Setting Condensing Boiler Flow Temperature

[ajrobb]

Rate of condensation is dependent on the return temperature, not the flow.

My limited understanding is that condensation only begins when then return temperature drops below 55°C. However, I thought it became more effective as the temperatures drop still further. For instance, my Keston manual only states efficiencies at two temperature return-flow ranges, 88% at 60-80 (non-condensing) and 96% at 30-50 (fully condensing).

 

[Chris_J]

Does your boiler support SARA?

Checking the manual, this boiler does support SARA. The boiler is currently set within the SARA range. So, could this mean that my efforts to regulate the return to below 55C are futile, since when it is calling for heat for a sustained period, it would crank the burners up anyway?

I'm starting to think that these modern boilers, particularly A rated ones, are programmed to run as efficiently as possible out of the box and will regualte the burners to control the return temperature, ensuring maximum condensing.

In the case of this boiler, I guess I could work on the principal of setting the temperature to the minimum SARA setting and let it do the rest.

 

[MJN]

I'm starting to think that these modern boilers, particularly A rated ones, are programmed to run as efficiently as possible out of the box and will regualte the burners to control the return temperature, ensuring maximum condensing.

They regulate to maintain the target flow temperature as set by you, unless it supports weather compensation or similar strategies as discussed whereby the flow temperature will be varied according to demand.

Mathew

 

[ajrobb]

They regulate to maintain the target flow temperature as set by you

The manual states otherwise.

Balancing radiators is a good idea. Whether you should de-rate your radiators significantly and go for a 20°C drop instead of 11°C is debatable. You might end up cranking up the flow temperature to compensate for de-rating the radiators.

 

[Agile]

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...!

Mathew

I always thought that was because the makers assume the installer is properly qualified and has attended the energy efficiency course and already knows how to install and commission a boiler.

They dont tell you how to solder a joint either!

Tony

 

[ajrobb]

In the case of this boiler, I guess I could work on the principal of setting the temperature to the minimum SARA setting and let it do the rest.

It's worth a punt.

 

[Chris_J]

Just done the boiler size calculator. According to this my heating requirement is 12.86kW.

My radiator outputs are 571W (1200x450) and 286W (600x450). Not sure what the towel rail would be.

So, would you say another 15kW for DHW, hence the boiler is probably a bit oversized.

 

[MJN]

I always thought that was because the makers assume the installer is properly qualified and has attended the energy efficiency course and already knows how to install and commission a boiler.

How do you know the target drop if it is not specified by the boiler manufacturer? As you know, the drop is dictated directly by the design of the heat exchanger. Many condensing boilers are simly rehashes of old non-condensing designs hence they have not been engineered for a 20C drop but must (should) remain at 11C.

Besides which, you might have a point were it not for the fact that it is clear from reading many a post on here that there are a great many installers do not have the sufficient knowledge to configure a heating system for maximum efficacy and efficiency. Present company excepted of course!

Mathew

 

[dumbrill]

Whether you should de-rate your radiators significantly and go for a 20°C drop instead of 11°C is debatable. You might end up cranking up the flow temperature to compensate for de-rating the radiators.

So if starting from scratch then it is my understanding that the radiators could be sized for a return temperature of 50°C and a 20°C temperature drop across flow and return. Thereby resulting in a deltaT of 40°C. According to the manufacturer's' details the radiators then have to be oversized because they will only provide 75% of the heat output (0.7482 to be exact). What's the flaw in this approach?

 

[MJN]

There's no flaw. The only drawbacks of larger radiators are increased capital costs and possible aesthetics issues.

Mathew

 

[dumbrill]

There's no flaw. The only drawbacks of larger radiators are increased capital costs and possible aesthetics issues.

Mathew

Good, as that's what I'm doing as I replace my radiators. I guess aesthetics only really kick in when the radiator turns from a single to a double. Although some people seem to get hung up about them whatever.

 

[ajrobb]

Looking at screwfix prices, radiators cost about £60/kW. Running at 75% rating will push this cost up to £80/kW. For 13kW of radiators (from your boiler size figure), this will cost £260 extra for something like 3%* improvement in boiler efficiency. Spread this over 10 years and your gas bills would need to average over £800/year to make a saving (keeping in mind that gas prices are rising). *The calculation is very rough as it is sensitive to the actual savings achieved. With SARA temperature modulation already standard on your boiler, the extra gas savings from larger radiators might be very small.

However, on the plus side, your radiators will have spare capacity to cope with very cold weather if you re-rate the temperature drop back down to just 11°C (your boiler already has plenty of spare heating capacity).

I think the Delta-T figure of 40°C is too low. With flow of 50°C, the maximum Delta-T figure (the excess radiator temperature above room temperature) is only 30°C. With a 20°C drop between flow and return, the mean Delta-T drops to less than 20°C. Your radiators would need to be huge.

I think of condensing efficiency as a free gift (the regulations have forced you to buy a condensing boiler anyway). You're going to get some saving with a normal system, but it's not worth paying more than you have to.

 

[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.

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

i think you mean under sized, if its oversized the water will come back quicker, as the smaller the rad the greater the restriction....

 

[MJN]

I think the Delta-T figure of 40°C is too low. With flow of 50°C, the maximum Delta-T figure (the excess radiator temperature above room temperature) is only 30°C. With a 20°C drop between flow and return, the mean Delta-T drops to less than 20°C. Your radiators would need to be huge.

You may have got your figures mixed up there. A Delta-T of 40°C gives an average radiator temperature of 60°C which can be achieved with a flow/return of 70/50°C.

Mathew

 

[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

i think you mean under sized

No, oversized. If they were undersized then there would be less (inadequate) heat dissipation such that the return temps won't be much lower than the flow. This results in the boiler having to modulate down or, if it cannot go any further, turning off (and cycling). It won't keep burning away as there's nowhere for the heat to go.

Mathew