Unvented hot water systems

[Teggs]

Guys,

I recently had a Megaflo unvented hot water system installed in my property. I must say that I am very impressed with the results.

I do have one or two questions with how this is set up though and wonder if one of you may be able to shed light on these. The unit came with a two port valve with has been installed just after my 3 port zone valve, what puprose does this serve. When I asked the installer he said that it was a necessary safety feature and had to be installed after the 3 port valve and wired into the junction box accordingly. Surely the 3 port valve is capable of stopping the ch system from supplying heat to the cylinder?

My other question related to the removal of the cold water storage tank. When this was removed the rising mains (in 15mm) was connected to the oulet pipework that came from from the cold water tank. These 3 pipes from the tank are all in 22mm. Should this have been replaced with 15mm to each outlet they serve or is there no overall affect?

Hope this makes sense,

Terry

 

[IanDB]

The unit came with a two port valve with has been installed just after my 3 port zone valve, what puprose does this serve.

I asked this question a while back as I had a problem with the valve (it had burnt out). Nobody knew the answer. I can't see what additional safety feature it provides. It is only another item to fail (as it did). I removed the head and left it in the open position.

I'm impressed with the Megaflo system. I've had mine for coming up for 7 years now. The only minor drawback is that you have to recharge the air pressure in the tank every six months. But that only takes about 15 mins.

I'll leave an expert to answer the other bit!!

 

[croydoncorgi]

I believe the second motorised valve is in case the first one fails unsafe (open) and by some mischance the water gets very overheated. Given the 2 PRVs on the cylinder this did strike me as 'belt-and-two-pairs-of-braces'!

Someone with an U/V system licence help out here!

 

[corgiman]

Croyden is totally right its to ensure the overheating will not take place

A two port valve ensures that when the cylinder stat is satisfied the cylinder flow is totally removed from the heating system, if you were not to have this set up and someones has just installed a three port valve get someone to change it NOW

the two two PRVs (which will trickle out if the cylinder over pressurizes) are no fail safe for over heating. if water is pressurized its boiling point increases (to appx 147.5 degs) so the unvented cylinder is fitted with a temperature relief valve which, unlike the PRV, in the event of over heating will open up to full bore and you get water at 90 deg C (temp that the valve is set at) rushing out of the boiler at full mains pressure. this is the reason that "belt and braces" is used and that most of the work in installing an unvented cylinder goes into the discharge pipes.

Hope that is a good enuff explanation?

 

[oilman]

The 2 port valve is there because it will shut if it fails, and so no heat can get to the cylinder. The 3 port valve can fail open depending on how it is connected, or its type. For instance if it is a "motored open/motored shut" type. To get over the problems, the manufacturers supply a 2 port valve with a known failure mechanism and it had jolly well better be fitted. An overheated cylinder has the ability to take down buildings, just like that!

So IanDB, get someone in to fix it, and make sure they have a G3 certificate. YOU are not allowed to work on unvented systems!!!! Not even your own.