Heating zone control

[Barley]

I have an S-plan system where one zone has 35mm outlets and another with 15mm outlets. It is impossible to balance, and the smaller pipe system is starved
I want to add a small pump to this circuit and then balance that pump with the other rads
What do you think

 

[D_Hailsham]

Can you supply a diagram of the system, showing valves etc?

 

[Barley]

Untitled

• Barley
• 16 Mar 2013
• 1

Thanks for reply, just spent two hours trying to get diagram on page
Hope the address above work
xxxxxxxxxxxxxxxxx

It shows the boiler output splitting into 35 & 22mm pipes

The 35mm splits into two motorised valves feeding two rad circuits - each reducing to 22mm, then to 15mm and both run 16mtr to 5 rads each

The 22mm pipe runs to a motorised valve then reduces to 15mm and runs underfloor for 18mtrs [in 15mm] to 7 rads

If I balance the 35mm circuit to ensure an adequate supply to the other 22 circuit, then the flow is starved to the 35mm circuit
If I don't restrict the 35mm circuit there is insufficient supply to the 22mm one

I am also concerned that a modulating pump will be confused and not supply to the smaller circuit

 

[mogget]

18 metres is a long way in 15mm. This pipe is undersized and you will have the devils own job to balance it (as you already know). The proper solution would be to re-do the run in 22mm but that might not be practical.

 

[D_Hailsham]

Your diagram shows a one-pipe system. Is this correct?

A 35mm pipe implies a very large (over 40kw) boiler. Is this correct?

View media item 17739
You will need to insert a balancing valve in the 35mm section after the junction with the 22mm so you can balance the flows in the two branches.

Are you sure that the pump is adequate?

Do you have a bypass?

 

[Barley]

Thanks D_Hailsham

Sorry for my sketch, it is a two pipe system, and yes it is a 41kW output, 51kW input Concorde CX140

I am looking to replace this with a Worcester 40Cdi, hence trying to decide the pumping and control circuits

The pump does seem adequate but is not reliable on the long 15mm circuit [sometimes good, sometimes not - it will circulate if I switch the pump on-off a few tims though]

The balancing valve in the 35mm would force the flow down th 15mm pipe but my concern is that this would reduce the flow to the rads on the 35mm circuit [this is 12 rads on zones plus 5 on an open circuit]

My thoughts were to add a pump to the 22mm circuit, with a balancing valve to restrict the flow, on the suction side. This pump would be linked to the motorised valve. Possibly fit 2 low reading pressure gauges to control the balancing?

There is no bypass at present but would be fitted with the new installation

 

[D_Hailsham]

it is a 41kW output, 51kW input Concorde CX140. I am looking to replace this with a Worcester 40Cdi

How do you know what size boiler you need? The Concorde may be over- or under-sized.

Use Whole House Boiler Size Calculator to find out what size boiler you really need. If you have an odd shaped house you may have to divide it into sections.

The pump does seem adequate but is not reliable on the long 15mm circuit (sometimes good, sometimes not - it will circulate if I switch the pump on-off a few times though)

I take it that you have just the one pump at present, so the question is: Why will the pump run OK on the 35mm circuit but not on the 22mm?

Your diagram shows the 15mm going to rads, yet you talk about underfloor heating. Which is it, or are there both?

Have you taken on-board Mogget's comment about 18m of 15mm pipe being too much; and what about the 15mm return pipe? How many rads or kW of underfloor is fed by this circuit?

The balancing valve in the 35mm would force the flow down th 15mm pipe but my concern is that this would reduce the flow to the rads on the 35mm circuit (this is 12 rads on zones plus 5 on an open circuit)

If that's the case, then the main pump is not correctly sized.

Has the system been balanced?

 

[Barley]

I have done the 'whole house calcs, and at first run it comes to 32kW -I assume we need some safety factor, and felt that the modulation would reduce the 40kW acceptably.

Yes just one pump, The answer to the question is unclear, it could be a balancing problem but to balance 18 rads to improve the circuit is a bit hit & miss. There are 7 rads on the 15mm circuit. All rads have TRV's and the whole has been power flushed twice.

There is no UFH -- the 15mm pipe is routed under the solid floor
I agree 18m is too long in 15mm, and that will cause a loss of pressure, that is why I considered a pump to increase it.

Yes the pump sizing may be in error, and would think one larger pump may well be a better route.

 

[D_Hailsham]

I have done the 'whole house calcs, and at first run it comes to 32kW -I assume we need some safety factor, and felt that the modulation would reduce the 40kW acceptably.

The 32kW already includes a safety factor. The only advantage of having a 40kW boiler would be that it can provide more heat when it is very very cold. e.g -6C.

Yes the pump sizing may be in error, and would think one larger pump may well be a better route.

Thinking about it further, I doubt if installing a larger pump will solve your problem; it may make it worse.

The most you should send along a 15mm pipe is 7kw, which means an average of 1kW per rad. If you have more than this, the water speed will be too high, meaning more friction and a greater pressure loss in that section.

I know you think balancing may not solve the problem, but its worth giving it a go. Here's a quick method:

Remove all TRV heads and set any wheel-head valves to fully open

Close all lock-shield valves then open them half a turn.

Check the return pipes of each rad, by feel. If any are too hot, close the LS valve an eighth of a turn; vice versa if too cold

The idea is to have the return pipe (cooler one) of each rad at approximately the same temperature.

Adjust one rad at a time and wait 10 minutes before checking.

Adjusting one rad may affect others.

 

[Tipper]

Great easy to understand balancing, well done! I find an Infrared temp gauge helps if you happen to have one available.

Just one small correction though

Adjusting one rad WILL affect others.

 

[D_Hailsham]

Great easy to understand balancing, well done! I find an Infrared temp gauge helps if you happen to have one available.

IR thermometers are fine, provided you understand that they are influenced by the surface they are pointed at. This is probably not important if all the rad pipes are painted the same colour. But point it at a shiny new copper pipe and you will get a ridiculously low temperature, when you know that the water is going through the pipe is at 75C. The way to overcome this is to wind black insulating tape round the pipe and measure the temperature off that.

Just one small correction though

Adjusting one rad WILL affect others.

Agreed; but whether it is noticeable on all other rads will depend on the relative sizes and locations of the rads.