Condensing Boilers

[cisco kid]

Hi - I understand that for a benefit to be obtained from a condensing boiler the return temperature which flows back into the boiler should be below 54C thus allowing the boiler to cool its gases to release "additional" heat through the latent heat of evaporisation.

My question is why don't engineers indicate through say an LED sited on the boiler when the boiler is actually condensing. My Bosch does not show anything apart from the boiler temperature outputted to the CH or HW, depending on what the valve has swapped to heating.

Also can anyone one extend the faq's for balacing radiators to be equally hot at the same time taking the return temperature of 54C into account as well as the pump speed.

Should the 54C be measured on the return pipe as close to the boiler as possible.

Has anyone any recommendations on where to get a cheap measuring device for this purpose and to find an 11C gradient across all the radiators for balancing.

[IMM]

Errr no. The heat exchanger is bigger so always more efficient. Below 54C the efficiency starts to rise quicker. It is no switch, which is either on or off.

Most modern condensing boilers will take a 20-24C differential temperature between flow and return. You set it up to this.

Most modern condensing boiler modulate the burner to lower the return temperature.

[Andy Hall]

An LED wouldn't be appropriate because there is not a sudden arrival in utopia at 54 degrees.

All that happens is that condensing *begins* to occur. The heat contribution will depend on the burn rate and the return temperature.

If you were to plot a graph of efficiency vs. return temperature, it will show an increase in the rate of increase of efficiency with falling temperature. There isn't a step change.

The Keston web site has an example of this.

The important thing is to try to have the intended temperatures across each radiator. Boiler modulation will influence what the boiler does, so there is no real point in attempting to control boiler return temperature as part of the balancing exercise.

Infra red thermometers from places like Maplin are a popular choice.

.andy

To email, substitute .nospam with .gl

[Christian McArdle]

Use an non-contact IR thermometer, they're quicker and more reliable.

For a condensing setup with correctly oversized radiators, you're not going for an 11C difference. 70/50 is about optimum with a 20 degree difference. This allows an average of 60C in the radiator (as opposed to 76C for old style), whilst giving 50C return temperature. You may be able to have the pump on a lower setting and even get away with smaller pipes as the bigger temperature differential gets more heat transfer down the same tubes.

Christian.

[Ed Sirett]

Agreed except for one aspect: the surface finish of the radiator. If they are all painted with the same paint then you should get consistent results.

[John Rumm]

I found taking a reading from a bit of sticky tape on the pipe itself enabled you to get a consistent reading without needing to worry about the surface emissivity since you were providing your own surface.