Blocked header pipe - consequences?

Hi, appoligies for the long post, please bear with me......

I'd appreciate any advice with the following, I'm a service engineer, not in this field but not completely clueless.

I have the boilermate II system, it's about 9 years old. About 4 years ago the heating circulation pump had to be replaced and due to a dodgy isolation valve the main tank had to be drained. When trying to refill the system I found the feed pipe from the header tank to be blocked - seems very common! Despite various attempts I could not un-block and resorted to filling the tank using the overflow pipe at the top of the header tank.

This was successful as far as I can tell, no issues since - no air in rads when trying to bleed, no issues with hot water and no 'trickle' sound from inside the main tank.

However, in the last 4-5 months 5 of my rads have burst! Mostly with pinhole leaks. The circulation pump has been clogged with debris and when I removed one of the rads there was quite a bit of fine debris clogged in the taps at the bottom. All seems to point to accelerated corrosion? It's possible there was little inhibitor in the system for the last 4 years, although 1 bottle was added to the header tank it may not have made it in to the system.

My question is, could the fact that the header pipe is blocked cause accelerated corrosion? Is it possible to run the Boilermate II with a blocked feed pipe long term? The overflow pipe is clear so as I understand there is room for expansion? I know of other conventional heating systems with little or no inhibitor that don't show any signs of corrosion.

I've been advised to get a Powerflush to remove the debris, although I'm woried that the other rads may also be weakened and doing this will burst them as well? I'm prepared to remove each one and flush individually, is this viable or is there likely to be debris throughout the pipes as well?

Appoligies again for all the text, any help or comments would be greatly appreciated.

Iain.

Hi,
Mike has some good advice on the boilermate 11 page regarding replacing the filler/expansion pipe. You are right the inhibitor proberly did not make it into the system. Rads are cheap, replace them and connect with telescopic tails, first flush all the pipework with 2 hoses, after that fit a magnetic filter to catch the sludge over the next few months. Replace any old pump valves left on system and refill, once leak free, drain off 3 litres. Add 3 litres of inhibitor or 1 for every 100litres of water in system inc pipes heat bank and rads, the vols of rads are on manufactures info. you cant over doses with inhibitor. Good luck Paul.

Thanks for the reply.

Isn't it unusual for the rads to corrode so quickly? Would you put that purely down to the lack of inhibitor or would the fact that the pipe between the expansion tank and main tank is blocked have an effect?

Cheers, Iain.

Hi, The filler does at least 2 jobs, it obviously facilitates filling and dosing, but the correct name is the expansion pipe, as the volume of the water increases with temperature approx 4% the excess volume will backup the expansion pipe into the expansion tank. If this pipe is blocked the system would pressurise, so a vent pipe is used for added safety. When the expansion pipe is blocked, the expanded volume will rise up the vent and fall into the expansion tank, the water will absorb air as it falls and splashs. This airrated water is what encourages corrosion, Sorry I cant expain why it does this.

When the expansion pipe is open only expelled oxygen bubbles will rise from the vent pipe. Together the two pipes ensure the safety of the water heater by holding the tank at atmospheric pressure. Note High pressure and high temperature is a lethal cocktail because, If pressurised hot water over 100degC is allowed to depressurise, say becauce of a fault like a leak or fracture then the tank contents would turn to steam expanding by 1400% instantly. The result would be an explosion which would distroy the dwelling and anyone inside it, Very scarry thorght!

Fortunitely this has not happened as far as I know in the UK since the 1930's when unvented heaters where 1st designed. It is the reason why they were outlawed in the UK until 1984. In conclusion anyone owning an unvented water heater should ensure the safety valves are operational by testing regularly, but to my knowledge one one ever does this!!

Rest assured a thermal store is the safest form of water heater because it is vented.