Manifold on the Run

25 May 2013
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United Kingdom

Thanks for the message on earlier post. Here's a modified question:

On my Axios UnderFloor heating manifold, if I close down each of the zone valves, but call for heat via the thermostats, after a few minutes, the actuators open, and hot water from boiler comes to manifold, as well, the return pipe (from the manifold, heading to boiler) becomes hot also. The temperature on the thermometer scale of the manifold is at 55C approx (which manufacturer says is correct).

However, once I open the zone valves, the temperature drops to 30C and the return pipe (of manifold to boiler), goes cold. Note, the little red things in the glass/valve does drop and water enters it.

Any idea? The manufacturer says that there may not be enough "push" from my main boiler pump, but this seems surprising.

One thing: The return of the boiler is a shared return of a radiator system. I know it's not ideal, but my plumber did it this way, and I only realised it's more optimal to have a dedicated return after my floor was closed, etc. The test written above was done without my radiators turned on.

Please help :)
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I don't think I have the full picture yet but here's some thoughts for you to be going on with.

What boiler do you have and did you have before that?

Perhaps there's a by pass you don't know about or maybe your plumber has not set a by pass valve correctly.

Some boilers have an anti cycle system and may not fire up for 5 to 20 mins after the required temp has been sensed.

Thanks for your reply.
Hmm, I have a worcester/bosch 440cdi, and it's new. All the plumbing is new.

It's really concerning to me that the return pipe does not get warm… but it seems there is flow b/c those little red things in the valves do drop a bit. The underfloor manufacturer says that if the red things drop a bit, then flow occurs, so they can't understand why the temperature is not rising beyond 30C, etc.

I am concerned that my return flow pipe (from the manifold, back to the boiler) has the problem, and the floor is closed so can't see it. Maybe there is air in the system somewhere.
How long have you left it running for and is it solid or suspended floor? Can you feel heat in the floor?

I know my system takes ages for the return to get hot maybe 1-2hrs if it hasnt been on for a few days (90m², pipes in 50mm screed).

Could it be you just havent had it running long enough to get the return temperature up?

With a solid floor, there is a huge mass sucking heat out of the water, so by the time it gets back to the manifold its cold. This gradually reduces as the slab heats up then return pipes start to feel warm.
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I have a suspended timber floor, not screed.

When I turn it on, 45min on, and the temperature on the manifold (there is a temp gauge) only reads 30C and the multiple flow pipes from the manifold going into the floor don't seem warm to me, and the major return pipe that links the manifold back to the boiler is not warm. The surface area is just a bedroom of 4x4 metres and a bathroom of 2x 3metres.

Interestingly, the floor sensor (in the thermostat) does show that the temperature rises, but I don't know if that's just coincidence with the flat below's heat.

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