Automatic bypass faulty?

[keynet]

I have a 5yr old Vaillaint Ecomax 622 system boiler with radiators, Megaflo cyclinder and under floor heating manifold, each controlled on a separate Honeywell valve. The system was new with the boiler (house renovation) and has MB1 in it.
Recently the boiler started short cycling on the UFH (only), manifold input only getting to about 45C before the boiler return gets to about 60C in less than a minute. The temp of the pipe on the bypass inside the boiler (where it meets the main return) is much hotter than the return, even when the hot water cylinder is on. When *only* the UFH is on, the main return is only tepid, the bypass pipe pretty much at flow temp - hot! I'm pretty sure the UFH circuit is clear, though the manifold will provide more resistance than the tank. Theres 1 bar of pressure in the circuit. I'm thinking maybe the bypass is bypassing when it shouldn't, as the return into the heat exchanger is significantly hotter after the bypass, even with the hot water on, which should provide a pretty decent return rate (and the cylinder does heat up reasonably quickly, boiler doesn't short cycle when the water is on). The boiler isn't reporting any faults.
So -
1) Is the bypass likely to have failed?
2) Any kind of tests I should run or other things to look at before replacing the automatic bypass?

Thanks for any advice!

 

[moony]

1) Is the bypass likely to have failed?
2) Any kind of tests I should run or other things to look at before replacing the automatic bypass?

Thanks for any advice!

1) doubt it but yes it could be that
2) is the pump for the primaries in good working order ?

 

[shorticus]

you don't mention whether the ufh is actually working or not, is the two port opening fully, is all the air vented, what about the rad circuit, is that ok?

 

[keynet]

Thanks for your replies -
shorticus - yes the ufh is working but not as hot as it should be as the input to the manifold is only reaching about 45C before the boiler short cycles out as I think it shuts down when it sees its return over about 55 or 60. It used to get to 60.

Now checked the rads in more detail, yes they are getting up to temp, but it takes perhaps twice as long - think the boiler is again short cycling, maybe not as much as when ufh only.

moony - pump is internal to the boiler, and on setting 1 (as originally), seems to be good, more flow (and more return/bypass!) when on setting 2. Not particularly noisy.

Thanks again for any feedback

 

[shorticus]

@keynet, I've had a look at the manual and the pump lift is a max of 25Kpa (1/4 bar) so the spring in the bypass valve is not going to be particularly strong. Your system pressure when cold should be set at 1.5 bar, this may have an effect so try that first. If you are 100% sure that all the motorised valves are opening properly, that you've vented all circuits and you're not sludged up (unlikely but it is possible) then you're probably right about the bypass being faulty. If it is the problem then it may just be a case of a bit of debris inside, can you get it apart for a clean/inspection?

 

[keynet]

@keynet, I've had a look at the manual and the pump lift is a max of 25Kpa (1/4 bar) so the spring in the bypass valve is not going to be particularly strong. .... If it is the problem then it may just be a case of a bit of debris inside, can you get it apart for a clean/inspection?

I'll see if I can get it apart this weekend and see if it's bunged up. Let you know what I find, maybe with pics even.

 

[shorticus]

I'll see if I can get it apart this weekend and see if it's bunged up. Let you know what I find, maybe with pics even.

The manual doesn't say whether it's servicable or not and there are no diagrams either, I guess it'll be a question of having to suck it and see, pics would be great as I'm now as keen as you are to know the cause.

 

[keynet]

The manual doesn't say whether it's servicable or not and there are no diagrams either, I guess it'll be a question of having to suck it and see, pics would be great as I'm now as keen as you are to know the cause.

Hi shorticus,
It was the bypass - had a lump of black crud keeping it slightly open. As you mentioned, it's a simple animal with a fairly weak spring, not much to go wrong unless something gets jammed in it.
See pic below - sorry camera wouldn't focus properly.
Looks like I need to do a good system flush again shortly. The water that came out wasn't gungy, it was clear apart from a few black gritty particles. Not sure how normal this is.
So I'm now back to temperature and no short cycling
Just 1 further issue - bizarrely the service value at the bottom of the boiler is now dripping - started as soon as I closed it. Grrrrhh!

 

[shorticus]

many thanks for the update, amazing how something so simple can cause so much grief, glad you sorted it. Shame about the service valve but it's always a risk using a valve that's not been moved in a while, sounds like you're pretty confident with a set of spanners so it should be no drama to replace. Even sealed systems dosed with inhibitor will get foreign bodies floating around in them, if it's only five years old I doubt that you'd need anything more than a simple raw water flush and a re-dose.

 

[keynet]

Well that lasted about 2 weeks! Then same issue again.
So as I didn't fancy a complete drain down/flush in this weather, I decided to install a Fernox TF1, as filters appear to be standard fitment these days to prevent long term build up, despite the chemical inhibitors. The TF1 has lived up to expectations - easy to install and clean, and it's catching the crud and slowly removing it. I found that my UFH overheat valve was bunged up with magnetite as well, this was helping the bypass to bypass of course - needed exercising a couple of times, and the boiler overpressure valve was weeping, explaining a slow loss of pressure, that needed similar couple of pings to make it seat correctly. All in all, this shows the importance of a clean primary circuit, so in the summer there'll be a major flushing exercise, meanwhile I'll be cleaning the TFI every few weeks - only takes 5 mins.

 

[SafeGasInstall]

before tf1 you have to power flush !!!!