Does Slower pump speed= better condensing

[Cubbs]

evening guys. On our condensing oil boiler will turning the pump speed to 1 increase the boilers condensing? That makes it more efficient?

thanks

 

[Dan-HybridHeat]

Running your system at a lower temperature will increase the efficiency

 

[Chalked]

Sorry, but the boiler needs a high flow to condense.

 

[Cubbs]

Boiler stat is on max but I've turned the pump to one. Rads are all still getting hot. There's plenty of condensate running.

So what would be best for how much or little the boiler is cycling?

Cheers

 

[newcastle phill]

You need to monitor the return temp and get that as close to the dew point as possible.
Do that by the means at your disposal while maintaining a comfortable temp.

 

[townfanjon]

Dew point Is 54 degrees

 

[Worcester]

Dew point Is 54 degrees

I thought it was 55° as that what most sites say, I am happy to be corrected

So you need a return flow temperature less than that (and it's the temperature not the flow volume that matters).

Heat emitted (crudely) = flow rate x temperature drop (deltaT)
Hence with heat pumps when we go for a deltaT of 5° we have a higher flow rate.
Biomass boilers with their high temperature output you can sometimes go with a deltaT of 20°
Condensing boilers normally a 10° deltaT.

So, to get your return flow at or below 54° you can balance flow rate (aka pump setting) and boiler temperature, getting this correct will also reduce the boiler cycling.

Of course your actual deltaT for a given flow and boiler temperature will depend upon the heat (kW) required and that depends upon radiator sizes, desired room temperature and outside temperature. (Assuming you are not changing the insulation

p.s. according to industry research out there most people haven't adjusted to the lower flow temperatures required of a condensing boiler so most of them don condense! (How many of you still design rads on a deltaT of 50 ?? just think about it - you should be designing for a deltaT of 40 on a condensing boiler.

Is your condensing boiler actually condensing? | YouGen Blog | YouGen, Renewable Energy Made Easy

 

[doitmyself]

Sorry, but the boiler needs a high flow to condense.

Where did you get that idea from?

Condensing starts when the return temperature is at the dew point (approx 54C) and increases as the return temperature lowers. So you get very little condensation at 54C return and much more when the return is 24C.

If you reduce the flow rate the temperature differential will increase at the boiler, so the return temperature will be lower. The only problem is that the rads at the far end of the system may not get hot. But this can normally be corrected by balancing the system.

 

[oz-plumber]

Where did you get that idea from?

Condensing starts when the return temperature is at the dew point (approx 54C) and increases as the return temperature lowers. So you get very little condensation at 54C return and much more when the return is 24C.

If you reduce the flow rate the temperature differential will increase at the boiler, so the return temperature will be lower. The only problem is that the rads at the far end of the system may not get hot. But this can normally be corrected by balancing the system.

So based upon that statement, what would your design parameters be on a new heating system installation?

 

[doitmyself]

I thought it was 55° as that what most sites say, I am happy to be corrected

It depends on the exact composition of the gas.

Most boiler manufacturers (including your namesake) now expect a differential of 20C max. This requires a lower flow rate, so the head of the system is reduced which means a lower pump speed. The problem then is that radiator outputs are reduced, but this is not normally a problem as installers tend to over-specify radiators.

If you design for a 70C flow and 50C return, rad outputs will be 73% of the BS EN 442 value shown in catalogues, so ypou need rads which are "oversized" by 38%.

 

[doitmyself]

So based upon that statement, what would your design parameters be on a new heating system installation?

Does my post above answer your question?

 

[TVP&H Ltd]

Try not to let the return temp drop below 40 or this causes condensing in the primary heat exchanger and therefore shortening its life span, as it is only constructed of mild steel where as secondary hex (condenser) is made of stainless because of the acidity of the condensate.

 

[SimonG]

If you are concerned about efficiency ensure that it is serviced and combustion adjusted every year to help maximise performance.

 

[doitmyself]

Try not to let the return temp drop below 40 or this causes condensing in the primary heat exchanger and therefore shortening its life span, as it is only constructed of mild steel where as secondary hex (condenser) is made of stainless because of the acidity of the condensate.

That can't be correct when you consider that boiler manufacturers have to state the output with temperatures of 80°/60° and 50°/30°. Some manufacturers also provide data for 40°/30°.

Which boiler has a mild steel primary hex and stainless secondary hex?

 

[Sparkgap]

Slowing the pump will not necessarily make the boiler condense any better. You're better off turning down the boiler stat a bit. Heat output into a room from a radiator will depend on rad design, room temperature and mean temperature through the rad. Turning the pump down (assuming it still manages to pump enough water through the whole system and satisfies the boiler flow requirements) will probably slow the flow through the rad, meaning there is a greater temperature drop across it and mean temperature goes down as well as return temp. Lower mean temp equals lower output from the rad and lower return temp back to the boiler means more condensing.
As Doitmyself says, these days the temperature difference across a system is normally 20C instead of the old 11C, which gives lower flow rates. Generally, if I'm adding an extension onto an old system designed for 82/71C I'll design for 85/65C. In that case the mean temp drops by 1.5C so the rad output is hardly changed but the pipe and pump sizing is smaller. If you want the boiler to condense more I would look at boiler compensation (assuming you're not wanting to heat a HW cylinder) with a limit on the return temp back to the boiler.