Boiler pressure sensor

[Kevin Poole]

My son's Glow-worm 30cxi condensing combi has performed perfectly for 3 years.

Today, it didn't fire up. The display was flashing a pressure of

0.4bar, so he opened the filling valve. He heard some water flow in, but the pressure barely changed, and the flow soon stopped.

Power cycled, and it showed 0.6 bar, and fired, but still wouldn't accept any more filling. Let a bit of water out through the prv - no change of reading, but it would then admit a small amount to refill.

It sounds like a faulty pressure sensor to me, but I suppose it could be a circuit board fault.

Can the pressure sensor be tested other than by substitution? Can any dummy load be applied to the circuit board to give a nominal pressure reading?

Pressure sensors seem to be around the £70 mark, so he'd rather not buy one if the fault's more likely to be elsewhere. Glow-worm part

2000801911, GC E83997.

Any ideas, please?

[John]

Are both valves opened when you fill it?

[Kevin Poole]

Yes - and we tried both the internal and external filling loops.

[John Rumm]

It sounds like you have pretty poor mains pressure. A typical result of opening a filling loop and leaving it on would be to raise the pressure to the point the PRV operates. Have you got an gauge on the external; filling loop?

[Tony Bryer]

Until you said this, I would have suspected the internal filling tap. On my Cxi this broke internally so that turning it on did nothing. In the best tradition of spare parts, a replacement cost £21 - for a little all-plastic tap.

[Kevin Poole]

No mechanical gauge at all, I'm afraid. The question of why the prv didn't operate is one that's been bothering me in the dark watches of the night. Starting fault-finding at 10pm in someone else's house is not ideal for a day-person, so I didn't think to check the flow rate at his cold tap, either. The prv isn't stuck - I manually released it OK, so what pressure do they normally blow off at?

Mains pressure is _normally_ fine - he's at about the lowest point in the county.

[John]

I thought it best not to use PRV as it can lead to them weeping if grit gets into seating.

[Doctor Drivel]

If the pressure sensor is £70 then yiou need a pressure tester.

?_dyncharset=UTF-8&fh_search=82412&searchbutton.x=0&searchbutton.y=0&searchbutton=submitFit a washing machine tap on the CH return pipe and use this as the test point for the system. Use a 3/4" BSP brass cap on the washing machine tap.

You can test the water mains pressure with this too, from the garden tap or the washing machine tap. This will determine if the internal gauge is faulty, or if it is faulty and the boiler still works then use this gauge to set the CH system pressure at 1 bar and save £70.

[Kevin Poole]

?_dyncharset=UTF-8&fh_search=82412&searchbutton.x=0&searchbutton.y=0&searchbutton=submit

Thanks for the suggestion - a gauge like this, or one I mackle up to do the same job, would certainly establish what the pressure in the system really was, but if the built-in sensor is faulty it would only be a permanent solution if I bypassed the sensor completely - at the moment the sensor, or rather its processed output, does two things: it gives a reading on the display, and if it thinks the pressure is too low, it locks out the boiler from firing.

[Doctor Drivel]

?_dyncharset=UTF-8&fh_search=82412&searchbutton.x=0&searchbutton.y=0&searchbutton=submit

I did make the point that if the boiler still runs, and the pressure is not being displayed, all you need do is attach the pressure gauge and set it to

1 to 1.25 bar. If it locks out on pressure out then you need a new assembly. But check that you have enough pressure from the mains first and this gauge can do that. Your mains may have dropped.

[<me9]

I've almost never had enough pressure to lift my PRV which is set at 3.5 bar. My mains pressure seems to top out at 2.5bar. At one time (before they built a local service reservoir) it would occasionally get high enough in the dead of night to lift the PRV.

I gather that in some areas the water companies are limiting pressure in order to reduce leakage.

[Doctor Drivel]

Do you keep the filling loop tap permanently on?

[John Rumm]

Approx 3 to 3.5 bar typically.

[Kevin Poole]

Thanks for that, John - higher than I was expecting. Checking again in daylight, I confirmed that the supply pressure is (unquantified) reasonably high; that the prv does just start to dribble if the filling loop valves are left open; and that the boiler display reads the same

0.4 bar regardless of the pressure in it, from very near zero to mains water pressure. The only way to get a lower reading is to disconnect the sensor from the main board, when it briefly shows 0 before giving the expected F9 fault code.

It's looking like the sensor, I reckon. What on earth possessed them to use such a sensor instead of a mechanical gauge and pressure switch?

[John Rumm]

Sounds like a sensor or PCB fault then...

Probably because the boiler would need a low pressure detection capability of some sort anyway, so it was cheaper to use that twice rather than provide a separate gauge!

[Kevin Poole]

To conclude the tale:

Much checking around found prices for a new sensor varying by a factor of more than 4. MJT Controls in Bristol

were by far the cheapest at £21.58 plus delivery and VAT, and they gave an excellent service.

The tip of the old pressure sensor was covered in rusty gunge, and I suppose a through clean may have unblocked its most intimate passages, but with a new one to hand, I just fitted it. All is now well.

Moral: when, a few days after installing a heating system, you hand your son a bottle of inhibitor and tell him to add it to his system (and how to do so), don't assume he will do it. I think it's now due for a dose of X400 or similar.

But I still don't know how the sensor works.

[Doctor Drivel]

Was it the gunge causing the problem?

As long as heat comes out they don't care, so ignore it. Then, when it is clogged up and they find out how much it will cost to unclog it, they get a wake up call. Like those who run around in cars with warning lights on and ignore them, then get upset when a big garage bill comes in.

The Glow-worm 30cxi condensing combi is a good boiler. The older one with the plastic hydro block were a little dodgy - Glow Worm replaced them for free. The later models are brass and far better. If this combis has the weather compensation option, have it connected. They really perform well.

I like the new Broags. Quality, great value and highly specced to.

[Kevin Poole]

I don't particularly want to disturb the boiler again to make certain, but I strongly suspect so. At first, I thought the old sensor might be of slightly different construction to the new, with some sort of deliberate constriction at the "wet" end, perhaps to dampen fluctuations, but then I dropped it and a plug of rusty gunge fell out. So I guess we may have wasted 33 quid.

We've been pleased enough till now. Easier to fit than the Buderus I was recently involved with, and the Buderus needed three warranty calls in the first 4 months.

I don't think it has, and in the totality of circumstances of its use, I don't think I could justify the cost.

[Doctor Drivel]

That is odd. Buderus are generally well made and "well designed " boilers. The heat exchangers clip in and out.