Low hot water pressure All aspects of plumbing questions and answers, help, tips and information

[juddster]

Hi All,

I hope you can help me solve this problem. The basic problem is:

- When water in cylinder is hot we have very low hot water tap pressure
- When water in cylinder is cold the hot water tap pressure is OK

The pressure in all the hot water taps in our house has suddenly become so low that it was virtually a trickle. I thought that it was an air lock at first so let the hot water run for a while to try and clear any air then tried the mixer tap trick but neither worked.

I tried the water again later on (the water in the cylinder had by now cooled) and the water pressure was much better. I let it run for a while and it spluttered a little as if air was coming out and the pressure gradually increased to a reasonable level. I thought it may be a cylinder thermostat problem (the water getting too hot and steam getting into the tank??) so I turned the temperature on the cylinder thermostat down to 50 degrees and turned the hot water on. This seemed to partially work in that the water pressure when hot was better but over the next day or so, it went back to the same problem of low pressure when the water is hot.

Anyone got any ideas what it could be?

Cheers.

PS: It’s an indirect heated hot water cylinder.

[rosebery]

Sounds to me like an air lock. You may not have enough differential pressure to shift it at the BSM as that relys on the static head from the CWST to the tap - probably only about 0.2 - 0.25 bar. Have you tried the washing machine trick? THis works if there is plumbing for both hot and cold supplies to the WM/C but not if cold only. Turn off the taps. Cross connect the cold mains pressure feed to the hot feed. Open the hot tap then open the cold tap. This will direct mains pressure (probably iro 3 bar of higher) up into the hot pipework and usually will blast any air out. You can also try this at the kitchen taps if the WM/C plumbing is cold only.

Cheers

[juddster]

Thanks for your reply rosebery.

I haven't tried the washing machine trick but I did try the kitchen sink mixer tap trick (cover the outlet, turn on hot tap then cold tap) but that didn't work.

I'm not sure it is an air lock because when the water in the cylinder isn't hot then the pressure is fine - it's only when the water in the cylinder is hot that the pressure goes really low.

I think I need to call a plumber...

[htg engineer]

tank full ?
ball valve filling at a decent rate ?

probably best to get a plumber, will save time

[rosebery]

"I'm not sure it is an air lock because when the water in the cylinder isn't hot then the pressure is fine - it's only when the water in the cylinder is hot that the pressure goes really low."

Perversely an airlock doesn't always provide a total obstruction. It's quite possible to have an airlock present which the water gets past. Tinkling noises in the pipework lik a little bell ringing are a indicator of this. Air expands as you heat it so will occupy more space in the pipe and therefore restrict it more.

"I think I need to call a plumber..."

Nevertheless I agree with HE and you. You should. Let us know how you get on.

Cheers

[CC]

[Hi Judster

I've got just the same problem!
When the water gets hot - particularly on a hot day the water flow is non existent in the tap but returns the following day when it has cooled down. It's definitely not a conventional air lock - The gas engineers hadn't a clue. I think I may have the Thermal Store System with a thermostatic mixer valve. It's one of these modern systems which is more trouble than its worth. Have you got a modern (last 7 years) system? with the above. My header tank is full so thats not it, I can't think of anything else it could be. Any comments from anybody please on this.

With many thanks,
CC

[juddster]

First off, thanks for your responses 'rosebery' and 'htg engineer' . 'CC' - I’ve got to the bottom of the problem I have - hope it helps you.

Just for info, the cylinder we have is a Fortic Type 3 (about 12 years old):

The plumber came round today to have a look and determined that the cylinder was clogged up with limescale and that was causing the problem. To work this out, he attached a pipe to the cold water inlet (from the expansion tank to the hot water tank) and blew down it. He could hear the limescale sloshing about and was expecting water to come out of the vent from the hot water tank to the expansion tank but it didn’t.

After he left I managed to clear some of the limescale from the vent pipe by poking coat hanger wire down it. Lots of air bubbled out of the vent pipe for about 15 mins and the hot water pressure is now back to normal, but I guess it’s only a matter of time before it happens again.

Basically, we need a new cylinder.

Thanks.

[CC]

[Well done Juddster. I'm glad you've sort of sorted it out. I too have sorted ours out. The whole of the header tank has seized up and our plumber has now fitted a new one and the water has returned. My ideas in my last quote were wide of the mark for which i apologise and must admit I didn't think this was the problem. Limescale is our problem too and the sooner you replace your cylinder the better. Good luck!!