Worcester Greenstar Heatslave 18/25 Loss of Pressure

[ericp]

I have a problem with a Worcester Greenstar Heatslave 18/25. I've read all the related threads but need some clarification on the PRV/Expansion Tank issue.

If I leave the sytem running contiuously, pressure stays up at 1.5 to 2.0 bars for a week no problem. If however, I run it on the timer, I have to get up at 6.00 am every morning to repressurise the system which by then is down to zero. It then runs fine all day at around 1.5 to 2.0 bars. (I pressurise to about 1.0 bar when cold). If I don't fill it up, the system starts up, air seems to get into the pump (how?) and after a while the overheat cut out operates. In the past I was releasing steam from the end screw on the pump but have stopped that by re-pressurising before it occurs.

There is some evidence of a leak in the prv - I put a freezer bag over the outlet and when working normally I can get about a cupful of water in 24 hours. When switched off however, a pint or so collects in the bag.

The boiler is relatively new (2-3 years?) and been regularly serviced. The guy who serviced it a couple of weeks ago reckons it may need a secondary expansion tank since the house is quite large, but since the pressure does not go above 2.5 bars when hot (that's what the handbook says), I'm not sure this is the root of the problem. The other parameter in the book is if the system is bigger than 180 litres which I don't think it is.

I have tried letting off the pressure using the prv to try and flush out any dirt, but looking at previous posts, this may have made matters worse with dirt getting stuck.

If someone can help me understand why the loss of pressure is happening when cold and why air gets in when the system cools down that would be a step forward.

There are no leaks visible in the rads or pipework (microbore - horrible I know but that what the guy installed and it seems to work).

The engineer checked the expansion tank simply by pressing the valve to check air came out and not water. He didn't check the actual pressure so I am going to try that next. However I'm not sure what to pump it up to. The installation manual quotes initial charge pressure of 0.5bar for a 180 or 107 litre system (initial system pressure of 0.5 and 1.0 bar respectively) but 1.0 bar for both system and charge pressure for a 138 litre system.

I don't know the capacity of my system but there are 12 rads plus a towel rail fitted.

Regds

EricP

 

[payney1974]

I think 1 bar is pretty much an unwritten rule to stick to. You do know to recharge the expansion vessel you have to isolate the boiler and drain it down and leave the drain point open don't you??

 

[ericp]

Ah, now you say that, I did read the installation instructions this morning, but the filling sequence just says to pressurise to 1-2 bar, vent everything, then check and adjust the expansion vessel pressure. Then connect the drain hose and drop the pressure down to the correct level.(I would have thought that with all the venting going on, several fills would be needed and there wouldn't be much pressure to drop at the end).

I think what you are saying is that I need to get rid of the water in the expansion tank before pressurising it whereas in the installation stage the air is already there - is that right?

Since I'm still getting some water from the pressure relief valve, I've either got to flush the muck out (tried 3 times) or maybe replace the valve.

Thanks for the tip

Eric

 

[payney1974]

Yes if you test the pressure of the expansion vessel with the system full it will just read the same as what the guage on the front of the boiler reads, with no water in the boiler and the drain point open you will get the proper reading and if it doesnt hold the pressure it is obviously split/perforated. An easy way to check if its perforated is to just press the pin on the expansion vessel valve (shrader valve) very quickly and see if water spits out.

 

[ericp]

Many thanks. The service engineer did give a quick press on the valve to make sure no water came out so it's probably ok (unless it's lost pressure over the last couple of years).

My post was originally on another thread (as a reply because I didn't know how to do a post) and someone else has suggested that since I need about 1 litre of expansion per radiator and have 12 rads, I probably need an external expansion tank. That makes some sense but I need also to cure the leak from the pressure relief valve. Maybe that is why I am not seeing the over pressure which I would otherwise be getting from too small an expansion tank.

Regds

Eric

 

[wes]

let's put thing right. isolate the boiler close the flow and return, fill the boiler to a 1.1 bar of water pressure. leave for a day and ckeck the pressure the next day. just to see if you loose any presure. if not the open the valves and leave for another day if you loose the presssure, then the leak in on the pipe work. all this in cold if you say that the prv leak please change it. to ckeck the expansion vessel pinche it on the little nipple, if not water comes out is fine if water comes out is broken well if no water comes out to precharge is easy fine the tyre pump electric one. in cold again
close the flow and return open the prv or the d.o.p of the boiler leave open connect the air pump to the vessel you will se how much pressure you got on the vessel on the air pump manometer precharge to 1 bar only or have a look at he expansion vessel data sheet to what precharge pressure should be once the pressure is reached unplug the air pump fill the boiler to 1.1 bar bleed the circulating pump and open a bit the prv to bleed te boiler too. done that open flow and return fill to 1.1 agian open screw of pump to bleed and prv again and that shoul do it.

 

[ericp]

Many thanks Wes.

It has kind of been behaving itself recently. By keeping the pressure up to about 1.5 to 2 Bars it keeps going for a week or more. However, I notice that the pressure rises by about 1 Bar when hot - ie up to 2.5 to 3.0 bars - not enough to blow the PRV but more that the 0,5 bar rise which I understand is reasonable.

No sign of leaks in pipes which is 10 mm microbore. I have recently had floorboards up doing some flooring repairs and had sight of most of the pipes -all ok.

Looks like an external expanasion tank would help which I can get fitted when the weather warms up. In the meantime I will check the expansion tank as you suggest and see what comes up. That will have to be next week as I have rather a lot going on this week.

Best Regards

Eric