Restricted flow on upstairs rads conundrum

[evb222]

Symptom:
Boiler fires up for 30 seconds, raises the flow temperature too high and then cuts out. The return pipe eventually gets to just barely warm (after hours of the repeating cycles like that). The first 2 – 3 radiators upstairs heat up ok, with the following 2 – 3 being just barely worm, and with the last 3 being completely cold. The upstairs circuit is separated from the downstairs circuit through a two-port valve.
Theories explored:
1. Radiator Balancing. Not an issue, as the boiler never has a chance even to get to a full power. The flow sensor senses temperature rise too quickly and shuts the boiler down.
2. Pump. Not an issue. There are no problems in the downstairs circuit.
3. Blockage in the pipe work upstairs. Possible, but is it likely? The boiler is a new install (combi for combi swap). The old system worked just fine. The system was power flushed prior to the boiler swap, and it had been clean to begin with anyway. The Magna Clean filter which was installed at the same time was virtually free of anything after 2 weeks running, indicating clean system.
4. Non-return valve installed for the upstairs circuit but not removed prior to closed system upgrade (by prior owners) and the flow/return pipes where mixed up during the boiler swap. This was the best hypothesis, but reversing the pipe connections did not produce any improvements.
The problem appeared after the boiler swap, which was done in the summer months and therefore not properly tested. The original installers refuse to investigate, claiming that they only did the swap and the problem is not boiler, but the central heating system related, which they didn’t touch.
Other heating engineers are puzzled. As described, they have attempted to swap the pipe around hoping to get around a potential non-return valve. It didn’t work, and apart from the lifting carpets and floorboards upstairs, they do not have other suggestions.
Can anyone offer any ideas or tests? What could the boiler swap people do wrong to cause a fault like that?

 

[D_Hailsham]

What was the old boiler?

What is the new boiler?

Which pump do you have?

 

[evb222]

Old boiler - Vaillant 835 (Combi), which has had a corroded heat exchanger and was more economical to replace (clearly not in the hindsight)
New boiler Atag A325EC (also Combi)
Pump - which ever one is built into the boiler, so don't know really know.

 

[D_Hailsham]

Thanks for the info about the boilers. I had wondered if the new pump was less powerful than the old, but, having checked the specs, it isn't.

Boiler fires up for 30 seconds, raises the flow temperature too high and then cuts out. The return pipe eventually gets to just barely warm (after hours of the repeating cycles like that).

You can get this effect if the boiler produces more heat than the radiators can dissipate. Check your radiator outputs using Stelrad Elite Catalogue

Check you heating requirement using Whole House Boiler Size Calculator. SEt the HW requirement to 0kW as you have a combi boiler.

Please post both results.

The problem appeared after the boiler swap, which was done in the summer months and therefore not properly tested. The original installers refuse to investigate, claiming that they only did the swap and the problem is not boiler, but the central heating system related, which they didn’t touch.

That's no excuse. The installers should have checked everything worked correctly, and that means the entire system, not just the boiler. It would be like a garage installing a new engine and then not road-testing the car.

 

[evb222]

D_Hailsham,

Thank you for your reply. I will check as you suggested, but here are some initial thoughts. The circuit upstairs is roughly the same size to the circuit downstairs. The downstairs circuit functions without any issues. Also, the new boiler is capable of modulating to a lower output than the the old boiler, in fact Atags start at the lowest level and then depending on the return temperature start modulating upwards, which is opposite to what Vaillaints do (at least the older ones) Finally, the return flow from the upstair circuit is completely cold to touch, so all of the heat is being completely dissipated.

Regarding the installers - of course I agree, but there is nothing I can do to force them come back.

 

[hertsboilers]

Check if there is any blockage near the two port valve to upstairs.

 

[evb222]

Check your radiator outputs using Stelrad Elite Catalogue

Upstairs rads add up to 9.1 kW. Downstairs is about 11 kW.

Check you heating requirement using Whole House Boiler Size Calculator. SEt the HW requirement to 0kW as you have a combi boiler.

17.99 kW for the whole house.

 

[evb222]

Check if there is any blockage near the two port valve to upstairs.

Checked, even the flow and return pipes swaped over to get around a potential non-return valve somewhere in the system. Same issue, irrespective of the flow direction and no blockage near the two port valve.

 

[telegan]

turn off all downstairs rads see if upstairs improve if not look for upstairs manifold and check for blockages

 

[evb222]

turn off all downstairs rads see if upstairs improve if not look for upstairs manifold and check for blockages

The downstairs is zoned out from the upstairs via another two port valve. Whethe the downstairs is on or off makes no difference for the upstairs.

Regarding the blockages - I have no idea where to start looking. Does this mean lifting all of the carpets and floorboards?

 

[Tom1981]

Try this:

Turn off all the radiators in the house and leave only one open upstairs. Don't just rely on the zone valve, close the flow side of each rad off.

Now set the boiler to a low heat (incase you do have a blockage, you don't want the boiler to fire straight onto overheat). Set the clock to heating and see if the pump pushes the hot water around the radiator upstairs. What you trying to do is release any potential airlock in the upstairs circuit.

Once, if any, airlock is released, open another upstairs rad and see if that heats and so on

 

[D_Hailsham]

The downstairs is zoned out from the upstairs via another two port valve. Whether the downstairs is on or off makes no difference for the upstairs.

Can the downstairs and upstairs circuits can be operated completely independently of each other?

 

[evb222]

The downstairs is zoned out from the upstairs via another two port valve. Whether the downstairs is on or off makes no difference for the upstairs.

Can the downstairs and upstairs circuits can be operated completely independently of each other?

Yes. Different valves, different thermostats, etc.

 

[D_Hailsham]

Can the downstairs and upstairs circuits can be operated completely independently of each other?

Yes. Different valves, different thermostats, etc.

OK

You originally said:

Boiler fires up for 30 seconds, raises the flow temperature too high and then cuts out. The return pipe eventually gets to just barely warm (after hours of the repeating cycles like that). The first 2 – 3 radiators upstairs heat up ok, with the following 2 – 3 being just barely worm, and with the last 3 being completely cold.

What happens if you run the downstairs circuit on its own?

What happens if you run the upstairs circuit on its own?

 

[evb222]

Can the downstairs and upstairs circuits can be operated completely independently of each other?

Yes. Different valves, different thermostats, etc.

OK

You originally said:

Boiler fires up for 30 seconds, raises the flow temperature too high and then cuts out. The return pipe eventually gets to just barely warm (after hours of the repeating cycles like that). The first 2 – 3 radiators upstairs heat up ok, with the following 2 – 3 being just barely worm, and with the last 3 being completely cold.

What happens if you run the downstairs circuit on its own?

What happens if you run the upstairs circuit on its own?

The downstairs circuit on its own functions just fine: normal long burn from the boiler, the radiators get hot everywhere and stay for as long as thermostat is asking for it.

The upstairs circuit on its own runs with the same problems: boiler starts up at the lowest power for 15-30 seconds and then cuts out.