Ufh not heating properly - boiler goes on / off every minute

[pototea]

I have under floor heating in my kitchen. My boiler is a Worcester Bosh 24 CDI and the ufh pump is a Grundfos. The radiators are on a different zone powered by a different thermostat.

Recently the ufh incoming flow seems to be pumping hot for one minute followed by luke warm water for another minute and repeates this cycle. This is reflected when I check the boiler light - it is on for a minute then goes off. Regardles of what temperature I select on the room thermostate eg 30'C the problem is the same. The floor does not get as warm as it used to. It still gets warm but barely and not evenly. I have changed the grundfos speed setting to 1/2/3 and the problem is the same on all 3 settings.

I had a plumber come and change the pump but the problem is the same. He has no clue what the problem could be.

When I put the radiator circuit on, there is no problem and the boiler is powered on constantly until the rooms get to the desired temperature and all radiators get hot.

There are no pressure issues on the boiler and all other functionality eg hot water works ok.

Anyone have any idea what the problem could be?

- boiler (is this likely given the radiator circuit works fine)?
- pump installation setting?
- thermostat next to the underfloor heating pump? This is set to 45'c. the problem moved it to 60'c which made no difference
- room controller?
- return pipe blockage? UFH was fitted when I had my kitchen built 2 years ago. All new pipework
- airlock on the return?

The problem started after my previous pump broke down. A plumber changed it and this new problem appeared. He changed the pump again but the same problem of the flow going on / off is still there.

Any help would be much appreciated.

 

[ericmark]

Seem to remember maximum temperature permitted for under floor heating is 29°C, so in this weather could simply be the floor is warm enough.

 

[pototea]

Hi, the floor is getting no where near that temperature. It barely gets warm and it's patchy.

It was working 2 months ago before the pump broke down.

Here are some more photos

 

[ericmark]

To work the under floor heating draws cold water from the radiator return, unless there is cold water it can mix with the hot water it can't control the max temperature, it's designed to all work together, not work independently.

 

[RobTi]

Could the pump be fitted the wrong way round ?

 

[Dan Robinson]

To work the under floor heating draws cold water from the radiator return, unless there is cold water it can mix with the hot water it can't control the max temperature, it's designed to all work together, not work independently.

edit

 

[Dan Robinson]

Could the pump be fitted the wrong way round ?

That was my first thought.

 

[pototea]

Hi there, yes it's certainly a possibility it's the wrong way round. When the plumber fitted the first pump that broke down he originally fitted it the wrong way round. It was only when I called another plumber who identified and corrected the issue that the heating started working again until the pump broke down.

I got the original plumber back to change the pump. He assured me several times this new pump is the correct way. From my point of view the heating is displaying the same issues as the previous pump he fitted incorrectly.

Is there a way to tell from the photos I've posted if the pump is fitted in the right direction?

 

[Dan Robinson]

No - we need to see the side of the pump.

If yours is this way round it is wrong.

 

[pototea]

Hi there,

Photos attached. The arrow is pointing down into the floor.

 

[Dan Robinson]

When the UFH is on, I take it there is a green light on the pump?

 

[pototea]

Yes there is a green light and it gets hot.

The plumber changed the pump twice so we've ruled out a pump issue.

 

[Dan Robinson]

Then it could be an issue with the blending valve above the pump. They have a finite lifespan.

You should have around 6 degrees difference between the flow and return pipes going into the floor. the blending valve mixes the floor return water into the flow to limit the temperature and once up to the rigth temperature closes off the supply from the boiler flow.

 

[picasso]

I reckon its got air locked

 

[Dan Robinson]

Possible - pumping down is never a good idea.