UFH & Combi problem

[T90]

Hi, I have an old Ferroli Modena combi serving a 1 bed flat. Never any problems. I’ve just installed underfloor heating. It was working fine whilst I had the ufh and a last temporary rad connected up. I’ve just removed the last rad and now I’m having a problem with kettling. The manifold has a mixing valve and pump. There is only 1 zone, very simple, (no zone valves), so I’ve wired the boiler pump and the manifold pump to run continuously whilst the nest thermostat calls for heat to always have flow around the system. Pump also has an overun. Set both the boiler stat and mixing valve to 40 degrees. I’ve fitted an auto bypass under the boiler. The boiler keeps kettling briefly (very loudly) before its internal stat switches off. The ufh is warming up nicely and the gauge reads 35 degrees. The bypass is on its maximum setting, but nothing flows through it (it stays cold even when the kettling starts). I’ve had it on for hours fiddling, and the return feed to the boiler feels cold to the touch, whilst the supply is very hot, so definitely nothing going through the bypass at any point. If I turn the boiler stat down anywhere below 40 degrees, it cycles on for maybe a minute, then switches off completely and doesn’t seem to want to come back on even though the pumps are both still circulating. I’ve sat here waiting about an hour and it still hasnt come back on unless i turn it up above 40. Baffled. Could the auto bypass be faulty? Do old boilers not like being set to less than 40 degrees? Have a feeling the mixing valve must be allowing just enough flow to stop the bypass working, but restricting it just enough to kettle the boiler. I’m considering replacing the auto bypass with a gate valve and balancing it out manually. Any thoughts?

 

[DP]

Old boiler or whatever, bypass should be opening to allow circulation. Is the valve fitted correctly to allow circulation- it is not bidirectional

 

[Madrab]

If the boiler is overheating and kettling then there isn't enough flow through the boiler to allow the heat to be dissipated quickly enough. What type of flooring do you have over the UFH, running the UFH @ 30Deg is quite low? Have you also turned the output of the boiler down? Now that the UFH is the only output then it only needs 50 odd Deg max. If the water entering the blending valve is too hot then it will blend more cool water from the return on the UFH and then the boiler can overheat. Sounds like the system needs set up correctly.

All that being said if the blending valve is doing it's job properly then it should be restricting the hot flow enough to create enough differential pressure to open the ABV. So as suggested is the ABV installed in the correct orientation? If so then I'd be tempted to swap it out for another one just to be sure.

 

[T90]

thanks, just read through the boiler manual. It says if you turn the dial below 50 degrees, that's "summer mode" apparently for hot water only. They "don't recommend" running the heating at less than 50 degrees for some reason. That's prob why it keeps switching off. The UFH is under engineered wood and tile, wood flooring manufacturers said max 35 degrees, but I've set it to 40, which is more comfortable. The blending valve came factory fitted to the manifold, and maintains a perfect temp. I'm using one of these https://www.toolstation.com/22mm-automatic-bypass-valve/p42638. It's 1 directional. I've fitted it so the arrow points from flow to return. I'll try swapping it for a more expensive brand one and see if that makes any difference. I've fitted it just below the boiler, reading up online, some say it should be as far away as possible from the boiler by the manifold? The flow and return rise up into the ceiling, across and down to the understairs cup'd, maybe the natural rising effect in the flow pipe is stopping it travelling down to the bypass?

 

[T90]

So, update, I tried changing the auto bypass for a different type one. And setting boiler to 50c. Still kettling even with bypass on max setting. Out of interest, tried taking it out and putting in an ordinary gate valve, open 1/4 turn and the whole system works fine. Don’t understand why the bypass valves aren’t opening. Should I just leave a manual gate valve in? It’s not going to be any more or less efficient than a bypass valve open all the time really is it

 

[T90]

I’ve solved it. Moral of the story, never assume a piece of red tape somebody’s put on a pipe at some point means “flow” And italian boiler diagrams are looking top down, not bottom up. feel really stupid now