Flow and return differential

[Handyman]

Is the difference between flow and return water temperatures relevant to the efficiency of the system? If so, why? The system in question is a sealed system heated by a condensing boiler.

 

[meldrew's_mate]

Lower return water temperatures promote greater heat transfer from heat exchanger gases into the water, hence greater efficiencies. With a condensing boiler the latent heat in the products of combustion (methane + oxygen gives CO2 + water as steam) starts to be recovered when the return water temperature falls below about 56C. If flow temperatures are maintained at 70 - 75C it follows that flow-return differentials must be higher than the traditional 10 - 12C in order for condensing conditions to occur.

Hope this helps.

 

[Onetap]

Is the difference between flow and return water temperatures relevant to the efficiency of the system? If so, why? The system in question is a sealed system heated by a condensing boiler.

The return temperature is more relevant;lower return temperature, allow the flue gases to be cooled more, so more heat can be recovered from the flue gases and so the greater the boiler efficiency. Modulating boilers allow the flow temperature to be reduced for part load conditions, giving a lower return temperature.

The temperature differential would depend on the amount of heat being dissipated by the radiators.

The condensate is acidic and flue-gas condensation in a non-condensing boiler would cause back-end corrosion and ruin the boiler.

 

[Water Systems]

Is the difference between flow and return water temperatures relevant to the efficiency of the system? If so, why? The system in question is a sealed system heated by a condensing boiler.

Non-condensing boilers were designed to operate at high temperatures and a high return temp so as NOT to condense. The peripheral equipment was designed around this, with 70C return temperatures required from Cylinders and radiators, which also sepecified a Delta T (temp differential) of 10C. The boilers are designed for 80C flow and 70C return – why they put adjustable boiler stats on is a mystery. I have seen some boilers corrode inside that are condensing because the stat was set too low by the user who though they were saving gas.

Condensers are the opposite. You keep the return temperatures from cylinders and radiators as low as possible, using quick recovery cylinders and larger rads. Although in 95% of cases lower temp in rads doesn’t make that much difference in existing rads designed for a delta T of 10C. Most decent condensing boilers are designed with a Delta T of 22-25C. A boiler can only raise the temp of the water a certain level, let’s say 25C, and the temp differential (Delta T) is roughly set to this. The heat exchanger is also designed to cope with this delta T too. So increasing the delta T it may cause long term heat exchanger harm. They don't over engineer the heat exchanger, as doing so would raise costs and boiler price.

Always keep within the Delta T. With one boiler I was putting on a heat bank I contacted the makers who said that as the boiler was direct to the heat bank and always at full water flow through the heat exchanger (many are not at full flow because of TRVs closing), I could raise to 29C from 25C. I did some tests and the boiler raised the water 28C, so I set the delta T to 28C. That is I set the TMV flow/return blender to 47C, when I knew the boiler would pump into the top of the cylinder 75C, the Heat Bank setpoint.

BTW, I have gained extra efficiencies from a non-condensing boiler having a TMV on the return set to just above condensing dew point – 60C. Lower temp and no condensing. Set to delta T of 10C, 70-60C.

 

[Handyman]

Excellent answers, and fully understood. Thank you.

 

[croydoncorgi]

Excellent answers

except for confusion about the dewpoint value inside a boiler.
I believe it's between 55 and 56 C degrees, not 60.

Also, if you run a condensing boiler with the Return JUST below dewpoint you won't achieve much useful condensing because only a small part of the heat exchanger will be below or 'at' dewpoint. To condense, fluegas has to run very close to a surface below dewpoint (forgetting about details such as turbulent vs laminar flow). What often happens is that fluegas that HAS cooled below dewpoint mixes in the flue with other parts of the flow that hasn't, the temperature of the whole volume goes just below dewpoint and you get a big plume of water vapour out of the terminal. OK - the flue is getting warmed a bit as a result and this heat is eventually passing into the intake air but the most efficient situation is when the condensate mostly runs down the drain and does NOT appear as a plume.

 

[Water Systems]

Excellent answers

except for confusion about the dewpoint value inside a boiler.
I believe it's between 55 and 56 C degrees, not 60.

The dew point of 57C, is the theoretical temperature that natural gas flue gasses start to condense. In oil boilers it is 47C. The return water temperature needs to be a few degrees lower to cool the exhaust gasses to 57C. The further the temperature of the flue exhaust gasses drop below the dew point temperature, efficiencies rise much sharper. The tell tale condensate plume from the exhaust starts to form below the dew point.

Look at the Keston site they have an efficiency graph. Dew-point is the fulcrum when efficiencies rise sharper. A degree or two above dew-point and the efficiency is about the same as when the plume starts to pour out.

If you have a pluming problem and the neighbours are starting to complain (the council can make you take the boiler out), it is worth installing a TMV (a UFH one) on the flow and return and ensuring the return temp is set just above what causes the plume and the efficiencies will still be greater than a non-condensing boiler.