Water overflow from CH expansion/fill tank

[Chis]

I have intermittent problems with hot water flowing into my central heating expansion/fill tank from the vent pipe sometimes when the system starts first thing in the morning causing the tank overflow to run. It does not happen every time and it only seems to happen first thing and not when the system starts again later in the day. I've checked the expansion tank level before the system starts and it is OK - well towards the bottom of the tank. I've also checked it later in the day and again it's OK. I've tried lowering the level but this has not cured the problem.
The system seems to be unusual. The boiler (Grant oil-fired) is on the ground floor. From the boiler outlet the hot water flow goes through a Honeywell mid-position valve (V4073A) into two separate loops. One loop is for the radiators as usual, the other is quite a long loop which goes only to the hot water tank. The returns from both loops join up just before the pump (Grundfos UPS2) beside the boiler.
Any help someone could give me to trouble-shoot the system would be appreciated.

 

[exedon2]

Could do with some pictures of where the cold feed from the tank connects into system also vent pipe.

 

[rpm]

Any recent plumbing work carried out, no matter how small ie new tap or shower?

 

[Chis]

Could do with some pictures of where the cold feed from the tank connects into system also vent pipe.

Thanks for the reply. It's difficult to get photos because the connections are in the back of the airing cupboard. However, both the vent and feed join the system adjacent to the hot water tank which is on the first floor and directly below the expansion tank. The vent is teed into the CH hot water feed to the HW tank and the fill line from the expansion tank is teed into the CH line out of the hot water tank.
[automerge]1604999552[/automerge]

Any recent plumbing work carried out, no matter how small ie new tap or shower?

Latest work was a couple of TRVs. Before that, the boiler was replaced. With the old boiler, there were 2 pumps - one for the CH loop and one for the HW loop. Now there is only one pump plus a mid position valve. The controls (time switch and thermostat) were also replaced.

 

[exedon2]

That sounds rather like cold feed and open vent are still as they would be for gravity circulation are the connections of top of tees?
Reason I asked question I've seen a number of grant boilers over last 2/3 years that have left with this problem.

 

[SJB060685]

If the pump is situated by the boiler and the vent and cold feed are positioned as you say then this is wrong and potentially dangerous as the vent which should be unrestricted and positioned before the 3 port is now positioned after it. If this is the case then the vent and cold fill and pump need to be reorientated in the right positions, this will also stop the venting over.

 

[exedon2]

If the pump is situated by the boiler and the vent and cold feed are positioned as you say then this is wrong and potentially dangerous as the vent which should be unrestricted and positioned before the 3 port is now positioned after it. If this is the case then the vent and cold fill and pump need to be reorientated in the right positions, this will also stop the venting over.

Totaly agree! We have had the misfortune to find over 20 systems with cold feed and open vent still on cylinder connection's as you might expect with fairly disastrous results to boiler.
Laziness or lack of knowledge im not sure 2 installers Involved one now retired other now employed. Not that it makes much difference but at time both oftec registered .
To get back to op I would suggest you get a competent heating engineer to check your system

 

[John.g]

If I am interpreting this properly, the vent passes up close to the HW cylinder and the hot feed to its coil is Teed off from this and the cold feed goes to the coil return, this would probably work OK even with a S system if the HW zone valve is installed close to the cylinder coil inlet and will not interfere with the vent but IMO the vent is now effectively blanked off by the diverter valve with CH only selected?.

 

[SJB060685]

Yes John. If the valve was to fail in CH position then there is no safety vent. As I mentioned above someone who knows what they're doing needs to rectify.

 

[Chis]

If I am interpreting this properly, the vent passes up close to the HW cylinder and the hot feed to its coil is Teed off from this and the cold feed goes to the coil return, this would probably work OK even with a S system if the HW zone valve is installed close to the cylinder coil inlet and will not interfere with the vent but IMO the vent is now effectively blanked off by the diverter valve with CH only selected?.

Thanks for the input John and others. Before the boiler was replaced, there was no mid-position valve and there were two pumps on the system - one for the CH and one for the HW, both adjacent to the boiler. This would presumably be safe because the HW pump would never totally blank off the vent. When the boiler was replaced, the pump on the HW system was removed and replaced by the valve.
I see two possible solutions - either put a new pump in the HW loop adjacent to the boiler (where the original HW pump was and all the existing wiring is) or move the valve to a position close to the HW tank so that the vent can function correctly whatever the position of the valve. Any further comments would be most appreciated.

 

[SJB060685]

If I was to undertake this job I would recommend repositioning the 3 port in the airing cupboard along with the pump and repipe the vent and cold fill to enter on the flow before the pump at a maximum spacing of 150mm apart. This not only gives you the unrestricted vent pipe, which is solely there for safety and should never be compromised by a valve or whatever but it will also create a neutral point where the vent and fill meet and a positive pressure after the pump and throughout the system, this in turn prevents any chance of air being drawn in. There are a couple ways to pipe open vented systems but the above has the desired outcome.
The other option is to pressurise the system and do away with the header tank, cold fill and vent.

 

[exedon2]

If I was to undertake this job I would recommend repositioning the 3 port in the airing cupboard along with the pump and repipe the vent and cold fill to enter on the flow before the pump at a maximum spacing of 150mm apart. This not only gives you the unrestricted vent pipe, which is solely there for safety and should never be compromised by a valve or whatever but it will also create a neutral point where the vent and fill meet and a positive pressure after the pump and throughout the system, this in turn prevents any chance of air being drawn in. There are a couple ways to pipe open vented systems but the above has the desired outcome.
The other option is to pressurise the system and do away with the header tank, cold fill and vent.

Could not have put it better! if rads and valves are in good condition I would go for sealed system option not forgetting the prv needs to be 2.5 bar

 

[John.g]

Edit: Just saw above posts.

You could consider just sealing the system with a expansion vessel and filling loop and leaving your diverter valve as is OR a semi sealed, just remove and blank the vent at the cylinder, install a NR valve on the cold feed (preferably a swing check, horizontally) with a 12 litre expansion vessel pre charged to 0.5 bar, this will then only rise to 0.75/1.0 bar with hot system, with either system ensure that a 2.5 bar PRV is installed at/on the boiler.