Viessmann Vitodens 100 low temperature cycling

JamesKnight

Vitodens 100 WB1B heat only boiler with external primary circulation pump, low loss header and a separate circulation pump for the 6 circuits which it feeds (1 hot water cylinder coil, 5 heating circuits).

Primary circulation pump is on the return side of the low loss header. Secondary circulation pump is on the flow, so path is low loss header -> pump -> 6 x circuits with zone valve on each -> 6 x return tee -> low loss header.

All works perfectly most of the time. But then, after 2 or 3 days, I have no domestic hot water and the house is chilly. The boiler is cycling between around 35 and 45 degrees C, and occasionally I get a short burst of heat on the flow from the low loss header (2-3 seconds), the boiler then fires up briefly for 10 seconds or so, heats up to mid-40s and then turns off again. This repeats, constantly, but there is never enough heat in the system to do any good - and it can be like this for days. Power cycling the whole system resets it to working condition straight away.

Boiler has been re-built by Viessman (fixed cost repair including control board, insulation on combustion chamber flanks, electrode and fan). Primary heat exchanger works fine. All zone valves are working correctly. There is power to the boiler and both circulation pumps.

Trying to visualise the flow, I think that I have water flowing round and round the short primary circuit through the low loss header, which periodically drops enough temperature to make the boiler turn on, but there's not enough heat being drawn off for the boiler to detect any meaningful heating need, so it powers down again. That leads me to think that no water is flowing through the main heating circuit in the house, except in occasional very short bursts.

That (I think) points to a problem with the circulation pump and/or a blockage in the secondary (whole household) circuits. But. If I turn off the system for just 20 seconds or so, and turn it back on again, everything returns to normal. Heat flows through all the circuits, the house is warm and I have an enjoyable shower again.

I've tried draining off the bottom of the low loss header, and it's clean.

Does anyone have any insight? Thank you.

psb75

"Boiler rebuilt by Viessmann" ? Assuming you mean they paid for the parts and labor? Also assuming whomever was "working for Viessmann" tested and reset the boiler etc. So is the boiler showing any codes or info during its operation? Have you consulted V. tech support? Text them first. This boiler must be on their 'radar' at this point after all of that work.

GGross

I would start by checking cold static fill pressure/leaks (hard to do during heating season), expansion tank pressure/location. These boilers will not fire or give an error code if they don't prove flow through the heat exchanger. If the temperature in the boiler remains hot, while the heating circuits are not heating the space I would look for air in the heating circuits, or some other blockage. If during one of these failures the boiler temperature drops below its setpoint and doesn't fire back up I would be looking at something stopping flow through the heat exchanger (usually air, no pressure etc). I'm thinking you don't have flow through the boiler as you said the DHW also drops out, but you may have the DHW piped as a zone on the heating circuit side instead of piped off the boiler loop.

@psb75 in many areas throughout Europe it is not uncommon for boiler manufacturers to have their own service techs that do on site repairs.

JamesKnight

Thanks for the replies - this was indeed caused by a blockage, as my engineer finally discovered today. There was such a solid mass in the bottom of the low loss header that it didn't even get into the drain-off point, hence I was fooled by getting cleanish water from it. A proper poke around with a screwdriver revealed a mass of metallic sludge, no doubt caused by a lack of inhibitor. Having cleared the blockage (and added inhibitor), the boiler now appears to be running cheerfully in the high 70s.

For posterity, in case this helps anyone else, note that this is a heat only boiler, with the DHW piped as a zone on the heating circuit. The boiler was getting plenty of flow through its primary circuit, but there was a periodic blockage in the domestic (secondary) circuits which prevented water from flowing around the house.

There were no error codes at any time - the boiler just ramped up a bit, and then back down again, every 60-90 seconds. No doubt it would have been a different story without the low loss header to provide hydraulic separation.

And yes, in the UK Veissmann have an engineering team who carry out a fixed price repair which covers everything except the primary heat exchanger. And I do mean everything - the engineer replaced the lot.

Thanks to psb75 and GGross for responding. Case solved, though I'm unsure how to mark it as such as a newbie to the forum.

GGross

I am glad they were able to fix the issue. On your next regular service call you may request that the tech blow down the low loss header again to see if any more junk has settled there. In addition I have seen some techs here in the US attach a magnet to the bottom of the low loss header creating a makeshift magnetic separator, that metallic sludge can cause many issues if it gets into other components, the best place it can be is in the low loss header, though it will need a blow-down every now and then to remove the stuff. Thanks for letting us know the issue was resolved!