UFH with 3 manifolds cools hw cylinder

[rk05]

Hi there

 

I have a 3 UFH manifolds, each has its own pump connected to a system boiler Vaillant 630, 30 Kw and a Joule 250L unvented cylinder. The thermostat on the cylinder has a digital temperature display. 
 

In the recent cold spell I found that when all 3 manifolds were on and water is being heated, the cylinder temperature never went above 45-47 degrees even when heating the water for 4 hours. 
 

Touching the flow pipe into the cylinder it’s not as hot as the manifold pipes. The pipes that go to the manifold and cylinder all come off the boiler flow pipe. 

When one of the manifolds is off then the water heats up much quicker and gets into the 50s. When a third manifold is then turned on the the cylinder starts dropping temperature with no hot water being used.

 

I would appreciate any ideas on the cause of this and how it can be resolved. 
 

thanks in advance

 

[PeterW]

S Plan or Y Plan setup .?

 

Most likely the 3 pumps are back syphoning through the coil in the cylinder due to a valve letting by. Non return valve (flap type) would stop it in the flow to the cylinder.

[rk05]

21 minutes ago, PeterW said:

S Plan or Y Plan setup .?

 

Most likely the 3 pumps are back syphoning through the coil in the cylinder due to a valve letting by. Non return valve (flap type) would stop it in the flow to the cylinder.

Hi, S plan I think. There are 4 2 port valves one for each manifold and one for cylinder. 

[PeterW]

No programme overlaps on the system ..?? So DHW is off when heating is on ..?

[rk05]

2 hours ago, PeterW said:

No programme overlaps on the system ..?? So DHW is off when heating is on ..?

Hi,

 

Yes there are overlaps.

 

The DHW is on a timer. 
 

For UFH there is a thermostat for each zone (Heatmiser), which is set to different temperatures at different times of day. 
 

This issue occurs when all 3 manifolds are calling for heat and the DHW turns on from the timer.

 

Thanks

 

[PeterW]

1 minute ago, rk05 said:

This issue occurs when all 3 manifolds are calling for heat and the DHW turns on from the timer.

The manifold pumps are overpowering the DHW / Supply circulator - what are they all set on .?? Sounds like you need to up the speed on the main circulator or do some timing changes.

[Nickfromwales]

Needs to be set up as W-plan aka DHW priority not S or Y plan   

[Nickfromwales]

The 3x 2-port ( UFH ) valves go downstream of the W-plan 3-port DIVERTER valve on the heating out port. 
Cylinder to HW port.

Problem solved. 
Depending upon number of occupants / build type / DHW consumption, this may need a decent sized buffer to maintain heat to the house whilst DHW recovers fully. 

[PeterW]

51 minutes ago, Nickfromwales said:

Needs to be set up as W-plan aka DHW priority not S or Y plan   

Yeh that’s designed properly !!! OP is in for some serious re-piping and rewires to drop this to what you suggest.
 

Poor man’s W-plan is to not overlap the programme timings and tbh there are 3 manifold pumps here vs. one system pump and it’s going to be an uphill battle anyway !! 

[Nickfromwales]

5 minutes ago, PeterW said:

Yeh that’s designed properly !!! OP is in for some serious re-piping and rewires to drop this to what you suggest.
 

Poor man’s W-plan is to not overlap the programme timings and tbh there are 3 manifold pumps here vs. one system pump and it’s going to be an uphill battle anyway !! 

Yup. 
Problem swerved is the cheapest option, just need to be sure with a heat demand over 3 manifolds, that it is manageable / practicable. 
A lot of programming can resolve it. 
Quite odd that it’s been set up to overlap tbh!!

[ProDave]

44 minutes ago, Nickfromwales said:

Yup. 
Problem swerved is the cheapest option, just need to be sure with a heat demand over 3 manifolds, that it is manageable / practicable. 
A lot of programming can resolve it. 
Quite odd that it’s been set up to overlap tbh!!

You don't want the heating to stop whenever the DHW timer is on, only when the timer is on and the tank thermostat is calling for heat.

 

A relay, energised when DHW is calling for heat, to cut off power to the heating would do the job.

 

Or you could be more clever and use the changeover contact of the tank stat to feed DHW or heating. Would need a little more thought depending exactly what timers etc you have.

[Temp]

3 hours ago, Nickfromwales said:

Depending upon number of occupants / build type / DHW consumption, this may need a decent sized buffer to maintain heat to the house whilst DHW recovers fully. 

 

Used the recharge time calculator here..

https://myboiler.com/calculators/hot-water-recovery-time-calculator/

 

30kW boiler, 250L tank. Time to raise temperature from 35 to 60C worked out at 15 mins which isnt too bad.

[PeterW]

2 hours ago, ProDave said:

A relay, energised when DHW is calling for heat, to cut off power to the heating would do the job.

 

Or you could be more clever and use the changeover contact of the tank stat to feed DHW or heating

Or just wire it as a standard W-Plan with a diverter valve (not a mid position) and use the satisfied connection on the tank to trip back across to heating. No need for relays as this is all standard stuff.
 

Used to be used a lot but not as popular now since multi zone S Plan has been used.

 

[ProDave]

13 minutes ago, PeterW said:

Or just wire it as a standard W-Plan with a diverter valve (not a mid position) and use the satisfied connection on the tank to trip back across to heating. No need for relays as this is all standard stuff.
 

Used to be used a lot but not as popular now since multi zone S Plan has been used.

 

But the OP says he has four 2 port valves, so that would need some plumbing changes. Agreed, an off the shelf solution otherwise.

 

Just another example of a system being "designed" with no proper thought.

[PeterW]

Quickest is just to remove the overlap and do DHW at say 05:00 and then again late afternoon so you’re missing the heating periods. 

[Nickfromwales]

19 hours ago, ProDave said:

But the OP says he has four 2 port valves, so that would need some plumbing changes. Agreed, an off the shelf solution otherwise.

 

Just another example of a system being "designed" with no proper thought.

Yup....

 

18 hours ago, PeterW said:

Quickest is just to remove the overlap and do DHW at say 05:00 and then again late afternoon so you’re missing the heating periods. 

....and yup.

[Nickfromwales]

21 hours ago, Temp said:

 

Used the recharge time calculator here..

https://myboiler.com/calculators/hot-water-recovery-time-calculator/

 

30kW boiler, 250L tank. Time to raise temperature from 35 to 60C worked out at 15 mins which isn't too bad.

Yup.

[rk05]

Hi All

 

Thanks for all the replies. I will adjust the timing to avoid overlap 
 

The manifold pumps are set to max speed. Should these be turned down, how is the speed determined? 
 

Thanks again

[PeterW]

1 hour ago, rk05 said:

The manifold pumps are set to max speed. Should these be turned down, how is the speed determined? 

Each loop should be adjusted to the correct flow rate to deliver the heating required. If you turn the pumps down and check the flow rates, you may find they are still ok as per design.