Unvented over heat stay tripping intermittently.

[Fireboy24]

Hi there,
Third call back today. Bit of help please. Installed Telford easy fit unvented cylinder 3 weeks ago. Over heat stat has started tripping on cylinder leading to no hot water. Primary circuit for the coil set at 73*C so no where near manufacturers cut out temp of 83*C.

When I'm there and have the hot water on if I turn the cylinder stat down it closes the 2 port valve and turns the boiler off so it's wired in correctly and that side of it is ok.

Presuming it's a faulty stat but just wondering if there's anything I might be missing???

Cheers

 

[ShaunCorbs]

You set the hot water temp to 73 ?

 

[Fireboy24]

Not the cylinder stat. That's set at 65*C. I'm taking about the temp for the flow on the boiler being set at 73*C. The cylinder stat sits next to the coil in the cylinder.

 

[ShaunCorbs]

Bring the cylinder stat down to 60 should sort it

 

[Gasmk1]

My thoughts also too high a temp lower it to 60 and try that go to 65 max

 

[snowhead]

What's the temperature of the water at a tap, assuming it doesn't have TMV at cylinder?

If the stat is low down in the cylinder even 60 is too high.

 

[Fireboy24]

Thanks guys,

No TMV fitted. I'll try nudging the cylinder stat down and making sure the DHW outlets are achieving 60*C minimum.

I think I'm over thinking this one a bit. Either a faulty stat or cylinder temp set to high.

 

[Fireboy24]

Yes SNOW HEAD. Stat is low down located near the coil.

 

[Chuck]

Thanks guys,

No TMV fitted. I'll try nudging the cylinder stat down and making sure the DHW outlets are achieving 60*C minimum.

I think I'm over thinking this one a bit. Either a faulty stat or cylinder temp set to high.

Store temperature should be 60°C. Outlet temperatures (assuming you mean taps) need to be 50°C within one minute. See:

HSE - Legionnaires' disease - Risk systems - Hot and cold water systems

for the details.

If you have G3 ticket, you should know this stuff in detail already. If you don't have a G3 ticket you're breaking the law by working on unvented systems.

 

[Fireboy24]

Chuck everyone here knows the basics of what temp stored hot water should be at. The thread is about the problem with the stat.

 

[Chuck]

Chuck everyone here knows the basics of what temp stored hot water should be at.

Not if the cylinder stat was deliberately set at 65 they don't.

Are you sure that the electric immersion heater is wired and set correctly and not causing the problem?.

Check, by measuring the pipe temperatures, that the 2-port valve is closing properly and the measured flow temperature is no higher than 75°C. There have been some cold nights recently so the CH may have come on at some point.

For low-tech check, attach a non-reversible temperature sensitive label to the primary flow. If the system trips and this does not show above 75°C then the upper limit thermostat is the likely issue.

Otherwise, attach a datalogger to the system. Leave it monitoring three temperatures: the cylinder temperature, primary flow and primary return.

If you don't have a datalogger already, the Extech SDL200 is a good choice for this sort of thing:

Extech SDL200 4-Channel datalogging thermometer

get a couple of K-type pipe clamps, generic ones are cheaper. It won't matter in this case, but you can avoid Galvanic crosstalk by using a single-ply sheet from a Kleenex to insulate the clamps. Exetech mains adapters for this device are not not easy to come by but this one:

PP8 UK 9V MED | Friwo, 8W Universal Travel Adapter 9V dc, 800mA Level V 1 Output, Switched Mode Power Supply, Medical Approved | Friwo

works excellently. Note that it's a medical grade supply to avoid leakage current issues. Don't use a random wall-wart for this sort of application.

 

[Best]

I thought 65 degrees was fine for unvented stored hot water?

 

[ShaunCorbs]

I thought 65 degrees was fine for unvented stored hot water?

Depends where the stat is could be top half is getting close to 80 if the stat is in the bottom etc

 

[Chalked]

Had this before when the immersion was left on!

 

[Fireboy24]

Cheers guys,
Great advice.

Cheers chuck.
I've got the pipe clamps and have been carefully measuring the flow & return temps. I've set them at 71 max now so thinking if the stat still trips after that then it's defo faulty.
Also adjusted the auto bypass a bit to make sure the temp isn't rocketing up when the valve starts to close and shooting water down the coil so hot it trips the stat.

 

[Chalked]

Boiler stat is fine. It's the cylinder stat that needs lowering.

 

[Vee]

Did you fit new zone valve with cylinder? If you left an existing one in place could be letting by and excessively heating cylinder. Just a thought?