Bath fluctuating hot water

[robbo83]

This is what I did previously in post #8 with the video on youtube. My wife is at the bath saying how the water feels but it may be difficult to hear.

Here is how it goes:

The water feels hot at the start of the video and then at 33secs in you can hear the boiler click off and the waters stop circulating inside but the light on the front is still on. It then clicks on and off a few times over the following 30secs and then stays on but the temp drops from 88° down to 56°. The temp then increases again up to 78°-80°. During this time my wife still feels hot at the bath.

These are my wifes comments on how the water feels and when it starts going cold:

1min 09secs cold
1min 37secs luke warm
1min 41secs warm
1min 45secs warm
1min 51secs luke warm
1min 59 hot
2min 5secs hot
2min 12secs end of video

 

[Johntheo5]

Can you repeat that test, don't have to feel the temp now but note both the heat exchanger temperature i9 and the DHW temp i11. The DHW displayed temp will tell alot.

 

[robbo83]

I have just retested and recorded a video of it:

Start of video HEX 79C and DHW 50C

For the next 1m30s the HEX temp slowly increases from 80C to 88C and the DHW from 50C to 55C

At 1m 35s the boiler starts cycling off and on during which the DHW temp is jumping up and down by 5C steps between 30C and 45C.

I have also just noticed that the CH flow is getting hot when the DHW is on. Could it be the diverter is faulty?

 

[Johntheo5]

Ok fine thanks, will be home in a few minutes to do a few calcs, it could be the diverter (pretty sure its not though) but you will be able to confirm that if you just repeat those tests on the bathroom basin which runs steadily, note, again, i9 & i11 and just feel the actual HW temp say once duing this period and we can more or less pin down the problem IMO.

 

[robbo83]

With the basin hot on full the i9 HEX temp is stable around 67C-68C and the i11 DHW stable at 54C-55C. I can upload a video if required.

Thanks for your help with this, really appreciate it!

 

[Johntheo5]

Right, the basin flowrate of 4.6LPM in heating from 20C to 55C requires 11.23kw and the primary/DHW dT of 13C, 68C/55C, is what I would expect, even if a bit on the high side, now again to rule out anything strange at the boiler, run both the main bathroom basin (again) PLUS the kitchen sink as both of them at (4.6+4.8), 9.4LPM will be almost exactly the same as the bath mixer flowrate of 9.5LPM. and note i9 & i11 displayed temps again.

 

[robbo83]

OK so I did kitchen sink + bathroom basin and i9 and i11 temps were 80C-82C and 54C-55C. The issue didn't occur during a test duration of 2m30s. I also checked i6 which is the DHW flow rate in LPM and it was around 7.6LPM which is a bit lower than the bath mixer which usually shows around 8.6LPM.

But I also did a test with shower and bathroom basin and the issue seen with the bath mixer did occur during this test. The i9 temp was 83C-85C and i11 temp 54C-55C. The i6 DHW flow rate shows around 8.2LPM.
After around 1m 30s the boiler started cycling off and on and the i11 reading drops and is jumping in 5C increments between 30C and 45C, same behaviour as with the bath mixer.

 

[Johntheo5]

Thats very interesting, I know/knew that the boiler primary flowtemp increases with increasing DHW demand but also thought that the boiler firing rate would stop increasing when the boiler flowtemp reached a max of ~ 80C but it seems not the case, at least on this boiler. So one final test on the bath mixer only, reduce the flowrate from its present 9.6 to say 4.0 to 5.0 LPM, you can do this initially with the boiler switched off, then once you get this flowrate, fire up the boiler again and take another set of readings.

 

[robbo83]

With the bath mixer on hot but not fully open with the i6 reading showing 4.5LPM it is fine, it behaves like the basin, i9 at 67C-68C and i11 DHW stable at 54C-55C.

 

[Johntheo5]

Great, your perseverence has paid off, certainly now looks definitely like a boiler problem.
You can rule in/out two possible issues yourself.
Diverter Valve: If the heating system is zoned, manually latch open one motorized valve, run the boiler on DHW and after say 5/10 minutes see/feel the boiler flow pipe and rad flow pipe and see if they are getting hot, if so, diverter valve problem.
DHW temperature sensor: Get hold of a thermometer and measure the actual temperature of the water and compare it with the displayed DHW temperature i11, your "ear" thermometer may do if its scaled to 50C, reduce the DHW temp setpoint to say 45C and compare, its vital that you do measure the temperature.
If the above check out OK then more than likely a fouled PHEX, you could get a gas registered to remove and clean it but probably far better to just get it renewed in view of the labour cost.

In the mean time you will probably get away with running the bath mixer at full flow if you reduce the DHW SP to 50C.

 

[robbo83]

Diverter Valve: If the heating system is zoned, manually latch open one motorized valve, run the boiler on DHW and after say 5/10 minutes see/feel the boiler flow pipe and rad flow pipe and see if they are getting hot, if so, diverter valve problem.

Sorry didn't quite follow the above test. Which motorized valve are you referring to? When doing this test shall I do it with the bath mixer on or the basin?

I have ordered a meat thermometer to come tomorrow to do the temperature test.

Thanks again for all your help with this.

 

[Johntheo5]

Some houses have the upstairs/downstairs controlled by zoning and roomstats, if say the downstairs is programmed to come on in the morning then a timer tells a zone valve to open, this is a ball valve with a motorlzed actuator on top which opens the valve, you can manually open the valve by means of a lever on the side. You may have none of these controls, just run the DHW to the basin and feel the boiler flow pipe to the CH after 10 mins and see if it gets hot. Will post a photo of a motorized valve shortly, or just Google 2 port valve.

 

[robbo83]

Morning! No we don't have anything like that in the house, just a single thermostat in the hallway and all rads come on when it comes on.

So I ran the DHW on the basin and the CH flow pipe was warm but it was only on the first 3-4 inches are pipe, after that it wasn't warm. So obviously none of the rads were warm.

I also ran the shower which has a higher flow rate and the pipe felt hotter but again it was only the first 3-4 inches of pipe.

Sounds like it is probably not the diverter then as would expect more of the pipe get hotter?

Under normal circumstances should the CH flow pipe not change temp at all?

 

[robbo83]

Right, the basin flowrate of 4.6LPM in heating from 20C to 55C requires 11.23kw and the primary/DHW dT of 13C, 68C/55C, is what I would expect, even if a bit on the high side, now again to rule out anything strange at the boiler, run both the main bathroom basin (again) PLUS the kitchen sink as both of them at (4.6+4.8), 9.4LPM will be almost exactly the same as the bath mixer flowrate of 9.5LPM. and note i9 & i11 displayed temps again.

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.

 

[Madrab]

Sound like it could be the same issue that happens sometimes, when a shower will run hot and cold. Does the boiler cycle on and off when it's doing it and the CH isn't on?

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?