Setting Condensing Boiler Flow Temperature

[Chris_J]

I've just had a whole new boiler system and controls fitted, consisting of the following:
Vokera Compact 29A combi condensing boiler
Honeywell CM927 room stat.
TRVs on most rads

The boiler has the normal flow temperature controls on the front for setting the main boiler running temperature and hot water temperature.

Is there an optimum setting for the flow temperature to ensure the boiler is running in condensing mode as much as possible? I initially thought that running the boiler temperature high and letting the TRVs and room stat do the work was the correct was to do it, but I hear that doing this would result in the boiler not being able to condense properly.

The boiler doesn't acutally give a readout for the flow and return temps, but I'm able to measure it myself at the pipes.

Any advice?

 

[MJN]

Hi Chris,

Is there an optimum setting for the flow temperature to ensure the boiler is running in condensing mode as much as possible?

As low as possible whilst still meeting the demands of the environment. In practice you may find that too low a flow temperature will result in inadequate heating performance on really cold days.

Boilers with external weather compensation controls are able to dynamically adjust the flow temperature in accordance with outside temperature. Thus, they can maximise condensing opportunities wherever possible (sometime you have to let go of ultimate efficiency in favour of adequate performance). Without such controls you have to either accept a compromise or be prepared to manually adjust in accordance with the outside temperature.

What are your current flow/return temps? Condensing of exhaust gases occurs at around 55C (return) and increases almost linearly the further you go below this.

Whatever you set the flow to you ought to ensure your system is properly balanced as an unbalanced system can produce unnecessarilly high return temps.

Mathew

 

[Chris_J]

Thanks for your reply.

The guys re-balanced the system when they fitted the TRVs, as they opened all the lockshields when the system was flushed a week before install of the boiler.

The system is only small and didn't need much balancing. I balanced it myself a while back and most of the radiators heated up at around the same rate.

The small system causes a bit of an issue with the return temperature. It's quite hard to get a 20C drop. At the moment, the return is running at ~60C. Looks like I need to just tweak that down a little bit.

The question is, should this be done by adjusting the CH temperature on the boiler, or turning down the pump speed?

Also, I assume you do all this with the TRVs wide open?

 

[DaveSteveBob]

Did you ballance the system the correct way to achieve a 12 Deg C drop at each rad. My system is also fairly small and I am able to achieve a 65 flow with about 54 return. My Baxi Duo-Tec will not allow turning the pump down.
In the interim I have ordered the optional weather compensator and look forward to further economy once fitted.

 

[MJN]

As DaveSteveBob(!) says it is often not possible to alter the pump speed if you've got a combi (it needs the pump on max to ciculate the small primary circuit fast enough to transfer sufficient heat in DHW mode).

Hence, without changing pump speed you are somewhat limited with regards to the absolute drop across all the rads - you're pretty much stuck with the consequences of whoever designed the system. Hence, the aim isn't nirvana - merely getting the best you can out of it - concentrate on the same relative drop across all radiators whilst ensuring one, the so-called 'index' radiator (this usually corresponds to the the one requiring most flow due to its size or distance from the pump) is fully open (still the same drop though). Throttling all rads will be over-restrictive and may cause excessive noise.

If you can't achieve the design drop for your boiler (usually 11C or 20C) then there's little more you can do. Of course, if you were designing a system from scratch you'd specify the correct sized radiators in the first place to achieve the drop your boiler is designed to provide.

And, yes, always balance with the TRVs - not just open as if the rooms start getting hot you may still find they start closing without you realising... that'll throw you right back you back to round 1 or cause you to endlessly chase your tail! (been there done that!)

You mentioned a return of 60C, what's the flow temp?

Mathew

 

[Chris_J]

My old combi used to have a pump speed adjustment. Checked the pump on the Vokera though and you're right, there's no adjustment.

I've had a bit of a tweak this evening and I've managed to get the flow to 68 and return to 55. That's wilth all the TRVs on full.

The rads are only single panel and there are 4No. full length, 3No. half-length and 1No. towel rail in the system. I guess the system is a bit undersized, hence the low flow/return differential.

 

[MJN]

My old combi used to have a pump speed adjustment.

It may well have done, however the installation instructions may also have said it must be left on max.

I've had a bit of a tweak this evening and I've managed to get the flow to 68 and return to 55.

By doing...? Is that with the boiler stat at minimum?

Have you checked the installation instructions to see what drop it is designed for?

20C drops, if that's your target, do usually require large radiators if a sensible flow rate is to be maintained.

Don't agonise to much though - there's a limit to what can be achieved unless you are designing from scratch.

Mathew

 

[Chris_J]

I've just been adjusting the flow temperature at the boiler, whilst everything else was at full demand (i.e TRVs and room stat.)

Had a look at the instructions and the only thing I can find is "Maximum Temperature Differential" which quotes 35C, which I don't think is what we're looking for.

As you say, it's probably not worth fussing about. I was just surprised when someone told me that it won't actually condense properly is the return is too hot.

Thanks for all your help.

 

[MJN]

I've just been adjusting the flow temperature at the boiler, whilst everything else was at full demand (i.e TRVs and room stat.)

That's fine. (Be careful with just having TRVs at max - some can still influence the valve if it gets hot enough)

Had a look at the instructions and the only thing I can find is "Maximum Temperature Differential" which quotes 35C, which I don't think is what we're looking for.

I have noticed that some manuals don't give balancing targets/guidance which is odd, but perhaps that's because a lot of installers don't understand it and/or don't do it properly anyway so why confuse the poor b*****s...!

As you say, it's probably not worth fussing about. I was just surprised when someone told me that it won't actually condense properly is the return is too hot.

There are still efficiency benefits of a (properly designed) condensing boiler given the bigger heat exchanger regardless of whether it is operating in condensing mode or not.

Mathew

 

[mysteryman]

As somebody said so rightly, you need weather compensation to do it automatically.

You need the flow temperature below 53C for full condensation.

 

[MJN]

You need the flow temperature below 53C for full condensation.

Rate of condensation is dependent on the return temperature, not the flow. Of course they are intrinsicly linked but a flow temperature alone is meaningless as the differential depends on the system.

Mathew

 

[onlyfitidealboilers]

silly this, if you have a boiler with a huge temp diff the boiler will never get to temperature thus cooking the heat ex as the boiler will never go off.

iv seen this lots of times on ideals & worcesters as these have aluminum

 

[ajrobb]

Does your boiler support SARA?

"The automatic temperature control function (SARA), permits the boiler (when the heating temperature selector is set within 4 and 6 sector) to automatically adjust (raise) the heating. The activation and the disable of the function is visualized by blinking the green led."

"the SARA function permits the boiler (when the set-point is within the SARA range) to automatically adjust (raise) the heating flow outlet temperature should the room thermostat contacts remain closed for more that 20-minutes."

 

[MJN]

silly this, if you have a boiler with a huge temp diff the boiler will never get to temperature thus cooking the heat ex as the boiler will never go off.

That's why the radiator drop is set as per the design specification of the boiler. Furthermore, all boilers specify an absolute maximum drop in order to prevent that occuring.

iv seen this lots of times on ideals & worcesters as these have aluminum

The radiators must have been too oversized, or the flow rate too low (i.e. too low a pump speed, blockage etc).

Mathew

 

[D_Hailsham]

Do a Boiler Size Calculation and find out the output of your rads using the Stelrad Elite Catalogue (very little difference between manufacturers).

Then post the results.

PS I know the OP has a combi boiler. The calculator will give the heating requirement.