, 84.0-54.5, 29.5C, (boiler output calculated 19.74kw)
"it should be capable of 13LPM @ 40C rise." calculated boiler output, 13*60*40/860, 36.28KW which (36.0kw) is the rated output of the boiler when producing DHW.
As stated previously, since the primary water flow is constant then the boier flowtemp has to rise to give increased hex/DHW dT so the hex (and boiler) output increases but if the PHEX is fouled then you just won't get the performance unless the the primary flowtemp rises to unacceptably high temperature, but as stated its essential that you actually measure the DHW outlet temp to confirm the displayed reading.
The very max constant output is 19.74kw (54.8% of the rated outout), you will get a improvement if you drop the DHW setpoint temp but thats not a fix for the problem.

There may be some way in the boiler parameters that you can limit the primary flowrate temperature to say 82C (while on DHW), at least then the boiler will not cycle untill you address the problem.

Yes sorry I understand the 13C was from my readings but I wondered if there is a way to calculate what dT if the plate HEX was not faulty? When I get the plate HEX replaced I assume at 8.2LPM then the hex temp won't be as high as 83C-85C? Just wondered if there is a way to calculate what the HEX temp should be under normal conditions.

I am just waiting for the thermometer to arrive, once here I will carry out the temperature readings. After that I will drop the DHW setpoint as suggested until the plate HEX is replaced.

So just to confirm, you don't think I need to replace the diverter?

Annoyed the plumber didn't diagnose this issue properly!

BTW is it normal for a plumber to phone Worcester for help when diagnosing an issue? Seemed odd to me?

Again, thanks for all your, greatly appreciated!

 

[Madrab]

Yes it does seem that it is the plate HEX, my only doubt was the CH flow pipe getting hot.
Yes we live in a hard water area. The system water is actually pretty clean now but it has been drained a few times since we moved in 2 years ago. The first time I drained it down it was pretty dirty. Thinking back I now suspect the issue has been brewing for a while because when we had the bathroom done a year ago we had the exact same issue with the bath mixer going hot and cold. The plumber who did bathroom said the mains pressure was too high for the boiler and it wasn't capable enough to heat the demand from the bath which is why we had a PRV fitted on the mains. Once that was installed it was working fine but it was probably because the flow was lower so the plate HEX could cope? Now it seems to have got worse as it is struggling with 8.6LPM now.

The first few inches of the CH flow pipe can get warm by convection, especially if the diverter is getting so hot that the systems is overheating. if there is no flow out to the rads then chances are the diverter isn't passing, that is if there is 'demand' at at least one rad's TRV/stat.

If the issue has become progressively worse then it can only be down to something in the chain becoming progressively worse (cause and effect). If the shower has been checked and it's clean then that's ruled out, pipework will rarely cause these symptoms so it has to come back to the boiler. If the boiler is overheating and then cycling then that is an atypical symptom of the system not being able to get rid of the heat it's generating, through the Plate, quickly enough. Normally the system would regulate it's heat input until it balances out with the output demand but if it can't transfer that heat fast enough then as it can't regulate correctly, it overshoots/heats and then shuts down, then when the HW temp drops enough it fires back up and so the cycle repeats.
If everything else is ruled out then the prime suspect would be a restricted Plate, more so if the system is/has been dirty on the input side and compounded by hard water scaling the plate up on the output side. Doesn't take much to at least have the Plate looked at.

A boiler engineer would usually call WB if they aren't familiar with the boiler or they've run out of fault finding ideas but the symptoms being experienced are quite common so shouldn't need a call to tech unless he's had the plate out and confirmed that's ok.

 

[Johntheo5]

Yes it does seem that it is the plate HEX, my only doubt was the CH flow pipe getting hot.
Yes we live in a hard water area. The system water is actually pretty clean now but it has been drained a few times since we moved in 2 years ago. The first time I drained it down it was pretty dirty. Thinking back I now suspect the issue has been brewing for a while because when we had the bathroom done a year ago we had the exact same issue with the bath mixer going hot and cold. The plumber who did bathroom said the mains pressure was too high for the boiler and it wasn't capable enough to heat the demand from the bath which is why we had a PRV fitted on the mains. Once that was installed it was working fine but it was probably because the flow was lower so the plate HEX could cope? Now it seems to have got worse as it is struggling with 8.6LPM now.



Yes sorry I understand the 13C was from my readings but I wondered if there is a way to calculate what dT if the plate HEX was not faulty? When I get the plate HEX replaced I assume at 8.2LPM then the hex temp won't be as high as 83C-85C? Just wondered if there is a way to calculate what the HEX temp should be under normal conditions.

No, the hex temperature definitely wont be as high with a new Hx but can't calculate it.

I am just waiting for the thermometer to arrive, once here I will carry out the temperature readings. After that I will drop the DHW setpoint as suggested until the plate HEX is replaced.

So just to confirm, you don't think I need to replace the diverter?

You can check that out yourself, first see if there is some form of CH control system like the zone valves shown above, if not you may have TRVs (thermostatic rad valves on all rads), if so just turn a few of these fully anticlockwise (open) and fire the boiler on constant DHW output to say the bathroom basin and see if any of the rads are getting hot, if so you need a new diverter valve.

Annoyed the plumber didn't diagnose this issue properly!

IMO that wasn,t that easy to diagnose,

BTW is it normal for a plumber to phone Worcester for help when diagnosing an issue? Seemed odd to me?

He was perfectly right in contacting them IMO.

Again, thanks for all your, greatly appreciated!

 

[Johntheo5]

Again re the hex/DHW dT, there are probably a few people reading this who, if their boiler parameters allow them to read the hx flowtemp etc, might care to post while running the DHW at 55C.

Should have asked you if you can read the primary return temp when on DHW?. would be able to calculate the primary flowrate then.

 

[robbo83]

Thank you both for the replies, appreciate it, this has certainly been a learning experience !

Thermometer arrived and I have just done the temperature readings:

DHW setpoint @55C the DHW actual was 55C and temp measured at basin as 53C
DHW setpoint @45C the DHW actual reading at boiler was 45.5C and temp measured at basin as 44C

Seems normal?

 

[robbo83]

Again re the hex/DHW dT, there are probably a few people reading this who, if their boiler parameters allow them to read the hx flowtemp etc, might care to post while running the DHW at 55C.

Should have asked you if you can read the primary return temp when on DHW?. would be able to calculate the primary flowrate then.

When running the basin hot tap with DHW setpoint @ 55C I get the following:

i9 (HEX flow temp) 68C
i11 (DHW flow temp) 55C
i14 (return temp to main HEX) 50C

Should i14 not be lower than that as the difference between i9 and i14 is only 18C but the temp rise on the DHW is 36C (my mains cold is 19C)?

 

[Johntheo5]

No, the DHW flowrate is 4.5LPM with a dT of 36C so the flowrate through the Hx at a dT of 18C must be 4.5*36/18, 9.0LPM. this is a very low flowrate IMO, should be ~ double that, (which would give much lower primary flow&return temperatures), could be caused by a partially blocked primary side PHEX or, if installed (like on some boilers) a blocked canue filter as they are called which are installed on the PHEX flow. WB might advise. Could also be caused by the circ pump if partially sludged up. Is your CH performing OK, normally with the rad valves fully open would expect ~ 10/12C difference between the flow and return temps. Is there any info on the pump in the parameters?. sometimes the pump speed can be set from the menu.

 

[Johntheo5]

This will tell you the % pump speed.

 

[robbo83]

No, the DHW flowrate is 4.5LPM with a dT of 36C so the flowrate through the Hx at a dT of 18C must be 4.5*36/18, 9.0LPM. this is a very low flowrate IMO, should be ~ double that, (which would give much lower primary flow&return temperatures), could be caused by a partially blocked primary side PHEX or, if installed (like on some boilers) a blocked canue filter as they are called which are installed on the PHEX flow. WB might advise. Could also be caused by the circ pump if partially sludged up. Is your CH performing OK, normally with the rad valves fully open would expect ~ 10/12C difference between the flow and return temps. Is there any info on the pump in the parameters?. sometimes the pump speed can be set from the menu.

Thanks for that I follow now. This boiler doesn't have a canoe filter. With the basin on hot the i16 reading for pump modulation was 100%.

I also noticed on i2 which shows the last error it reports 277 E9 which is 'Safety sensor temperature exceeded 95°', so another data point that it is overheating.

 

[Johntheo5]

There is a filter on the mains inlet to the PHEX, no harm to get that cleaned if changing/cleaning the PHEX.
What kind of indicated flow i6 and temps i9, i11 & i14 are you getting at a DHW setpoint of 45C if you keep opening hot mixers until the hx temp i9 reaches say 80C?

 

[robbo83]

I did a test at with DHW setpoint at 45C and get the following readings:

i6 9.2LPM
i9 71C-72C
i11 44C-45C
i14 42-43.5C

It doesn't reach the point where it cycles off with the setpoint at 45C.

 

[Johntheo5]

That gives a boiler output of 16.37kw. The hx temp was 82/83c some posts back with a boiler output of 19.7kw which is its ~ max at the moment to maintain a steady output without tripping, this requires a DHW flowrate of > 11.0LPM at 45C, you might try and increase the DHW flowrate to achieve this and also include pump speed i16 and a i10 with is the actual hx temperature and a good indication of any main hx fouling.
i10 is the safety temperature sensor mentioned in post #55.

 

[robbo83]

Reporting back as we have now had the plate heat exchanger replaced and boiler is working great and bath hot water stays consistently hot now! Hurray!

With the DHW setpoint at 55C I am getting the following readings:

i6 9.6LPM-10.2LPM
i9 64C-65C
i11 54C-55CC
i14 49C-50C

Previously i9 which is the temp from main heat exchanger was getting up to 88C so now much more sensible temps.

Just wanted to thank everyone on this thread for their help in figuring out the issue, especially Johntheo5 with all the suggestions on tests & investigation, it really is appreciated!

Cheers!

 

[old duffer]

Below is a photo of the mixer. It is a Grohe 33963000 Concealed Mixer. It's not thermostatic. I took it all apart and there wasn't much crap in there just a few tiny bits of black stuff.


View attachment 311288

View attachment 311289View attachment 311290

I replaced all of the showers in the house with these simple mixers. Cartridges always fail and can be very expensive. I use a continuos water heater for the showers so we tell it what temperature it should be and it will never go higher or lower (flow rate might change)