Unvented cylinder not reaching temp when running on boiler

[Joseph83]

Hi.

I've got a Pullin Evolution 250l indirect unvented cylinder, connected to a Vaillant ecotec plus 637 system boiler.

It heats from the gas boiler normally, but also has an electric immersion for emergencies.

My boiler flow temp is set to 75, the HW dial on the boiler is set to 70, the stat on the cylinder is set to 70.

But when the boiler heats the water in the cylinder, it only heats the water to 53.9 (measured at the tap with a thermometer). When the immersion heats it, it gets to 70 measured the exact same way. Because I do get 70 at the tap when heated by immersion, I think this rules out a bad stat and heat loss due to pipe run?

I replaced the cylinder stat anyway, and it made no difference.

If it makes any difference, I've measured how much hot water it holds and I get 230L at a steady 53.9, and by the time 250L has come out, the temp is 53.0.

If it isn't the cylinder stat, what could be preventing it from getting the water to the target temp when heated by the boiler?

 

[Johntheo5]

Hi.

My boiler flow temp is set to 75, the HW dial on the boiler is set to 70, the stat on the cylinder is set to 70.

If, and it normally is, the coil flow is from top to bottom, then there is allways a temperature difference between Hw temperature in the top and bottom of the cylinder, 5 to 10c would be normal, you say the boiler flow is set to 75C and the HW to 70C, above, a bit confusing if not a combi boiler, have you checked the actual boiler temperature?. by looking at the parameter d.040 in the diagnostic menu.

 

[Joseph83]

you say the boiler flow is set to 75C and the HW to 70C, above, a bit confusing if not a combi boiler

Thanks. Yeah the boiler has a dial for HW and a dial for radiator temp. I was told that the HW dial does nothing when its used as a system boiler, its just there for when its used as a combi? Either way, I have it whacked to max just in case.

have you checked the actual boiler temperature?. by looking at the parameter d.040 in the diagnostic menu.

Yes, this is usually between 74-77 when its running. I also used a thermal imaging camera to check the temp of the flow pipe taking water into the cylinder - its 74 when the boiler is running.

there is allways a temperature difference between Hw temperature in the top and bottom of the cylinder, 5 to 10c would be normal

I had thought about that, but no matter what I set the boiler to (60, 75 or 85) the tap water is still 53.9. Adjusting the HW dial down/up also makes no difference.

 

[Johntheo5]

Is the motorized valve opening/closing? with HW timed to be on turn the cylinder stat up/down to test it.?

 

[JonathanM]

Are there any rules about working on unvented cylinders? What can a DIYer do?

 

[roguetrader]

Ignore the hot water temp dial on the boiler, it doesn’t do anything. It's a generic facia.

Cylinder temp should be set at 60°c. 65° max.

 

[ianmcd]

I replaced the cylinder stat anyway, and it made no difference.

you shouldnt have you are not qualified to do so Im afraid

 

[Joseph83]

Is the motorized valve opening/closing? with HW timed to be on turn the cylinder stat up/down to test it.?

So it actually looks like the cylinder has 2 stats - one for the immersion heater (blue dial, numbers 1 - 5), and a second for when it heats from the boiler (gold screw, numbers 1 - 5).

Anyway, I turned the blue dial up/down and the valve didn't move. I turned the gold screw to "1" and the valve closed. Turned it to "5" and it opened again. When its heating from the immersion, the blue dial is the one that causes it to start/stop heating as you can hear it go on/off.

you shouldnt have you are not qualified to do so Im afraid

Did not know that. I thought it was just fitting/removal that was regulated. Ah well, its done now.

 

[ianmcd]

So it actually looks like the cylinder has 2 stats - one for the immersion heater (blue dial, numbers 1 - 5), and a second for when it heats from the boiler (gold screw, numbers 1 - 5).

Anyway, I turned the blue dial up/down and the valve didn't move. I turned the gold screw to "1" and the valve closed. Turned it to "5" and it opened again. When its heating from the immersion, the blue dial is the one that causes it to start/stop heating as you can hear it go on/off.



Did not know that. I thought it was just fitting/removal that was regulated. Ah well, its done now.

you shouldnt have the immersion and Gas heating of the cylinder on at the same time, the immersion is a backup if the gas boiler fails

 

[Joseph83]

you shouldnt have the immersion and Gas heating of the cylinder on at the same time, the immersion is a backup if the gas boiler fails

They're not on together. I've only used the immersion once, to test it, and I made sure the boiler was off when it was on.

 

[Johntheo5]

What setting was/is the blue (immersion) stat set to and what setting was the gold stat set to? and what is it set to now?.

 

[Joseph83]

What setting was/is the blue (immersion) stat set to and what setting was the gold stat set to? and what is it set to now?.

Originally (before I did anything):

Blue - 3
Gold - just under 4

Then about 3 days ago (when I started trouble shooting):

Blue - I changed to 5
Gold - I didn't know it existed so it stayed at just under 4

At this point, the water was 53.9 when heated by the boiler, but 70 when heated by immersion.

As of just now:

Blue - 3
Gold - just above 4

I'm an idiot though, because this is the stat with the blue dial: https://www.advancedwater.co.uk/product/thermostat-tse00187-turquoise-cap-red-adj-TSE00187 and its clearly labelled as "Immersion Stat", so that should have been a clue! According to that, the numbers on the immersion stat correspond as follows - 1 = 10°C, 2 = 25°C, 3 = 40°C, 4 = 55°C, 5 = 70°C.

I imagine that same scale applies to my gold screw stat as well because now I've set the gold screw to just above 4, I've just let the boiler heat the water and its now around 61 at the tap, which is perfect.

I think we can call this one solved. Thanks for the suggestions - if it wasn't for the comments here I doubt I would have found the second stat!

 

[Joseph83]

Here's the thing I refer to as a "gold screw" (and you can see the blue immersion stat dial in the lower left hand corner too):