WB 30CDi advice on recommended CH temperature differential.

[plb]

In the commissioning notes (page 34) it refers to a differential of 20 deg C across the heating system. Is this a misprint and should it read 20 Deg F which would equate to 11 C which I understand is the normal differential.

 

[Blackcatgas]

No misprint, you need this diferential on all condensing boilers to achieve dew point (57 degrees) on the return.
Your quite right about the lower teperature needed on non condensing older boilers.

 

[plb]

OK thanks for the explanation.
So I take it that when balancing the radiators I should try and achieve a 20 Deg C temperature differential between the flow and return on each radiator.

BTW, I will be using Drayton TRV’s and their Commissioning instructions quote setting lockshields to achieve an 11 Deg C differential which is a little confusing as I assume that their instructions should be relevant as regards the current range of condensing boilers.

 

[PlumbBobBob]

i don't think you will get the same temperature at the radiator as at the boiler because of heat loss on the pipe run, so you won't be getting the same differential between flow and return. imho

 

[doitmyself]

So I take it that when balancing the radiators I should try and achieve a 20 Deg C temperature differential between the flow and return on each radiator.

BTW, I will be using Drayton TRV’s and their Commissioning instructions quote setting lockshields to achieve an 11 Deg C differential which is a little confusing as I assume that their instructions should be relevant as regards the current range of condensing boilers.

To be fair to Drayton, what they actually say is: The lockshield valve needs adjusting to achieve the design temperature drop between flow and return (usually 11°C).

Most systems will have been designed for an 11C differential as radiator outputs are stated when it is that (actually 10°C, but who's arguing about 1 degree!). However if a 20°C differential is used, this is significant as the radiator output will be nearly 20% less than that shown in the manufacturer's catalogue.

Of course if it is an existing system, installed many years ago, and the insulation of the house has been increased since installation, the rads will now be oversized. So balancing for a 20°C differential may be possible.

It's more important that the differential is the same, within reason, than that it is a particular value.

 

[doitmyself]

i don't think you will get the same temperature at the radiator as at the boiler because of heat loss on the pipe run, so you won't be getting the same differential between flow and return. imho

The actual temperatures may vary slightly due to losses in the pipes, but the differential should be nearly the same.

To give a simple example:


Assuming you have a 15kW boiler with a 20°C differential, the flow rate will be 15 ÷ (20 x 4.18) = 0.18 litres /sec.

The heat loss from uninsulated 22mm copper pipe with a 55°C differential between the water (75°C) and the room (20°C) is about 40W per meter.

If you have a three metre length of pipe, the heatloss will be 120W. The temperature drop along the pipe will therefore be 0.12 ÷ (4.18 x 0.18) = 0.16°C. The total drop around a 50m circuit would only be about 2.5°C.

It would be different if it was a one-pipe system.

 

[PlumbBobBob]

i stand corrected

 

[plb]

Thanks for all the useful and informative replies.
Just to be clear this is a new install and will have slightly oversized radiators.
So, as I understand it I will be ok to use say a 10 – 11 Deg C temperature differential as a 20 Deg C differential may be difficult to achieve.

 

[doitmyself]

Just to be clear this is a new install and will have slightly oversized radiators.
So, as I understand it I will be ok to use say a 10 – 11 Deg C temperature differential as a 20 Deg C differential may be difficult to achieve.

I assume you have the Regular version of the 30CDi (not the combi or the system) with a separate pump.

Follow the balancing instructions and set the pump so you get as near to 20C as you can or slightly less. Then balance the rads to the same temperature. You will find that the temperature differentials across the boiler and rads will have changed when you get to the last rad. So you will have to go round several times to get them all approximately the same.
If necessary, adjust the pump speed to maintain the correct differential.

The important thing is that the differentials are, if possible, the same. This then ensures that they are all getting their fair share of the heat.

You say the rads are slightly oversized; how were the sizes determined? They may be more oversized than you think.

 

[plb]

I assume you have the Regular version of the 30CDi (not the combi or the system) with a separate pump.

Follow the balancing instructions and set the pump so you get as near to 20C as you can or slightly less. Then balance the rads to the same temperature. You will find that the temperature differentials across the boiler and rads will have changed when you get to the last rad. So you will have to go round several times to get them all approximately the same.
If necessary, adjust the pump speed to maintain the correct differential.

The important thing is that the differentials are, if possible, the same. This then ensures that they are all getting their fair share of the heat.

You say the rads are slightly oversized; how were the sizes determined? They may be more oversized than you think.

Apologies, I should have stated that it is a WB 30CDi Combi. :blush5:
The Pump Map is set at default (Proportional pressure high)
The radiators have been oversized by approximately 25% using an appropriate software package.
BTW, do you guys recommend using one of those Infrared DigitalThermometers for measuring the temperatures. If so can you recommend one that is reasonably priced and reliable.

 

[PlumbBobBob]

got mine off fleabay about a fiver from china. works really well.

 

[doitmyself]

Apologies, I should have stated that it is a WB 30CDi Combi. :blush5:
The Pump Map is set at default (Proportional pressure high)

The pump curves in the Service manual don't make sense! The 30cdi has a CH output of 30kW, which means a flow rate of (30 x 3600) ÷ (4.18 x 20) = 1290 litres/hour. But the pump runs out of guts at 1400 lph (no available head). It would be even worse at an 11C differential as the required flow rate is 2350 litres per hour!

The radiators have been oversized by approximately 25% using an appropriate software package.

That should be sufficient to allow a 15-20C drop across the rads.

The only thing you have to remember when using IR thermometers is that the temperature displayed will vary with the surface (dark/light. shiny/matt). If you want consistency, wrap some black insulating tape round the pipes where the enter the rad and measure the temperature off the tape.

You also need to realise that the "collection" area depends on how far you are from the pipe; think of it as a torch beam. The closer you are to something the smaller the area covered by the torch. The IR themometer will give a coverage figure, e.g 1:8. This means that it will cover an area 1 metre wide at a distance of 8 metres (1cm at 8cm). The higher the ratio the better.

 

[mylesy]

there ok the lasers but reflect a lot off shiny copper, you sometimes have to wrap a piece of thin tissue around the pipes to get a reading. the best way is to use an analyser with a temp diff application on it.

 

[mylesy]

with clamps

 

[mylesy]

by the way i was under the impression that the CDI range only put out 24kw for central heating regardless and the higher KW is for hot water output only??

 

[mylesy]

just checked its 30KW out put for 30-37 and 42, new it was something like that

 

[doitmyself]

just checked its 30KW out put for 30-37 and 42, new it was something like that

You are quite right. I have just realized I had been looking at the HW output figures! No wonder the input and outputs were the same.

I have edited the figures in my earlier post.

 

[Mrs Tara Plumbi]

I have edited the figures in my earlier post.

I love the maths even though i wouldn't know how to actually do any of it.
What do you do it fitting new boiler to old systems sized for 11C differential?

 

[doitmyself]

What do you do it fitting new boiler to old systems sized for 11C differential?

There's not much you can do as you are stuck with the rads which are already there. However, as the rads are probably oversized, either due to over-generous allowances when they were first installed or because the house has been insulated better, you may find that you can run the system at a higher differential than 11C.

Using the Boiler Size Calculator and the Stelrad Elite Catalogue will tell you how big a boiler is needed for heating and the outputs of the rads. You can then work out how much the rads are oversized.

 

[mylesy]

You can alter the central heating output on the CDI range in the service menu which is the spanner symbol on the CDI (it's not the boost like in the I series, strange?) think it's menu 1.A but I'll have to check

 

[doitmyself]

You can alter the central heating output on the CDI range in the service menu which is the spanner symbol on the CDI (it's not the boost like in the I series, strange?) think it's menu 1.A but I'll have to check

You are quite right. The outputs of CDi Combi and System boilers can be down-rated, but not the Regular boilers. It's on page 18 of the Service Manual.