Viessmann boiler failure

[boater sam]

If your cold water main pressure is over 4 bar be careful. Rads are normally tested to 100psi, 7 and a bit bar when new. Old ones will crinkle a bit at 5 bar. The problem with trying to test a whole old system is how fast you can get round and find any leaks. Even power flushing will not shift all the crud unless you do one radiator at a time so its actually better to take them out and just flush each one after you have flushed the system through the drain tap/s ( which will probably need a new washer once disturbed, available from a merchant for pence ) to clear the pipework.

In my opinion the power flush idea is just a lazy way to make money and not the best way to clear the crud which is surprisingly solid in the radiators. Its fast but expensive, the old way is cheap and sure, just takes time. 
The advantage is that you remake all the joints at the ends of each rad as you go,  so you inspect the valves too, which are the main source of minor leaks.

[ppppenguin]

Last month I finally got round to cleaning out the system. I fitted a temporary contraption of valves to alllow forward and back flushing. Then put Fernox DS40 in the system. At first I had trouble getting any circulation - it must have dislodged a lot of black gunge. Then it alll settled down and I ran the system gently warm for 3 days. Added more DS40 as I wasn't satisfied that it was 100% clean. After neutralising the DS40 with the sodium metaborate sachets I forward and back flushed each rad with mains pressure for at least 5 minutes. Added Sentinel X100 and refilled.

Lots of bleeding later the system was running nicely. Let it settle for a few days to let the dissolved air out of solution, bled again and set the cold pressure to 1 bar. To my astonishment the cold pressure rose to almost 2 bar over the next few days. I though the filler loop might be letting through but it wasn't and I left it disconnected. Then I thought about leakage from the DHW secondary. Not possible because the head is only about 18 feet, so the secondary pressure is only about 0.5 bar. Although I've got a minor doubt about gauge pressure and absolute pressure. The only other thing I can think of is that something chemical is going on. Which it shouldn't because there's ample X100 in there.

I let it run for a couple of weeks like this. Bled again and not too much air came out. Set cold pressure to 1 bar. After a few days the cold pressure is up to 1.2 bar.

I'm a bit confused and worried. I think I'll take a sample of water and put it in a closed jar with some nails, just to prove the chemistry is OK. Or get one of these test kits: http://www.plumbase.co.uk/sentinel-x100-...-0-2205364

[boater sam]

Thoughts.

1  Cold water in, house warmer= pressure rise?

2 Draw hot water , DHW, heats boiler and some pipework=pressure rise.

3 House sinking into hole=atmospheric pressure rise. ( Joking )

4 solar gain? Unlikely in our climate.

Would not worry about rising pressure, better than falling. If you have plenty of corrosion proofer in it will take care of the steel radiators and the ally bits. Boiler is mostly stainless steel, cu pipes will look after themselves.

Sometimes its good to be an ignorant know nowt Joe Public, saves fretting about the inconsequential things.

[ppppenguin]

When I adjusted the pressure last time I had to drain some water to do it. So no shock of cold water involved. When I check the pressure it's like with like, in other words when the boiler has been off for several hours. I appreciate that the hot DHW in the cylinder can heat the primary coil and induce circulation via the boiler but there's a closed motorised valve in the circuit so it won't happen.

Another minor oddity. The boiler tempearture readout never goes below about 30 degrees, even when the flow and return are cold, blatantly below that. I don't think it's affecting the operation of the boiler but I don't recall it was like that before I cleaned the system. I've used my infrared thermometer to confirm the pipe temperatures too.

I'll get a Sentinel test kit and give it another can of X100 if necessary.

[boater sam]

This boiler temperature readout and pressure readout, are they old fashioned guages or electronic displays?
If the later, gremlins in silicon.

[ppppenguin]

The pressure gauge is proper old fashioned. Works when power is off. Temperature is an LCD display.

Managed to get a Sentinel X100 test kit locally and have just tried it. From the colur of the urine test sample I reckon the horse had been dead for at least a week the amount of X100 is a bit marginal. Won't hurt to put another litre in.

[boater sam]

Would take the lcd reading with a pinch of salt, the thermocouple will be a really cheap one, read higher as they age.

[Mark Hennessy]

The flow temperature sensor will be an NTC thermistor - do these go wrong with age?

On that boiler, it ought be a decent one. Accurately knowing the flow temperature is an essential part of the "Weather Compensation" scheme that Viessmann do. In warmer times, the flow temperature can get as low as 30C, so it would have to maintain its accuracy down at those levels.

At the risk of stating the obvious, IR thermometers under-read from copper. A layer of black insulating tape is all you need to get an accurate reading. But certainly, I'd expect 30C to result in pipes that are warm to the touch compared to the adjacent gas pipe...

But having said all that, it's entirely possible that they cheaped out when doing the control electronics for that boiler, given that the 100 model was a cut-down UK special.

Anyway, it's easy enough to check, as the installation guide gives a chart of temperature vs resistance for the sensor (see attachment).

   

[ppppenguin]

I realise that a shiny surface can give a false reading but wasn't expecting much of an error. Good idea about using black tape.

That curve assumes you have an infrared or clip-on thermometer to check the temperature of the pipe.

[Mark Hennessy]

With copper, it's really surprising - many 10s of degree at normal operating temperatures. By comparison, the error from shiny bright aluminium heat sinks might only be a few degrees at a similar temperature.

Any half-decent heating engineer should have the gear to check against that curve - as do you - but time is money, so they'd just change the sensor (not their money, after all). Whereas we have the advantage of time and patience, plus a desire to only change components that have actually failed. Or to put it another way, be grateful that they included that curve - most manufacturers don't bother IME!