Unvented direct cylinder - tundish drip

[RobbiS]

Hi all,

We had someone out to our Oso direct unvented cylinder recently as it was dripping tepid water from tundish when running. He recharged the internal air gap and all was well for a few weeks, but then the same problem reoccurred - tepid drip when heating element running, although the water from the taps was much hotter than that. The drip stops within a couple of minutes when the heater is turned off.

Having sought his advice again, he suggests fitting an external expansion vessel and replacing the TPRV. The cylinder is about 12 years old.

There isn't much space where it's sited so an external expansion is going to eat into our already very limited storage space, could there be any other options short of replacing the whole cylinder? Is it going to be worth us sticking an external expansion on a cylinder of that age (not sure how long they're supposed to last anyway)?

Thanks in advance for any advice!

 

[Madrab]

Yes, nothing wrong with placing an external vessel on a cylinder that age. That being said I'd maybe look at replacing the internal bubble a couple of times 1st.
What model of OSO cylinder is it?

 

[RobbiS]

Yes, nothing wrong with placing an external vessel on a cylinder that age. That being said I'd maybe look at replacing the internal bubble a couple of times 1st.
What model of OSO cylinder is it?

Thanks for the response - it's a 20RD (170 litre).

 

[Madrab]

Personally I'd be checking the bubble again first, the OSO has a very specific draining & filling process to ensure the water drops below the T&P valve and dip tube to fill the space properly. Some Unvented need a recharge tried a couple of time to re-establish the bubble properly.

That being said, the OSO doesn't use a baffle, it uses a dip tube with the air being trapped at the top of the cylinder above the dip tube, so very little to go wrong with that.

If it still continued to drip then I'd look at the elements to see that they aren't set too high and cut off ok and then the relief valves.

Putting in a remote EV would be the last option IMO.

 

[ianmcd]

Personally I'd be checking the bubble again first, the OSO has a very specific draining & filling process to ensure the water drops below the T&P valve and dip tube to fill the space properly. Some Unvented need a recharge tried a couple of time to re-establish the bubble properly.

That being said, the OSO doesn't use a baffle, it uses a dip tube with the air being trapped at the top of the cylinder above the dip tube, so very little to go wrong with that.

If it still continued to drip then I'd look at the elements to see that they aren't set too high and cut off ok and then the relief valves.

Putting in a remote EV would be the last option IMO.

+1 the OSO is a decent cylinder, I wouldnt write it off, a decent enginer should be able to sort that out , without an external expansion vessel

 

[RobbiS]

Thanks for the advice, appreciated. I'll have a go at recharging again as a first step and ask about the valves if no joy there.

Am I right in thinking given that the tundish water is never more than lukewarm even when the water out the taps is hot that the likely source of the drip is expansion back up the cold pipe and out through the expansion valve i.e. a mix of cold coming in and warm backflow, as opposed to the TPRV?

 

[Madrab]

Yes, that a fair assumption.

Usually first thing to check would be the expansion valve first as that usually starts to lift at a lower pressure (same as what the pressure reducing valve is set at). With that version of the OSO though the expansion relief valve (ERV) is pressure rated the same as the T&P relief valve. If it's the T&P valve that's lifting I would expect it to be hot water in the tundish not cool/tepid. Follow the info in your MI to reset the bubble (Pg4)

If one of them still passes they can be checked individually to identify which valve is at fault. All this should be part of the testing/fault finding process the qualified engineer should run through though, IMO.

The relief valves may also require serviced - it actually details how to service the ERV properly which is a rarity. Again though a proper test and problem finding procedure should check all of this. Maybe time to find a new Unvented bod that wants to identify what's wrong 1st and not just chuck bits at it.

 

[RobbiS]

With that version of the OSO though the expansion relief valve (ERV) is pressure rated the same as the T&P relief valve. If it's the T&P valve that's lifting I would expect it to be hot water in the tundish not cool/tepid. Follow the info in your MI to reset the bubble.

My expansion valve is rated close to but a bit lower than the TPRV (8 bar vs 10 bar), another reason why I think that's what's passing.

Anyhow - sadly the attempt to recharge the air gap was a failure. Couldn't get a drop of water out of the commissioning cock to drain, nor the TPRV (though I was reluctant to turn that more than a little bit for fear of breaking it and leaving it permanently open, which would be problematic as the only way to isolate the cylinder from supply is by turning off the mains).

Have also contacted Oso for their thoughts on the situation. Will see what their recommendations are going to cost and weigh that up against getting a new one!