Balancing for condensing boilers and what pump speed

[sxturbo]

Boiler and radiator manufacturers' use delta T to mean different things.

To the boiler manufacturer it is the difference between flow and return temperature. So a Flow of 75C and Return of 55C gives a 20C Delta T (75-55)

To the radiator manufacturer it is the difference between Mean Water Temperature (MWT) and Room temperature. A radiator with flow of 75 and return of 55 has a MWT of (75+55)/2 =65C. If the room temperature is 20C the radiator Delta T is 45C.

As the heat output of a radiator depends on Flow, Return and Room temperatures, all rads have to be tested to a common standard, BS EN442. This specifies Flow = 75C, Return = 65C (MWT=70C) and Room =20C. Hence the rad Delta T of 70-20=50C.

If you want to run at a 20C boiler delta T, you have to take this into consideration when selecting your radiators as the output will be less than that quoted by the manufacturer. There is normally a table in the radiator literature showing the correction factor for different (radiator) delta Ts.

Thanks.

I'm not going to replace my rads to meet the maximum boiler delta, however will work out what my rads are (not sure what brand they are but all have fix stamped in them) and see if they are sized for the delta. If not the system can stay exactly as it is, as it is working the best it has ever worked at the moment, and that's what I'm most fussed about, although if I can get it right then I would prefer to do that.

 

[John D v2.0]

Don't worry about getting enough drop, just make sure it's not too much. You can keep throttling all your rads until you get the drop on any system, but then your average rad temperature will be lower and most of the water will be going round the bypass, meaning higher return temperatures to the hex compared with the average rad temp therefore less condensing.

Eg drop the flow temp by 5c and have the water racing round with 10c drop, the average rad temp is the same, the return temp to the boiler case is 5c higher, but when the lack of bypass is taken into account the return to the hex is probably 5c cooler. So the average hex temp is 5c cooler.

That bypassing water is heating your room instead of keeping the flue gases warm.

 

[fezster]

You can run your rads at 20 degree delta, but as the output is then lower from them, they will only be able to heat your house up to an acceptable temperature if they are adequately sized. As per previous post from D_Hailsham, the actual output can be calculated and adjusted for differing delta's. The long and short of it is that you can only run rads specced at an 11 degree differential at a 20 degree differential, if they are oversized.

The other thing to consider is that at the magic 55 degree return temperature, the boiler is only partially condensing. I believe the figure is 25%, but would have to check. So for optimum efficiency, you'd need to run the boiler at temperatures well below 55 - which isn't realistic. Which is why underfloor heating is more suited to condensing boiler efficiency as it's designed to run at lower temps.

If your house is heating up correctly and your boiler is functioning (ie. Not falling over because the flow/return temperature differential is too high), then I wouldn't get too hung up over it.

 

[John D v2.0]

magic 55 degree return temperature, the boiler is only partially condensing. I believe the figure is 25%, but would have to check

I am always surprised by the magic figure that's quoted at 55 or 56c. The only magic figure is the flue exit temperature of <100c. IE the boiling point of water at atmospheric pressure. Maybe that often happens at 55c return temperature, but I'm sure if you whack the pump on full and the burner on min you'll manage it at 65c return.

https://www.engineeringtoolbox.com/saturated-steam-properties-d_101.html
As you can see, the latent heat of evaporation is 2257kj/kg which is the source of additional condensing boiler efficiency. Cooling the steam (water) further to 20c only gains a further 167.6kg/kg so you can see why "condensing" is the important part.

given a condensate rate of 2kg/h that would be 1.25kw extra power that isn't go up the flue but went into your radiators.

I wouldn't get too hung up over it.

True well that's only 4p an hour or so even if you're going at it full heat.