Condensing Boilers

Rich L.

Heating energy is commonly measured in BTU's. A BTU is the amount of energy required to raise 1 pound of water one degree fahrenheit. Conversly if that same pound of water lowers 1 degree fahrenheit it gives up 1 BTU of heat. This is how you get BTU's of heat out of your radiant floor or base board hydronic system.

The same is true of your flue gasses. As they go across your heat exchanger in your boiler they are giving up BTU's of energy as they cool. But now the really cool part! When those flue gasses condense from a gaseous state to a liquid state they are going to give up 970 BTU's for 1 pound of water condensed! This is called the latent heat of vaporization. All this energy goes up the chimney in a non condensing boiler. What a waste! A condensing boiler captures this heat by allowing the gasses to condense before leaving the heat exchanger. Of course this is not "pure water" so the 970 is not 100% accurate but it's still very close.

Also regarding Stirlings comment on cooler return water temps, he's right on the money. The reason you get a more efficient heat transfer is because the rate (speed) of heat transfer is directly affected by the temperature differential. The greater the difference, the faster, more efficient the transfer. Therefore the cooler the return water, the better the heat transfer, the more condensing, higher efficiency, etc. you get the picture!

Hope this helps,
Rich L

MB_4

Condensing Boilers

I imagine that this question has been asked a thousand times on this website but at the risk of appearing stupid here it is anyway.
Are condensing, fully modulating boilers efficient because they condense or do they condense because they are efficient? Or is the answer possibly a combination of both?

Stirling Boston

Condensing

It's all about physics. When the flue products reach the temp that they change from a gas to a liquid, they turn into condensate. Condensing is a sign of efficiency.

Basically, if you can put so much heat into the heating system (or heating water) that the flue products reach the dew point temp, they turn into condensate. Exact due point temp is a complicated calculation that invloves such things as air temp, relative humidity, etc.

The cooler the return water temp, the easier it is for the btu's to force their way into the water. The more Btu's that can transfer, the more efficient the boiler has done its job. The fact that a boiler can modulate means that at lower firing rates, less Btu's are competing to move into the heating water. The surface area remains constant so it means at lower firing rates the Btu's compete less to transfer heat across the surface area into the water. This is why mod-cons are so effective. We always design to worse case conditions but how many days a year do we actually operate at this design outdoor air temp (i.e. 10 Deg. F.) and at full load on the building (all zones are calling for heat)? If we can modulate the delivery of heat into an partially loaded facility, we are basically able to match the heat loss of the building and maximize the amount of Btu's put into the building that are being taken away from the building though heat loss (through the windows, out open doors, across the walls, etc.).

I think the easy answer is that because they are so efficient, they cause condensate to form.

One word of caution though, all boilers CAN and WILL condense. Any boiler, no mater what design, mod-con, cast iron, steel tube, copper tube, etc. CAN condense and WILL condense under the right conditions. The question is... is the boiler designed to handle the condensate? A non-condensing boiler at roughly 130 deg. F. or less return water temp will "sweat" and form condensate. That condesate will come in contact with the flame of the burner and turn into soot. A condensing design on the other hand is designed so that the condesation occurs on a "secondary" pass of the heat exchange which does not come in contact with the flame and therefore is designed to effectively move the condensate away from the burner and not form soot. The condesing boiler obviously is built of materials that are not affected by the acidic affects of the condesate (i.e. stainless steel or other materials protected by coating such as herosite (sp?))and move the condensate out of the boiler without harming the boiler.

A good question, thanks for asking.


Stirling

MB_4

Condensing Boilers

Thanks for the technical info, I now understand the process much better.
As a follow up I have also been told that a large part of the efficiency is due to the codensate in the secondary chamber which adds to the effect of lower return temperatures. Is this true and if so is there an approximate % of the additional efficiency that can be calculated?

MB_4

Condensing Boilers

Thanks for the info Rich. Both you and Stirling have shed new light on my understanding of the process.

Stirling Boston

Rich's Response

I couldn't have said it better myself.

scott markle_2

11%

I believe that latent heat of condensation for gas(CH4)amounts to a theoretical 11% above rated caloric value of the fuel. Actual recovered value will be lower of course.

cowboy

so you are not in aggreement with Rich L. who says condensate transfers 969 more btu than pure water

scott markle_2

Aggreement?

Rich's figure relates to the energy in a quantity of condensate, My figure is the percentage above caloric value of fuel that condensate can theoretically contribute.