Tundish sometimes blows off..

[The Natural Philosopher]

System:

System boiler, pressurised DHW tank in loft..

Symptom: water stains around tundish area and occasional drips in pressure blow off exit pipe outside house.

Someone enlighten me as to whether the safety valve(s) operate on the DHW circuit, the CH/DHW primary pressure, or both...and what pressure they operate at?

Mains pressure is a stonking 2 bar plus...and I normally run the primaries at 1.5-1.8 bar...

I assume the boiler has an expansion vessel in it, but should there be one in the DHW circuit? I ocasionally get water hammer on one hot tap...

[Doctor Drivel]

Check the DHW unvented cylinder expansion vessel. If a silly Mergaflow, they have an air bubble, which has to be re-instated. Pressure is building up and opening the relief valve. It should not blow off any pipes, check the pipe fixings.

Safety valves on the DHW. The boiler primary side has its won safety blow-off valve. The boiler primary and cylinder secondary sections are separate.

2 bar is not much. Unvented cyldiners are pressure reduced to about 3-3.5 bar.

The boiler pressure should be charged to around 1 bar NOT, 1.8bar.

Should be, check for a large red thing at the back of it.

Yep, or if a Megaflow they don't have one using an air bubble that dissolves all too easily.

[Ed Sirett]

Most things you may need to know about the primary are in the sealedCH FAQ. However the tundish is fitted to the cylinder not the primary.

The commonest reason for discharge from an unvented cylinder is failure of ITS air bubble or expansion vessel. Not all unvented cylinders are fitted with a gauge - it is however a key tool for diagnosing problems with the cylinder.

PS the blow off pipe (for the cylinder) should have been installed so as to discharge safely to ground level not on the house.

[The Natural Philosopher]

Thanks.

Ok...see what you mean

It does...but there is spray over inside

Looks like some setting/testing of the HW circuit pressure is in order - thanks.

[Doctor Drivel]

Looks like it is undersized.

[Ed Sirett]

Assuming the tundish is 15mm in and 22m out the maximum length is 9m for the 22mm discharge pipe. Each bend takes 0.8m away from the limit. The discharge pipe must always go downwards. There must be a direct drop of 200mm after the tundish before any bend. Sometimes when a lot of lime-crud has built up on the tundish it then fails to work right.

[The Natural Philosopher]

Its almost new install, and its always blown off a little...never great clouds, just like a little phart now and again, so its not an issue of it failing to cope with the volume..

I can't see why it goes at all though..

[Ed Sirett]

I doubt very much that either blow off valve is defective. I strongly suggest that you obtain a pressure gauge (0-10 bar or thereabouts) less than £10 and with a 1/4" BSP connector. You will find that the there are very likely some tapping(s) on the main pressure reducing valve. The valve screws into one of those.

If the type you have is an internal air pocket the I suggest you renew the air pocket in line with the instructions. If the type you have is the other sort with an expansion vessel then check that vessel (as you would for a sealed CH system - see FAQ).

When you have the gauge you will be able to see if a) The blow off occurs at the wrong pressure (duff blow off valve) b) The pressure is always too high (duff inlet PRV). c) The pressure rises on heating are excessive (duff air pocket /expansion vessel).

HTH

[The Natural Philosopher]

Thanks Ed..its in the round tuit file..will do as suggested..