Unvented T&P relief valve on buffer store

[Jonesyme]

Hi,

I'm planning to install a thermal buffer store (to prevent my combi cycling) as part of my UFH on a building control ticket. I'm a professional engineer, and feel competent to install it safely.

However, I am unclear about part of the G3 requirements (so competency not quite there, but even if I get an installer to do it I want to understand):
"3.35 Temperature relief valves and combined temperature and pressure relief valves should not be used in systems which have no provision to automatically replenish the stored water (e.g. unvented primary thermal storage vessels). In such cases there should be a second non-self- resetting energy cut-out independent of the one provided in accordance with paragraph ..."

Can anyone explain the reasoning behind this requirement? I guess it is either a) dangerous to have the water vanish (in which case there should be no T&P valve, even though there is increased risk of BANG) or b) T&P valve is not sufficient to shut down the system in even of failure and could lead to failure elsewhere (in which case the T&P valve could still be present, but be backed up further as described).

I ask because I am re-purposing a (new) 100L water heater (electrics disconnected) to use as the thermal buffer store (water flows into the store in parallel to the UFH, and is then pumped back to UFH heating when temperature is reached, isolated by a zone valve). The system is fully direct, with the same water in the combi, store, and UFH.

I can satisfy the two indepedent cut-outs by having two non-resetting thermal switches to the n/c zone valve and a cutout to the boiler, however the cylinder already has a T&P valve mounted on the top. I could disconnect it, but this seems to remove a safety feature as it leads to a vessel exploding failure mode.

Thanks for any comments you may have.

 

[Dan Robinson]

To ensure against dry firing the heat source.

 

[fixitflav]

To ensure against dry firing the heat source.

My boiler, 20 years old, has a dry-fire cut-out. Don't all boilers have this nowadays?

In such cases there should be a second non-self- resetting energy cut-out independent of the one provided in accordance with paragraph ...

This doesn't specify it must work on pressure or temperature, so presumably either is OK, temperature for a dry-fire cut-out.

 

[Jonesyme]

Thanks, Dan and fixitflav. It may well be against dry-firing the heat source, but as you say most heat-sources have their own protection to guard them once this condition arises. To me it would seem the lesser of two evils to allow a dry firing vs a pressurised vessel exploding though.

On more reflection I wonder if it is to prevent a large air volume in the system that could hold more energy in the event of an over temp or over pressure failure condition. But then again the combi will also have a pressure relief valve anyway and could create the same condition. I'm still baffled! Also, still not sure if it would be OK to retain the valve if there were two other independent protections too.

 

[fixitflav]

Thanks, Dan and fixitflav. It may well be against dry-firing the heat source, but as you say most heat-sources have their own protection to guard them once this condition arises. To me it would seem the lesser of two evils to allow a dry firing vs a pressurised vessel exploding though.

On more reflection I wonder if it is to prevent a large air volume in the system that could hold more energy in the event of an over temp or over pressure failure condition. But then again the combi will also have a pressure relief valve anyway and could create the same condition. I'm still baffled! Also, still not sure if it would be OK to retain the valve if there were two other independent protections too.

I'm no expert on this, so I wouldn't try to advise, but I'm puzzled by the wording. If "Temperature relief valves and combined temperature and pressure relief valves should not be used....." how can there be a second non-self- resetting energy cut-out? What is the 1st one in those circs?

Just to be clear about terminology - when I worked as a process engineer, vessels were sometimes fitted with thermal relief valves. These relieved on pressure (in spite of the name, which confused some people) and were there to avoid pressure rise due to thermal expansion of vessel contents because of eg solar radiation. A small valve, maybe 3mm seat even for a biggish vessel, was usually OK. I suppose a temperature relief valve is the same, but I could be wrong. Sometimes, specially on offshore platforms, fire relief valves were needed, to vent in event of the vessel being exposed to fire. Much bigger valve, but also relieving on pressure. So I'm not clear what a combined temperature and pressure relief valve is. Is it like a temperature (or thermal) relief valve, but bigger, and still relieving on pressure?
Presumably these relief valves are on the pipework, remote from the boiler. The boiler control-stat, limit-stat and (if any) dry-fire stat are part of the boiler.
One question - why do you think you need the thermal store? Is it normal with UFH, perhaps because of low water volume compared with rads?

I can satisfy the two independent cut-outs by having two non-resetting thermal switches to the n/c zone valve and a cutout to the boiler, however the cylinder already has a T&P valve mounted on the top.

I don't follow that. I suppose you could wire the thermal switch contacts to stop the boiler, but would it do anything the boiler controls don't do? Also 3.35 refers to "e.g. unvented primary thermal storage vessels" so seems to refer to the vessel primary circuit. The T&P valve mounted on the top sounds as if it's on the body of the vessel, ie the secondary.

Sorry for a longwinded post!

 

[Roger928]

The t and p blows aff at around 5 or 10 big bars. Its a safety device to prevent the cylinder exerting pressure on the sides of the tank and exploding the ends aff.
Probably not need on a thermal store.

 

[fixitflav]

The t and p blows aff at around 5 or 10 big bars. Its a safety device to prevent the cylinder exerting pressure on the sides of the tank and exploding the ends aff.
Probably not need on a thermal store.

So it's only looking at pressure, not temperature? BTW, what is a big bar?