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Discuss UFH pump robbing from Rad system in the Water Underfloor Heating Installations area at PlumbersForums.net

Zencap

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17
Hi - been told that when the boiler runs in its normal mode the boiler plateaus at 64 degrees on the flow and returns at 55 degrees. I have no idea how to measure temp of UFH flow and return. Attached are photos of manifold and flow meters again no idea how to measure any more.

To confirm I know the UFH is robbing from Rad - when one is on fine, but when both are on only the UFH flows. Thus trying to reduce pump power of UFH ?
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John.g

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3,424
You will have to install a new manifold pump if you want to reduce the power but no guarantees that this is the fix. The best advice/reply is in post ~26 of the other thread.
Based on a few assumptions (can't see exact flowrates) but assuming a total UFH flowrate of 12LPM with flow/return temps of 45C/37C will result in a boiler demand of 6.7kw and (assuming a boiler temp of 65C) boiler flow/return temps of 65C/37C, dT of 28C. and a boiler flowrate of 3.43LPM.

Sometime maybe try and get hold of a thermal gun and take a few readings.
 

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John.g

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One more thought, if the manifold pump is robbing water from the rads by drawing it through the UFH return then easy to stop this by installing a non return valve where shown on the vertical pipe before the isolating lever valve.
 

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John.g

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Why are the Rads and UFH teed in here together and both feeding the pipe marked UFH. Shouldn't the Rads 2 port valve just feed the rads?.


UFH pump robbing from Rad system filename: {filename} - Plumbing Advice
 

Zencap

Messages
17
Why are the Rads and UFH teed in here together and both feeding the pipe marked UFH. Shouldn't the Rads 2 port valve just feed the rads?.


UFH pump robbing from Rad system filename: {filename} - Plumbing Advice
Yeah that is a trick of photography - only the bottom pipe UF feeds the UF, the top pipe feed the rads - they don’t connect. But as per original issue, when both are on the Rad pipe becomes lukewarm once UFH has been on for c 15 mins.
I still feel the UFH pump is just dominating vs this pump in the air cupboard but I can’t seem to get it to flow any lower
 

John.g

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3,424
Which rad pipe gets warm first, is it the flow or return.

You could throttle the pump discharge to get the flow meter adjusters full open more to maintain the same flow, it may help but don't really know if it will.
Have you tried the primary circ pump on speed 3?.
 

Zencap

Messages
17
Which rad pipe gets warm first, is it the flow or return.

You could throttle the pump discharge to get the flow meter adjusters full open more to maintain the same flow, it may help but don't really know if it will.
Have you tried the primary circ pump on speed 3?.
The flow pipe gets warm first

Afraid don’t understand the “throttle the pump discharge” comment.
The primary circ pump is on max speed 3 already and UFH pump is on lowest.
 

John.g

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3,424
The pump(s) havs isolating valves on the the flow and return sides so you can throttle the flow (discharge) one. (with the red handwheel).
 

John.g

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3,424
Can't remember if you can get the UFH working OK on its own, if so and if the TMV is OK then the boiler should only be supplying ~ 3.5LPM of hot water, if so then you will feel a big difference between hot feed to the TMV and the mixed flow to the manifold, if so then difficult to imagine why the systems won,t still work together when the rads are then switched in.
 

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