Unvented Cylinder - HW to Thermostatic Shower Problem

[Lexden]

At the outset let me say that I am not looking for self help advice: this is clearly a job for a professional.

I have a 250L OSO 20RD unvented cylinder. It has been installed for 15 years. It serves two Bristan thermostatic showers which recently have started to run cold. There is no pressure equalising valve between the cylinder and the two adjacent back-to-back showers.

I have re-charged the cylinder air gap and the shower will run hot for about 2 minutes or until someone turns on a hot tap somewhere else in the house. When, say, a bath has been run, I can hear the unvented cylinder topping up with cold water for a short period of time after the tap has been turned off. HW flow to the taps (up and downstairs) is good. Cold water supply is via a PRV and a Kinetico HF 2020C water softener.

Could it be the the unvented cylinder valves/filter need checking or cleaning, or should I be looking at something else? Logic, at least to me, would suggest that the problem is falling pressure in the Oso cylinder as hot water is dispensed.

Grateful for any thoughts/advice before I approach a G3 qualified plumber.

 

[dibby90]

have you had water discharging from any of the safety valves in to the tundish?

 

[Lexden]

have you had water discharging from any of the safety valves in to the tundish?

No - the tunidish is always dry; i.e., no evidence of a discharge. On first using the shower, the shower flow is always high suggesting that the air gap is charged. It then falls away, and I suspect that this is on the HW side. The HW shower elbow is always hot to the touch. I should add that I have changed the Bristan valve in one of the showers to rule that out as the issue.

 

[dibby90]

are your hot and cold supply's balanced so they operate at the same pressure? But it does sound like the strainer on the cylinder needs some attention

 

[Lexden]

are your hot and cold supply's balanced so they operate at the same pressure? But it does sound like the strainer on the cylinder needs some attention

That is difficult to tell with the shower but I would say that the basin hot tap in the ensuite (same room) has a slightly lower flow rate than the cold tap. This hasn't been an issue with the shower until recently.

 

[Agile]

Surely the softener supplies both cold and hot supplies?

The correct way to connect an unvented cylinder is to take the cold output from a connection after the cylinder PRV to the cold connection to showers so they then are supplied from the same pressure on both hot and cold.

An unvented cylinder is meant to have annual servicing which includes cleaning the filter.

Tony

 

[dibby90]

yes that is the correct way agile but I go to many that don't balance the supplies and have problems, as you say it sounds like the cylinder needs a good proper service

 

[Lexden]

Surely the softener supplies both cold and hot supplies?

The correct way to connect an unvented cylinder is to take the cold output from a connection after the cylinder PRV to the cold connection to showers so they then are supplied from the same pressure on both hot and cold.

An unvented cylinder is meant to have annual servicing which includes cleaning the filter.

Tony

All fair points but I suspect that hasn't happened. New build etc, 16 years ago. The original plumbers also omitted to put in any isolation valves for the taps, showers and basins.

I am happy to get a G3 engineer in to service the unvented side and see what happens after that has been done.

 

[Lexden]

Thanks both - I will let you know how I get on.

 

[Agile]

I generally find that new build is most often piped correctly.

Its the one off later UV installs which are often not connected to cold supplies in the house because that involves a lot of extra pipework and the customer would not know how it should be done.

Worse when the owner has asked for several quotes and accepted the cheapest ( which is then not done correctly. )

I find it so annoying when a quote for a proper job is set aside for a cheap but substandard job. Since the customer does not know how it should be done and thinks that the cheapest job is what he wants.

Tony

 

[dibby90]

thanks good luck hope a service sort your problem

 

[Lexden]

Update:

This has taken me longer to resolve than I first thought. My initial problem was finding a G3 qualified engineer. I eventually found two firms in my area. One firm carried out an on-site survey and based on their observations of water flow etc, the conclusion was that the problem was related to the combination valve. They also pointed out that the valve needed to be moved from its position on the floor to comply with Building Regulations. Unfortunately, the guy that quoted for the work wasn't the engineer that came out to do the work. The tasked engineer was insistent that the only work needed was a like-for-like replacement of the valve in its present position. I pointed out that the quote said otherwise. Needless to say, the work didn't get carried out.

A second firm also identified the combination valve as the likely problem. The engineer pointed out that I wouldn't be able to tell if this was the case unless I checked the external vent pipe for drips as the combination valve had been installed incorrectly. He pointed out that the PRV on the combination valve was connected to the vent pipe below the tun dish and minimum distances had not been satisfied. There was also evidence on the ground that a small amount of water was being expelled. The tun dish was dry so the conclusion was that the water was coming from the PRV part of the combination valve.

I am pleased to save that after some re-piping, a new valve and a bubble re-charge, I now have hot showers once again.

Thanks once again for your advice guys. Sadly, the challenge is finding a firm that one can trust to analyse what is needed and to do the work correctly. As a parting shot, the engineer that did the work quietly pointed out that the vent pipe should have been 28mm and not the 22mm that has been fitted (underneath the bathroom floor). He said that it was something that he came across frequently when installers failed to take into account the number of 90 degree fittings in their maximum pipe length calculations.