Definition of \"condensing boiler\"

Mark R.

If I hang around here long enough you guys are going to give me an education. I haven't had any problems, but wanted to know a little more about condensing boilers. My return water is too hot to hold you hand on the pipe. My guess is that I have about a 20 degree decrease from supply to return. My service man does draft and flue gas analysis when he tunes the boiler. Based on your comments I think my system is OK. I was thinking that return water that was too cool would cause condensation inside the firebox. I would think that any baseboard system would have cool water return when the circulator starts after an off cycle. I don't think there is a solution to that, other than continuous circulator operation. Thanks again for all your help!!

Mark R.

Condensing ?

Can someone tell me about a condensing vs. non-condensing boiler? I am not a contractor (obviously), and have a W-M WGO-3 oil fired boiler without a tankless coil. I have a small single story house with a single loop, and boiler is in basement. My boiler normally runs/stays around 170 degrees, and does a nice job. I would appreciate any help to understand the terminology. Thanks, Mark R.

Steve Ebels

Basically

A condensing appliance is one that extracts so much of the heat of combustion that the flue gases are lowered to the point where they condense. When condensation occurs, the latent heat which is in the flue gas is able to be captured and used through a condensing type boiler/furnace. These appliances are usually designed with a specific type of heat exchanger or secondary heat exchanger that causes or allows for this phase change to happen. Your boiler can be brought into a condensing mode by firing it into low water temps however it is not designed to live long and prosper while doing so.

Help?

Mark R.

Thanks...another question

Thanks Steve, I follow you I think. Then condensing boilers/furnaces have a built in means of getting rid of the condensate? At what water temperature would a boiler like mine start to condense? I presume that I am at a high enough temperature to prevent the problems you mention. I really appreciate you help. Mark R.

joe_14

condensate



as long as the return water stays above 140 degrees you will not condensate in the boiler.


hope this helps

joeg.

Steve Ebels

You're welcome

To answer your other questions, yes, a condensing boiler has provision made to discharge the condensate to a drain.

For the second question, you will usually find that a boiler like yours will operate safely down to 135-140* water temps. Anything below that and you are going to be borderline as far as safe, clean operation. Oil has a lower temp threshold for condensation than a gas fired appliance. This is due to the fewer (hydro/water) molecules contained in the fuel itself. In fact, if you read the specs for noncondensing boilers that can be fitted with either an oil or a gas burner, you will nearly always find that the oil unit will achieve an additional 2-4% efficiency.

There are some cast iron boilers on the market that are equipped with proprietary control setups that allow operation down to just over 100*. They will monitor the water temp and shut off the circulator when the boiler actually fires, allowing it to heat up quickly and avoid condensation problems. These controls typically run a program that causes the water temp to follow the outdoor air temp. The lower the temp outside the higher it will cause the water temp to run and vis-versa. This will run a cast iron boiler as efficently as possible.

Mark R.

Thanks Steve & Joe

Thanks again for your help. I don't have a good way to measure return temp, but my system stays well above 135-140 on the boiler temp, so I should be OK. You guys help is really appreciated! Thanks Again!

Mark Hunt

Oil


systems may condense at 155*.

Are you noticing something unusual?

Mark H

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munchkin-man

Additionaly it cools flue gas so that the stack eff. increases

this results in less heat going out the stack and being returned in to the heating system thus increasing efficiency

Jim Davis

Temperature of condensing

It is not the water temperature that determines if a boiler will condense, it is the flue temperature. Water heaters operate at 120 degree water and they don't condense. Proper firing rate,proper differentials & proper venting will allow equipment to run at temperatures 105-120 degrees water temperature and not condense.

Larry (from OSHA)

pardon my ignorance, but

so many of the folks posting here talk about rotted out boilers from too cool return water. I know (I think I know) that the amount of time spent in a "too cool" condition has a bearing on this, but quite frankly, I'm wondering about my own situation.

Does an induced draft boiler which has a stack temp of 300* + after a few minutes firing and a boiler water content of 2.7 gal. and providing heat to copper fin tube baseboard with a total content of about 10 gal. in three zones run a risk of condensation damage if supply temps run down to 110* during periods of low demand while utilizing an outdoor reset control?

Thanks for any perspective that can be provided.

Larry

Jim Davis

Why can a water heater not condense with make-up water temperatures of 45 degrees or are they made of more expensive material?

heatboy

They condense all of the time........

at the bottom sheet when you have water replenishing the tank. The reason they don't condense in the stack is due to the horrible efficiency, or lack of, they have. There is so much dilution air in the stack that it is almost impossible to get them to condense. CO2, at that point is probably down around 4%. Hard to condense with that low of a CO2.

hb

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Jim Davis

water heater CO2

Actually water heaters run with a higher CO2 than most any other equipment 8%-9%. They tend to be more efficient out of the box than any of the equipment I have combustion tested for the last 25 years. Scale is the biggest reason they get inefficient. The ones that do condense have venting issues or leaks!