Auto Bypass Letting by (apparently this is normal (?) )

[Ric2013]

Hi there,

I recently fitted an ABV to a Y plan system due to the fact that the heating side is full of TRVs and the boiler has a minimum flow requirement.

The boiler also has a requirement that if a bypass be fitted in a Y plan scenario, the bypass should be capable of dissipating 1kW of heat. Due to the fact that the primary runs to the three-port valve are long, they are well lagged, so I opted for the unusual arrangement of running the return from the bypass through a small radiator connected via male irons, just to help meet that specific requirement.

This worked well for two weeks. Today, I had to look at again because the radiator was getting hot, and now, despite the fact that the bypass is set to its highest setting (highest pressure required to open it) and that the pressure differential is low because the DHW coil is not a great restriction, nor is the old boiler very restrictive, the bypass radiator is warming up. The bypass is letting by. I had expected this could happen during central heating mode, but it shouldn't be happening now when only DHW is selected. Tried closing the cyclinder balance valve and re-opening in case some debris was blocking the valve, but still there is let-by.

So, either I drain the system and relocate the feed to the bypass from the radiator flow pipe (it currently tees off before the 3-port valve), or the bypass has developed a fault and is the problem.

Honeywell informs me that an ABV is designed to have some let-by. This isn't what I understood from college, in which I am taught that the whole point of an ABV is that is can shut off when bypass is not required which is why a fixed bypass is no longer acceptable.

What do people on this forum think?

Thanks for any comments!

R

 

[Ric2013]

UPDATE

I am of the opinion that the the lady in the technical department was fobbing me off.

I have since contacted Honeywell again by email. The first solution was that the valve calibration could be out if someone had turned it beyond 6, and then (when I pointed out that no-one had touched the valve except me) that the valve could be changed under warranty on the grounds that if the next one is the same then the problem is obviously in the system, not the valve. From this, I would assume that Honeywell entertains no possibility that there could be a faulty batch of valves.

I can't get Honeywell technical to answer my emailed question as to whether let-by is normal or not, as they answer around the question. An attempt was to make the question very specific and I've had no reply to that.

All I can assume is that the valve is not made to close enough tolerances to allow this fine a level of control as the valve itself is not much more sophisticated than a plastic disc bearing on a brass seat, and the plastic possibly is not smooth enough. Ceramic onto ceramic would have been what I'd have expected for the cost of Honeywell, to be honest.

I'll try to shame Honeywell into sending me a new plastic disc, though silicone grease on the old one seems to have improved matters significantly, and if not I'll buy a new ABV made by someone other than Honeywell and see how it compares.

 

[oz-plumber]

If you have a speed setting on the pump, reduce it when you are only heating the cylinder.
See if that stops the by-pass.

As for Honeywell admitting a fault with their product - no chance.

What size / type of pump have you got? 25/60. 15/50, there should be something on the pump with figures like that.

 

[Ric2013]

What size / type of pump have you got? 25/60. 15/50, there should be something on the pump with figures like that.

It's an old (pre-energy saving type) Grundfos Selectric UPS 15/50. Even on the slowest speed, it's overkill for my system, but even with a total blockage it shouldn't develop more than 2.5m head at this speed, and the valve was letting by when set to 6m head.

 

[Ric2013]

As for Honeywell admitting a fault with their product - no chance.

I'm actually fed up with Honeywell products. They can be beautifully engineered, but I always seem to expect more from them than I get and I'm fed up with the disappointment.