Radiant Combi exceeding 3 bar ... ouch!

[johnspark]

Surfing around I found this board and started reading the threads, found them most interesting and amusing. So much so I decided to chance my arm and leave a question:-

I have a Radiant RSF24, 27KW combi boiler, located in the integral garage and heating the house and supplying hot water. I was short of money about 8 or 9 years ago and installed the boiler and a few extra rads myself and got a corgi engineer to come and connect it to the gas supply and commission the boiler.

It was working fine until last year, then the pilot light kept going out. Not too difficult that one, faulty thermocouple. Those nice design people combined the thermocouple with the safety stats into one harness, so a little more tricky and expensive to replace than it need be.

Then a couple of months ago, hot water but no heating. Didn't take me too long to figure out the divertor valve wasn't working and replaced the syncron motor.

Last month the boiler kept going out, pilot light as well. Took me a little while to work out the exhaust fan was sticking. Stripped it down and left the bearings in WD40 for a couple of hours and despite the play on the shaft it has been running nicely ever since.

Most recent fault has got me complete stumped though. I will describe what I see as the symptons but I can't figure out the root cause, maybe one the more experienced Gentlemen can lend a hand ....

The system is pressurised to just over 1 bar. If hot water is commanded then everything appears to work normally and the system pressure is only elevated by about 0.2 bar, even with the hot tap running for 10-15 minutes at just over 10 litres per minute.

When the central heating comes on, the system pressure starts to rise gradually. Over a period of say 20 minutes the system has reached 3 bar and is blowing water out the pressure relief valve - had plastic bottle outside the house to make sure and it collected about half a litre.

When the system gets cold, the pressure is now below 1 bar and will not come back on due to "low water level". So why is the pressure getting too high?

At first I thought it might be the expansion vessel so I checked the pressure and it was still at the recommended 1 bar. I pumped it up a bit, let some air out (not water thankfully) and pumped it back to 1 bar.

If it is not the expansion tank them the water must be getting too hot?

I lowered the boiler output using the knob on the control panel label "heating temperature control". From a series of indicators on the control panel the boiler temperature was 10-20 degree lower and the boiler would turn the main gas burner off, but leave the pump running until the system's temperature was low enough to bring the main burner back on. The control system did seem to be working. Pressure was a bit better at 2.8 bar not as much heat at the radiators at the other end of the system.

I still can not figure out why the system is running at too high a pressure and I have had to fix a couple of leaks inside the boiler due to the higher pressure.

The radiators are getting hot, so I don't suspect the pump too much, could be wrong there though.

My best guess was the temperature sensor on the output side of the heat exchange is actually producing a wrong reading and the boiler stays on way too long. So tonight I use a temperature meter and thermocouple placed next to the heating sensor. Here's what I found ...

Open the tap in the kitchen and let the domestic hot water run flat out. The pressure increases from 1 to 1.2 bar. The control panel shows the temp increasing from 40-50-60-70 centigrade, the Fluke meter shows the output of the heat exchanger at 65C. To me everything is stable and we have lots of hot water in the kitchen.

Now go turn the room thermostat way up and command central heating. The temp drops to 40C and the pressure rises a little to 1.5bar. 5 minutes later the temp is at 50C and pressure at 2 bar. 10 minutes later the temp is only at 55C and yet pressure is not at maximum 2.5 bar. Test aborted as clearly the system is not over temperature.

The resistance of the temperature sensor near the heat exchanger is compared with the one at the output of the DHW exchanger and found to be within 10% during the tests.

So, I am stumped, why the increase in systems pressure during central heating. Of course this may have been a deteriation in the system over the past few weeks and it is not until the boiler doesn't come on one morning due to low pressure you realise something is wrong.

All thoughts and comments welcome, even funny ones
I think I have given way to much info, sorry for making you read it all, which of my assumptions are wrong?

Confused John of Essex ...

 

[croydoncorgi]

Weird!
Does the pressure keep on rising towards 3 Bar and the system CONTINUE to vent from the overflow? Or does it stop venting once it's got to the set temperature?
Are you sure the pressure ONLY rises when the heating is on?
There are very few ways this can happen if the expansion vessel is working correctly. One way is a leak across a heat exchanger from mains-pressure water into the CH plumbing which should be at 1 Bar. But with this type of leak, the pressure should rise continuously.

 

[PEDANTICVINDICTIVEMAN]

Jeez ANOTHER boiler I have never heard of, really does amaze me where these manufacters pop up from, of course they will have been making boilers since adam and eve were around and sell more than ANYONE ever and obviously have been making condensers since a million BC thas why we all have never heard of them.
ANYWAY, I would check whether the guage you are pumping up your xpansion vessel with is correct plus have you done the pumping up with the boiler drained and the drain valve left open so that when you are pumping it up it will force water out the drain valve, then close off drain valve before repressurising system to 1 bar with the filling loop.

 

[kevplumb]

me thinks ravenheat ??? arrrrrggggghhhh

 

[PEDANTICVINDICTIVEMAN]

Ravenshitte.

 

[keego]

Half empty one rad & run the heating, it will act as an expansion vessel. If it runs OK then change or add a expansion vessel.

 

[gasman]

You probably have a blockage in the pipe leading to the expansion vess. This would cause the system to over pressurise as it is doing and also the press in the exp vess would still have .5 to 1 bar. Probably blocked with iron oxide deposits, they usually are.

 

[raykeeley]

when you checked the charge of the expansion vessal , was the pressure at 1 bar accross the system.

 

[johnspark]

Firstly thank you all for the replies.

I have figure out what is wrong, the expansion vessel is loosing air ...

I pumped up the expansion vessel again with a foot pump to 1 bar, using the release valve on the boiler side to make sure the water was also at 1 bar as read on the pressure gauge, took a few iterations. I then run the boiler flat out and it never went above 2 bar. Yay!

Unfortunately, the boiler later did not come on due to low pressure. I have topped it up a couple of time today, so it must be loosing air from the expansion vessel

My fault before was not to ensure that on repressurising the expansion vessel to lower the boiler pressure back down to 1 bar, so I didn't have a full charge. Also borrowed a tyre pressure gauge that read in bar not psi this time.

For those who have not seen the boiler, here's their website. It's not a recommendation, just info ... www.radiant.it there is an English button luckily.

Maybe someone can confirm if its a Ravenheat ??

So leaking expansion vessel, most likely the schreider valve I would think. Will try some soapy water on the valve otherwise it looks like a new one.

How often do they go ??

---------------------------------------------------------------

Thanks to raykeeley for giving me a big clue, I knew 1 atmosphere was almost 1 bar so there was no point in draining the system. Just forgot to check the system pressure after the pump up.

Also to keego, I had considered empting half a rad. Nice one.

Also to kevplumb, I enjoy your comments

Also, PEDANTICVINDICTIVEMAN, yep, I got a better tyre pressure gauge.

Also, croydoncorgi, it vents until system temp stabilises and vented enough water to stay at 3 bar. Expansion vessels are more important than I thought.

Sounds like the Oscars ... John

 

[PEDANTICVINDICTIVEMAN]

Ohhhhh daaaaaaaarllllllllllllllingggg, luvvie, luv you all thank you so much your performance was maaaaaaaaaaaggnifficent

 

[PEDANTICVINDICTIVEMAN]

Radiant have ONE service engineer for the whole of England, what a blooddy joke, wonder how many days it takes to get him to come round in winter if you live in Scotland