UFH pump robbing from Rad system

Zencap · Jan 20, 2023

Hi

We have an UFH system part of the house (large kitchen / dinnner and utility room) and then Rad system for rest of house (10 rads).

We have noticed in this recent cold weather when both UFH and Rad are on (ie thermostatic driven) the UFH is robbing all the hot water and not getting round the rads.

The pump on the main system is Grundfos UPS3 15-50/65 and set to max. The UFH had a Grundfos UPM3 Flex AS 25-75 130 and we have managed to get it set to what appears to be lowest setting per attached but can’t seem to get it lower eg 1 green,1 amber light - have tried all settings and only goes higher. Any advise on this pump and if does lower to reduce flow /balance across both OR advise on if need to change pump for UFH to something else and what else?

Thanks

ShaunCorbs · Jan 20, 2023

You need x amount of flow rate through the ufh system else it won’t work properly

Best to look at the load of the ufh system and rads to see if it exceeds the boiler kw rating if it doesn’t it might want some separation eg ccts

John.g · Jan 20, 2023

Is this your manifold? from "View on Low loss header for domestic properties".

Also the manifold pump shown above is on constant curve 1 at ~4M, as you stated the lowest setting is proportional pressure PP1, one green+one yellow which would probably result in a pump head of ~ 2.5/3M depending on recirc rate.
What is the total UFH flowrate as per total of flowtubes?
what is the mixing valve set to?. It looks very low at ~ 32C?
what is the boiler flow temperature?
what is the boiler circ pump model and mode/setting?.
When you feel the pipes (or measure the temp is you can) do they correspond to their settings ie 2 port valve pipe at boiler temperature, the pipe (boiler return) less than 30C and the bottom "hot" manifold 35ish?.

Can you set the UFH to say 40/45C, and set the manifold pump to PP1

Zencap · Jan 21, 2023

John.g said:
Is this your manifold? from "View on Low loss header for domestic properties".

Also the manifold pump shown above is on constant curve 1 at ~4M, as you stated the lowest setting is proportional pressure PP1, one green+one yellow which would probably result in a pump head of ~ 2.5/3M depending on recirc rate.
What is the total UFH flowrate as per total of flowtubes?
what is the mixing valve set to?. It looks very low at ~ 32C?
what is the boiler flow temperature?
what is the boiler circ pump model and mode/setting?.
When you feel the pipes (or measure the temp is you can) do they correspond to their settings ie 2 port valve pipe at boiler temperature, the pipe (boiler return) less than 30C and the bottom "hot" manifold 35ish?.

Can you set the UFH to say 40/45C, and set the manifold pump to PP1

Thanks the temp is at c45C, the challenge I have is I can’t get this pump to change to PP1 - looking at instructions when try to - it shows 1 green 2 amber vs 1 of each. Anyone used or seen this specific pump in action and confirm?

John.g · Jan 21, 2023

If you search for "Grundfos UPM3 Flex AS 25-75 139 AZA" in DIY.not you will get some info on your pump.

The gist of it is that on this older type pump you can only access 4 settings presumably, if you keep the setting button depressed you will see a red LED and you should then be able to change it to one red LED + one green LED. with a green LED the yellow LEDs will light up to indicate the power level as shown below.
I gather that there are no PP or CP settings.
Yours is running between 25% & 50% speed and if you bring on/off rads/ufh etc the power level should change as indicated by the LEDs?.

Zencap · Jan 21, 2023

John.g said:
If you search for "Grundfos UPM3 Flex AS 25-75 139 AZA" in DIY.not you will get some info on your pump.

The gist of it is that on this older type pump you can only access 4 settings presumably, if you keep the setting button depressed you will see a red LED and you should then be able to change it to one red LED + one green LED. with a green LED the yellow LEDs will light up to indicate the power level as shown below.
I gather that there are no PP or CP settings.
Yours is running between 25% & 50% speed and if you bring on/off rads/ufh etc the power level should change as indicated by the LEDs?.
View attachment 80996

View attachment 80997

Thank you - yea I have done this and the “settings” is definitely to one red and one amber but the display/flow goes to one green and 2 amber. 25%-50%, I just can’t seem to get it to drop to the one green, one amber!?! Not sure if it possible??

John.g · Jan 21, 2023

The lowest power requirement is 13W at your selected pump curve which is 22.4% of max (58W) power so you would have to dead head the pump, normally by shutting the discharge valve to achieve this but you havn't got one and would have to mess around with the manifold valves/meters which is inadvisible IMO. The max power requirement is just under 25W so as both the min and max power requirements are very close to 25% then probably the reason for no change in the LED status?

However I wouldn't have thought that the 4M head should cause your problems, where is the rad(s) return and the UFH return located with respect to each other and also with respect to the boiler circ pump.

Also what are the flow tubes readings?.

Can't remember the model and setting of the boiler pump but would think its head should be > than the UPM 4m head as you don't want the UPM pulling any water from the rad system.

Zencap · Jan 21, 2023

John.g said:
The lowest power requirement is 13W at your selected pump curve which is 22.4% of max (58W) power so you would have to dead head the pump, normally by shutting the discharge valve to achieve this but you havn't got one and would have to mess around with the manifold valves/meters which is inadvisible IMO. The max power requirement is just under 25W so as both the min and max power requirements are very close to 25% then probably the reason for no change in the LED status?

However I wouldn't have thought that the 4M head should cause your problems, where is the rad(s) return and the UFH return located with respect to each other and also with respect to the boiler circ pump.

Also what are the flow tubes readings?.

Can't remember the model and setting of the boiler pump but would think its head should be > than the UPM 4m head as you don't want the UPM pulling any water from the rad system.

This is the airing cupboard where returns are. And this is pump for boiler.

I am not fully up with language - but the prob I have is the underfloor pulling from the rad system. I just need the UF pump to take less!

John.g · Jan 21, 2023

If the pump is pumping upwards then you are showing the Rad&UFH flow side and any returns should be before the boiler. Is it pumping upwards?.

If you can post the boiler flow and return temps and the UFH flowrate from the flowtubes + the manifold flow/return temps (if you can measure them) with rads (+UFH) on then easy enough to figure out whats being robbed or whatever.

Zencap · Jan 21, 2023

John.g said:
If the pump is pumping upwards then you are showing the Rad&UFH flow side and any returns should be before the boiler. Is it pumping upwards?.

If you can post the boiler flow and return temps and the UFH flowrate from the flowtubes + the manifold flow/return temps (if you can measure them) with rads (+UFH) on then easy enough to figure out whats being robbed or whatever.

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 ?

Zencap · Jan 21, 2023

Zencap said:
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 ?

John.g · Jan 21, 2023

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.

John.g · Jan 21, 2023

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.

John.g · Jan 24, 2023

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?.

e80046bf-1eae-41eb-a1da-e549f8aba915-jpeg.81004

Zencap · Jan 24, 2023

John.g said:
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?.

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 · Jan 24, 2023

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 · Jan 24, 2023

John.g said:
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 · Jan 24, 2023

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 · Jan 24, 2023

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.

UFH pump robbing from Rad system

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