Boiler keeps short cycling with only UFH is on

[John.g]

All very strange, the difference in pump heads should only increase the flowby ~ 15% so possibly blockage.
You replied to my post in that other site re reducing d.0 to ~ 7kw to get the boiler flow temp to fall, then increase it in 1kw steps until the flow temp returns to 70C, can then calculate the exact flow rate. Do this on CH only and on main pump red speed3. Close the manual bypass valve fully and then open it to its original setting of 1/2 turn open before starting.

 

[John.g]

Should have said shut the bypass fully and if the boiler keeps running then cary out the test above, ch only on, pump speed 3.

 

[windsor-tg]

Sorry @John.g, I am confused on what you are asking me to do. Are you John Carroll on the other forum (buildhub.org.uk) where you replied to my post? If so, would you mind listing the steps you want me to do?

If you’re not John Carroll on the other forum, if you can outline again what you want me to do, that will be very helpful

 

[John.g]

OK, with CH only on, ensure main red pump on speed 3 (8M), when the boiler flow/return temps are steady and when the boiler flow reaches ~ 70C (its present setting), shut the manual bypass valve fully and if the boiler continues to run continuously again note the boiler flow/return temps. Then reduce d.0 to say 7kw, the boiler flow temp should then start falling, then steadily increase d.0 until the flow temp again reaches ~ 70C, note the flow/return temps, you now know what output (within reason) the boiler/CH system requires, we can then calculate the actual flow rate quite accurately from the above.

When test completed reopen the manual bypass valve a 1/2T again and again note the flow/return temps after another say 5 minutes.

 

[windsor-tg]

Just a follow up question if someone can help.... if I have a system boiler installed with closed couple tees, do I still need an external pump(s)? I am aware that the existing UFH manifold pump is needed for the UFH system.

 

[ShaunCorbs]

Yes for the ufh manifold the pump isn’t rated for the heating pipework from the boiler etc

So you will need two or theee depending if it’s a system boiler/ combi or heat only

 

[windsor-tg]

Yes for the ufh manifold the pump isn’t rated for the heating pipework from the boiler etc

So you will need two or theee depending if it’s a system boiler/ combi or heat only

thanks @ShaunCorbs. Just to clarify, for a system boiler:

1. internal pump contained within system boiler
2. UFH manifold pump - pump already exists
3. is another external pump(s) required for central heating?

 

[ShaunCorbs]

point 2 the manifold pump isn’t rated to pull from the pipework so you will need another one here

Point 3 yes

 

[windsor-tg]

point 2 the manifold pump isn’t rated to pull from the pipework so you will need another one here

Point 3 yes

thanks @ShaunCorbs.

Just to add some context, I am looking to replace my heat-only boiler with a system boiler. I already have an external pump beside each of my zone valves - photos attached of zone valves marked 1 = HW, 2 = UFH, 3 = CH

The external pump is a UPS2 25-80 (photo attached). Any issues with this pump being used for the central heating?

From what you have said, the manifold pump isn’t rated to pull from the pipework so you will need another one here. Can you expand upon where another pump will need to be fitted?

 

[ShaunCorbs]

That pump is fine for the ch you will need a separate 15-5/60 pump for the ufh after the cct if the cct is above the boiler etc