Balancing issue since moving from vented to unvented.

[Ant1]

I recently had a vented indirect cylinder (fed by a tank in the loft) replaced by a unvented indirect one (fed by the mains).

The plumber left me with a leaking pipe causing the pressure to drop, not a great start. As he never came back to fix the issue, I had to drain the system to replace the pipe and olive. The leak has been fixed and now the pressure remains stable (making sure all the radiators have been purged).

I still have an issue though, it seems very difficult to balance the radiators since moving from a vented to an unvented system (I am using 2 thermometer to achieve a 12 degrees drop). 2 days ago I managed to get all the radiators to warm up ok, though I admit it was challenging, I struggled to get some of them to heat up properly.

2 days later 2 radiators would not heat up. If I completely shut off the radiator closest to the manifold and another one which is not really essential, the 2 affected radiators now heat up again.

Are there any tips to balance radiators on unvented systems? (I tried increasing the speed of the pump from 1 to 2 but did not help)

Thanks a lot!

 

[ianmcd]

You may need a flow restrictor valve on the return from the cylinder coil, when you switch it to Heating only do the rads then heat up with the HW demand off ?

 

[Ant1]

Thanks Ian, all the issues I described above were with heating only, i.e. with HW demand off. I could get all the radiators to heat up but very challenging, currently I had to shut off 2 radiators completely (TRV + lockshield) from the hallway to ensure the bedroom ones are working.

PS: I am not on site right now, but normally most of the pipes going to the radiators are very hot, this is the return pipe for some of the radiators remaining cold despite the lockshield being opened. I know the TRVs and lockshields are fine since the problem can move from one radiator to another one.

 

[ianmcd]

Ok thats fine you dont need a balancing valve, what boiler do you have ?

 

[Ant1]

It is a Baxi backboiler 552, old but reliable, it is being serviced every year, last serviced in September.

 

[ianmcd]

It is a Baxi backboiler 552, old but reliable, it is being serviced every year, last serviced in September.

yes great boilers, I think your problem is a blocked cold feed, when the system was drained to swap the cylinders it sounds like crud has been drawn into the system when re-filling, plent of posts on here of how to check if it is that

 

[Gasguru]

The plumber left me with a leaking pipe causing the pressure to drop, not a great start.

Does this infer the boiler has been converted to a sealed system..ie. you now have a red expansion vessel/gauge and safety valve?
These back boilers are only permitted to be run as a sealed system providing the additional overheat thermostat kit has been fitted to the boiler.
What was the setup with the old open vented cylinder...was the cylinder heated by gravity circulation or pumped?

 

[Ant1]

I do now have an expansion vessel (though it is grey) and a gauge which the plumber advise to get to 1 bar.

Previously the open vented system was fed through a pump (refer to the diagram below):



Still using the same pump, the only difference is that the old cylinder and expansion vessel were both removed, new ones installed (unvented indirect cylinder) connected to the main water rather than loft tank.

These back boilers are only permitted to be run as a sealed system providing the additional overheat thermostat kit

I will take a photo of what was installed and will upload tomorrow. Would you be able to show how a overheat thermostat kit looks like and where it is typically connected?

In the meantime I will add some Sentinel X400, I would agree with Ian, i.e. crud in the system might be the root cause, specially since the original expansion tank (Spartan ET50 Expansion Vessel) broke and seem to have "contaminated" the system.

 

[Gasguru]

So it would appear you had Spartan "semi sealed" system before and your plumber has now converted the boiler to a fully sealed system.
The Spartan units were filled via the large cold domestic water cistern and in theory there was an open pathway to vent of steam in case of back boiler control failure.
I'm not sure whether there was a requirement to fit the overheat thermostat kit for that arrangement but the installation manual certainly requires it with a fully sealed system that you appear to have now.

The overheat thermostat is fitted to the back boiler and it's probably not possible to see without removal of the fire front.

Did the old unit look like this...

Untitled

• Gasguru
• 30 Nov 2008
• 1

 

[Ant1]

@Gasguru I am very impressed, not many people know about this Spartan kit! A lot of the plumbers I got over the years ran away when seeing it. Interesting piece of kit (I attached a brochure if people are interested), but after so many years of loyal service it is now gone. Believe it or not I could actually have got a replacement part from a company within 5 days, though it made sense for me to move to a vented system since the shower is connected to it.

Coming back to my problem, the plumber never went near the boiler, only in the cupboard where the cylinder is located.

If I need to purchase an overheat thermostat kit, can I get a "generic" one or are they specific to the Baxi 552?

 

[ianmcd]

you should not be using the sytem till the upgrade kit is fitted, it is dangerous, and dont get the same "plumber" back to do it, they dont know what they are doing

 

[Gasguru]

Ta for the flyer...not seen that before.

I first came across them 20 years ago when I got into the trade and the knowledge back then was scarce....very few left now.
Without knowing whether you have the overheat thermostat fitted the back boiler is not safe to use.

The kit consists of a replacement pipe, overheat thermostat and cabling to interrupt the pilot in case of overheat and therefore shut down the boiler.
https://www.dhsspares.co.uk/product/baxi-230336bax-bermuda-o-heat-stat-1111996
Showing as obsolete but they may be around. Also not necessarily straightforward to fit now the back boiler is in position....it's strictly a Gas-Safe job and finding anyone to do it would be tricky.

The IGEM Gas Industry Unsafe Situations Procedure lists missing overheat protection as an "At Risk" fault...

Section 7.13 Appliances installed onto a sealed heating system:
Without pressure relief controls
With pressure relief controls, but without overheat protection where required.

This means that if we come across this situation we must label up the boiler, issue a warning notice, turn off the appliance and explain the risk to you and why it must not be used.

 

[Ant1]

@ianmcd & @Gasguru , I hope you can help me a bit more resolving the issue with my "blocked" cold feed, actually I am now sure it is blocked, here is the story so far.

Basically I have 2 Speedfit manifolds (refer to the photo below) serving a total of 7 radiators:



The first manifold (manifold 1) serving hot water to all the radiators is very hot.

The second manifold (manifold 2) returning the water back from the radiators is not hot.

Therefore I cleaned the system by connecting manifold 1 to the main water supply and by connecting manifold 2 to a pipe which drained into the batch. I cleaned one radiator at a time by shutting off both TRVs and lockshields on all radiators expect one. There was a bit of sludge but nothing unusual, clear water now goes through each pipe. I also visually inspected the inside of the manifolds.

However I still have the same issue, i.e. manifold 2 not returning much hot water.

My questions:
1) The main water supply is quite powerful (fed through a 22mm coper pipe in manifold 1), when draining into the bath (through a 15mm speedfit pipe on manifold 2, could only use that) I noticed a pressure drop, but I guess this would be expected as going through a radiator. Would you agree? The flow out would have filled a cup of tea in 3 second, not sure if that gives you an idea...
2) After draining the system, each of the radiators refill quickly when bleeding. Based on this and point 1 above, I can only assume there is no blockage. Is this a fair assumption?
3) I am going to put some Sentinel X400, I was going to feed the product through the filling loop, based on my problem would you advise to put some in each of the radiators instead?
4) If all of that fails, do you think a power flush would actually help? It is quite expensive, hence would need to make sure.

Hope this makes sense!

PS: I would have thought that by only leaving one radiator turn on (connected to either of the 4 x 15mm tubes) I would get hot water back on manifold 2, but this is not the case. I find it hard to believe all pipes are blocked. Everything would point to an issue with manifold 2 but it is clear and water goes through it. Strange...

 

[Gasguru]

But what about the elephant....?

 

[Ant1]

He wrote to me "all safety measures in place", I am still awaiting for him to confirm he either installed an overheat thermostat or if there was already one in place.

There was one day I was out when he was doing some work, so he might have installed it, though it does not sound promising as he is being too vague.