Indirect (Boiler Fed) Megaflo Doesn't Stop Heating

[Chuck]

Just a reminder that thermostats on unvented cylinders, such as Megaflos, must be serviced and repaired by someone who holds a current unvented hot-water systems ('G3') qualification.

 

[techweb]

Just a reminder that thermostats on unvented cylinders, such as Megaflos, must be serviced and repaired by someone who holds a current unvented hot-water systems ('G3') qualification.

Although the process of swapping it is straightforward, I will have my plumber who is suitably qualified firstly confirm my fault determination and then swap the stat.

 

[techweb]

And the display for heating is reading 70 c ?

Yes, although I took it back down after the test to what it was before, 65.

 

[ShaunCorbs]

Yes, although I took it back down after the test to what it was before, 65.

And you let it run for a couple of hours at 70 ?

 

[techweb]

And you let it run for a couple of hours at 70 ?

It never took more than 30-40 minutes to heat the tank and it had been running for 30 minutes when I increased the temperature for a further 15 minutes.

 

[John.g]

Well, i've eliminated the control valve or leak. If the stat fails, it may not tell the boiler shut off. I will adjust the stat down on the cylinder tomorrow but like I said, this hasn't been touched. I have my plumber coming to do some other work so will have him take a look and troubleshoot correctly.

Have you adjusted the stat??, with the boiler firing keep turning the stat down until the boiler stops firing, then turn it up slowly until the boiler refires, there should be ~ 6C to 10C difference (hysteresis) and you should also hear the stat clicking, if the stat has a index, note the two settings.

 

[techweb]

Have you adjusted the stat??, with the boiler firing keep turning the stat down until the boiler stops firing, then turn it up slowly until the boiler refires, there should be ~ 6C to 10C difference (hysteresis) and you should also hear the stat clicking, if the stat has a index, note the two settings.

Not on the Megaflo. I might give that go, I'm assuming this doesn't break any rules as I'm not G3 qualified but I see a thermostat as a user control.

 

[techweb]

@John.g @ShaunCorbs

So some more trouble shooting done this morning... I opened the cover on the thermostats for the Megaflo and the attached picture is what I have, indirect stat set at just over position 3 (this seems low). I reduced the temperature and after turning anticlockwise, I heard the 2 port diverter valve deactivate and the boiler ran for a few minutes on overrun until is shutoff. I then turned this setting back up to 3. The boiler fired back up but the cylinder stat did not shut off. After a while I increased boiler flow temp to 70 and then after a few minutes, once again I heard to 2 port diverter deactivate and the boiler shut down.

A few observations / thoughts here...

3 on the cylinder indirect stat seems very low but it is the same on the immersion. The water from the taps is very hot, you couldn't leave you hand under it for more than a few seconds, so it must be in excess of 60c. I know when using the immersion in the past that the water was also very hot. However, the recommendation for 60-65 from Megaflo is between 4 and 5.

I notice that although my Vailant boiler flow temp is set to 70, when the boiler stops heating the water but continues pumping, it lets the temperature drop to 54 before firing up again to bring the water back up to 70. Is this level of hysteresis typical?

Finally, unrelated to this problem, I noticed that an elbow on the copper pipe below the automatic air vent has a load of green verdigris on it. The rest of the pipe is free from this and it is insulated in foam. I took it off to feel the temp on the pipe back to the boiler. Is this anything to be concerned about? I can have my plumber take a look when I have him check the stat.

I hope that by doing what I've done today, I haven't broken any rules... After unscrewing the cover, it would be very easy to just swap this stat out myself but I believe that technically that would put in violation.

 

[John.g]

@John.g @ShaunCorbs

So some more trouble shooting done this morning... I opened the cover on the thermostats for the Megaflo and the attached picture is what I have, indirect stat set at just over position 3 (this seems low). I reduced the temperature and after turning anticlockwise, I heard the 2 port diverter valve deactivate and the boiler ran for a few minutes on overrun until is shutoff. I then turned this setting back up to 3. The boiler fired back up but the cylinder stat did not shut off. After a while I increased boiler flow temp to 70 and then after a few minutes, once again I heard to 2 port diverter deactivate and the boiler shut down.

A few observations / thoughts here...

3 on the cylinder indirect stat seems very low but it is the same on the immersion. The water from the taps is very hot, you couldn't leave you hand under it for more than a few seconds, so it must be in excess of 60c. I know when using the immersion in the past that the water was also very hot. However, the recommendation for 60-65 from Megaflo is between 4 and 5.

Most stats are like the Cotherm. below, 10C to 70C, so 4.5 would seem to be correct for a setting of 60C, and 3=40C, its strange how your setting, if original was set to 3. I presume your stat has a hi limit trip incorporated in that stat, what make is it?.

I notice that although my Vailant boiler flow temp is set to 70, when the boiler stops heating the water but continues pumping, it lets the temperature drop to 54 before firing up again to bring the water back up to 70. Is this level of hysteresis typical?

Thats normal when the cylinder temperature gets close to the flow temperature, the boiler cycles on/off as the coil cannot absorb the minimum boiler output, the circ pump continues to run but the burner cuts out at the SP temp+5C, the burner will then not refire until the anticycle time has elapsed.

Finally, unrelated to this problem, I noticed that an elbow on the copper pipe below the automatic air vent has a load of green verdigris on it. The rest of the pipe is free from this and it is insulated in foam. I took it off to feel the temp on the pipe back to the boiler. Is this anything to be concerned about? I can have my plumber take a look when I have him check the stat.

Yea, get that checked out.

I hope that by doing what I've done today, I haven't broken any rules... After unscrewing the cover, it would be very easy to just swap this stat out myself but I believe that technically that would put in violation.

Can't advice on swapping out but it is advisable IMO to renew it even though you can reduce the setting to give say 60C cylinder temperature.

 

[techweb]

Thank you.

My cylinder has two stats. The larger one at the bottom is for the immersion, set at 3 and the top one is the one for the indirect feed (boiler) which was set somewhere between 3 and 4.

Following this test, I put the indirect stat back at what it was and let the boiler run. It run for another 25 minutes at 70c and then the stat was satisfied. Water from the taps is hotter. Personally I think it's too hot. Anyway, I've ordered a thermometer off Amazon so I can check it and rule out any subjectivity.

I am tempted to swap out the stat anyway, they are inexpensive and it's operation seems to be suspect. What flow temperature should a system boiler ideally be set at? Is 70 the right setting anyway? We have 21 radiators on the system (not that they are in use now during summer). Boiler is in loft.

On the subject of the Verdigris on the pipe elbow. I'll ask the plumber to check it but there is no signs of leakage.

 

[John.g]

Is the one in the photo the indirect one and what is its make/model, what did you order??

 

[techweb]

Is the one in the photo the indirect one and what is its make/model, what did you order??

Although it's not that clear, if you look to the top right of the photo, you will see another screw driven thermostat. Can't see the make. If you look to the top left you can see the thermal cut out stat.

 

[techweb]

New stat on order and hopefully plumber comes next week to do it (even though I could do it myself). I'll have him check out the verdigris on pipe also @John.g @ShaunCorbs

I'll update after with results.

 

[techweb]

One thing I wondered, is whether where the stat is housed, ifr they can get scaled up so they don't detect the temperature so easily... i.e. the stat sees a lower temp than is correct. I checked the temperature of the water from a tap and it's 54c.

 

[John.g]

One thing I wondered, is whether where the stat is housed, ifr they can get scaled up so they don't detect the temperature so easily... i.e. the stat sees a lower temp than is correct. I checked the temperature of the water from a tap and it's 54c.

Its a possibility but only if scale/limestone is building up in the bottom, the lower stat pocket which is for the immersion? should be even more scaled if this is true so maybe switch this in overnight or today if not on "economy 7" or whatever its called now and measure the actual tap temperature when it cuts out. How far above the bottom of the cylinder coil is the cylinder coil stat located?, if the circulation through the coil is lowish then there could be a dT of 15C or more between the upper yliner temperature and the bottom. My (PT1000) coil probe is located ~ 1/4 way up the coil bottom and there is ~ 10C dT between the tap temp and the probe temp, tap temp is ~ 68/70C with a SP temp of 60C at the end of the reheat period.

 

[techweb]

Its a possibility but only if scale/limestone is building up in the bottom, the lower stat pocket which is for the immersion? should be even more scaled if this is true so maybe switch this in overnight or today if not on "economy 7" or whatever its called now and measure the actual tap temperature when it cuts out. How far above the bottom of the cylinder coil is the cylinder coil stat located?, if the circulation through the coil is lowish then there could be a dT of 15C or more between the upper yliner temperature and the bottom. My (PT1000) coil probe is located ~ 1/4 way up the coil bottom and there is ~ 10C dT between the tap temp and the probe temp, tap temp is ~ 68/70C with a SP temp of 60C at the end of the reheat period.

The coil(s) are located ~1/4 way up the cyclinder I would say.... Seems that tap temps are ~10c higher than stat temp.

 

[John.g]

Yes, seems fairly typical, how far above the bottom of the coil (where the coil return exits the side of the cylinder) do you reckon the probe is located?.

If this is typical then I wouldn't expect a very big dT but you can live with 10C anyway.

 

[techweb]

Yes, seems fairly typical, how far above the bottom of the coil (where the coil return exits the side of the cylinder) do you reckon the probe is located?.

If this is typical then I wouldn't expect a very big dT but you can live with 10C anyway.

View attachment 94760

Just a few centimeters, it's very similar to the picture above.

 

[techweb]

Thermostat changed... With the new one, it seems to need to be turned up a little higher to get the same result. But ultimately, there was nothing probably wrong with the old.

The pipe with verdigris I was told is just superficial corrosion and not to worry about it.