Waste flue heat recycling from steam boiler possible?

bruce h

Can you retrofit a flue pipe heat exchanger on a boiler flue to capture heat before it goes up the chimney? Sort of a secondary condensor... Who manufactures something like this? I am thinking it would be passive but could probably connect a fan to the hydrotherms electronics.

I have a hydrotherm boiler 2 pipe steam system that is less than perfect. I figure efficiency is 65% tops since the system is over 15 years old, has no damper, no electronic pilot, etc. So if i get 15-20% more efficiency out of this rig with a secondary exchanger, i still have enough heat to blow the exhaust up he chimney. Does this make sense? And if i'm at 85% efficiency, can i still not get the exhaust up the chimney?

Christian Egli_2

Economizers

It's done a lot on large installation, it can give these installations the same performance you get from the best modulating / condensing hot water boiler.

The reason it is not done for residences is the cost is hard to justify (the same argument you have to confront when buying a new hot water boiler)

How big is your boiler? How big is your heating budget for the season? What kind of smoke stack do you have? tall? large flue? lining?

What region are you located in?

bruce h

Its a Hydrotherm VGA 121 (200k in, 154k btu out, net IBT 115,500, built in 1987 on a 2 pipe steam system. Its vented to an unlined 77 year old brick chimney almost 40 feet high with no lining (big old masonry house). In January & February of last year with gas costing about $7/btu versus todays $13/btu, I had bills of about $450 USD/month. This is Detroit.

Tony Conner_2

Don't...

...confuse boiler efficiency with system efficiency. An economizer won't impact boiler eff (boiler eff is calculated off net heat output, and you can't count the "running start" from an economizer), but it will help with system eff. If you don't have a steam meter on this boiler - and it's highly unlikely that you do - then you've got no way to calculate proper "boiler efficiency", in any event. If, for example, you've got something like a lot of make-up water going into this system, and scaled the boiler up, that will have a big impact on boiler eff. But if you simply replace the boiler without fixing the problem that caused it, then it won't be long before it shows up again in the new boiler.

Installing an economizer means looking at the impact on the chimney, among other things. You can easily get tangled-up with several code issues. Plus, it's pretty easy to condense flue gases with economizers unless you take measures to prevent that from happening. I doubt you'd get any kind of realistic payback.

I'll bet you get a lot more bang for your buck upgrading the building envelope and fixing up the steam & condensate systems as opposed to doing anything with the boiler itself.

Constantin

If the chimney's unlined...

... and you want to recover latent heat from the flue gases, you're going to have to line that chimney or the flue gases will eat it for breakfast.

Your money may be better spent improving the insulation in and around the house, weatherizing, etc. and then evaluating what to do to meet the reduced heating load once the home is as tight as it's going to get. Two-pipe steam apparently allows you to meter locally, so you could even consider reducing the size of the boiler at that point.

Retrofitting TRV-vents to radiators may also allow you better control over where the heat is going.