Has my hot water cylinder coil broke?

[ShaunCorbs]

Your filling loop could of been passing eg feeding and back feeding the cold water

your heating pressure drains and refills when you drain or refill the cylinder

 

[Dave18183]

Your filling loop could of been passing eg feeding and back feeding the cold water

your heating pressure drains and refills when you drain or refill the cylinder

Thanks. least I know the problem, though gutted as it’s going to be a costly fix.
best find check my pockets for £1000+ lying around .

my Vaillant Boiler is old but working (borrowed time).

will I need to replace the cyclinder with a Vaillant one to go with the Vaillant boiler. I’ve heard they are on the expensive side “just because it’s vaillant”.

 

[SJB060685]

Pressure/pressures always try to reach an equilibrium. Your sealed heating system is usually pressurised to around 1 bar, depending on certain things, whereas your unvented hot water system will usually be restricted to 2.5-3 bar. If the filling loop fails your sealed heating system pressure will creep to whatever the pressure relief valve is set to, normally 2.5- 3 bar before it acts and discharges. If the coil splits (which seems to be the case) then the same will happen, the heating system pressure will rise until an equilibrium is reached or the pressure relief valve acts. Remember a lot about plumbing systems and thermodynamics is all about the equilibrium.

 

[ShaunCorbs]

You can go with another manufacturer but if your planning on going with a new vaillant boiler when the other one goes then it’s best to match as there controls work together

 

[Ric2013]

I've been thinking about this and how it all makes sense in hindsight. This would be my reconstruction, and hopefully people will tell me if they think it is wrong:

The OP had a sealed system (primary water) at 1 bar (estimated) pressure. That's the heating system. There was also a mains pressure cylinder. The two systems should not be able to mix at the coil, but the coil was starting to leak.

Presumably the mains pressure is not that high (or there is a pressure reducing valve) so the pressure in the cylinder was not much greater than in the sealed heating system so some kind of rough equilibrium developed without the heating system PRV having to open. Some mixing of the two water systems occurred as pressures fluctuated, but this was minimal as the 2 systems had roughly equal pressures. Enough to lead to active corrosion and brown water in the heating system (rather than a small amount of black sediment), but not enough for the OP to notice a problem.

Then the mains failed. At this point, the heating system pressure was much higher than the pressure in the cylinder and a large quantity of heating system water had to run into the cylinder to restore equal pressures. This may have enlarged the existing leak and noticeably contaminated the cylinder water.

If the OP does not intend to replace his boiler immediately, I would recommend that when he has the cylinder changed he makes some effort to improve the (probably very poor) quality of the primary water. At least several fills and draindowns or a mains flush, and then refill with quality inhibitor. It is surprising how long a boiler can run on borrowed time if looked after, if you're lucky!

 

[Dave18183]

I think Ric2013 has got what happened spot on.

I’m glad we have all got to the bottom of it and I really appreciate eveyone help. Cheers guys

———————

I have a quote for replacing the cylinder, and a power flush which I might consider but funds are tight. Does this seem reasonable?

it is not replacing for another vaillant but a good quality cyclinder.

“
Please find below our costs for above works,

• Drain, decommission and remove existing Vaillant cylinder.
• Install new Main 150 Litre unvented cylinder.
• Install new unvented safety controls including TPRV, PRV, 3bar safety set, expansion vessel.
• Alter existing pipe work and 3 way valve to suit new cylinder.
• Alter wiring of system in airing cupboard to suit new cylinder.
• Install new 2 port safety zone valve.
• Inhibit system.
• Test and commission to manufactures instructions.
• Waste removed from site.

£1008.11 + VAT

• Powerfush 13 radiator system using Kamco FX2 chemicals then system neutraliser, Then test and re-inhibit system.

£450 + VAT
”

 

[John.g]

Only query is why a 3 bar PRV, this PRV is I think normally set 1 bar lower than the TPRV which I would expect set to 7 bar

 

[Dave18183]

Only query is why a 3 bar PRV, this PRV is I think normally set 1 bar lower than the TPRV which I would expect set to 7 bar

Ok thank. I will query it. I was more querying price. There is a fair fee plumbers on here

 

[SJB060685]

Without revealing too much about it yes the PRV should be six bar. The combined temperature and pressure relief is 90-95°c and 7 bar.

 

[SJB060685]

Ok thank. I will query it. I was more querying price. There is a fair fee plumbers on here

That's a reasonable price for materials and labour.