System Pressure

[Shudavleftit]

Hi.
Hope someone can help please.
I have a Sime boiler running two radiators, two towel rails and my underfloor heating system (Polytherm). I have been having problems.
The boiler has a pressure gauge but there is also a pressure gauge at the filling point of the system which is actually on the end of one of the heating manifolds. I suspected a dodgy expansion vessel and replaced it.
After that I topped up the system to 1.1bar cold as per boiler manual. Turned on the heating. The pressure on the gauge attached to the manifold is now continually moving between 1bar and 2 bar (it takes about 10 minutes to go from 1 to 2 and back to 1 again.

 

[gblades]

And its doing that while the heating is on and you are not topping up the system?
What is the pressure showing as in the morning before the heating comes on?
Is the pressure gauge in the boiler showing the same pressure?

Check the PRV (pressure relief valve) outlet outside to see if it is wet. I am assuming as you thought the expansion vessel was faulty you were getting excessive pressure. If the PRV does blead off excess pressure they can stick and keep dribbling water.

 

[Shudavleftit]

Firstly, thanks for reply.
I replaced the expansion vessel and opened the filling line until I got 1.1bar. I have not touched the filling line since so correct I am not topping up the system. Last night as I was nervous about the whole thing I
switched off the room thermostats so effectively all the boiler did overnight was heat the two radiotors, two towel rails and the domestic hot water. As all the room stats were off the ufh circulating pump was off.

When I checked the dial before 07:00 it was reading something around 0.5bar.

The pressure gauge on the boiler shows about 1.6bar when the pressure gauge on the filling point/manifold shows 2bar ??? Would the circulating pump on the ufh add additional pressure that the boiler does not see ?

Based on other info on the net I have tried looking at both by PRVs. One is on the boiler itself the other on my systemzone. Here is info on the systemzone http://www.systemlink.ie/systemzone-product-ireland.asp
It links everything together ...
Boiler
UFH
DHW
I can't find any evidence of dribbling water from either PRV.

 

[twgas]

Is the pressure vessel correctly sized and charged, does the pressure rise, from 1bar cold, as the system heats up - and if so, to what pressure, and when the pressure is up do you get water from PRV(s)?
Tie a plastic bag over the pipe from prv to see if it collects water.

 

[Tipper]

Air in the system?

An air pocket somewhere can cause pressure fluctuations as the air expands and contracts when heated/cooled. Air/gas expands more than water.

 

[Shudavleftit]

After getting some advice from another very helpful gentleman (www.makewrite.demon.co.uk) on Friday I went home to look at this again. As it turned out I think twgas and tipper were both correct.

When installing the new expansion vessel I thought it was a case of swap the new for the old as both were labeled to have a precharge of 1.5bar. I had previously straight swapped the vessel on my DHW in this way. In actual fact I was supposed to set the pressure on the vessel to match the lowest operating pressure of by boiler (1 bar). In the instructions with the vessel it clearly explains all this and indicates the minimum setting should be 1 bar.Maybe I'll change my name to Shudavreadtheinstructions.

By the way it talks about static head etc. but how does this apply to me as I have a pressurised system with no tanks etc.

Once I did this and filled the system to 1.1bar cold, everything returned to normal. The pressure holds at 1.1 bar cold and runs at 1.9bar hot. I think these are ok (I have underfloor heating) but please confirm. So to twgas, no the vessel was not charged correctly and to Tipper yes there was air in the system !

Thanks to all.