Boiler bypass saves fuel?

Spudwrench

I have read claims that a boiler bypass line will save fuel on a gravity conversion system... I understand the other benefits of a boiler bypass (preventing flue condensation & thermal shock to the boiler) but I can't understand how a boiler bypass would save fuel. Can anyone elighten me?
Also, how does a simple boiler bypass with valves to balance compare to doing the job with P/S pumping or a variable speed injection pump? I'd like to keep the controls simple and low cost.

Thanks,
Nathan

Firedragon_4

Essentially,

it's because you're making better use of the heat exchanger.

Spudwrench

but...

If you're transferring so much of the heat from the combustion to the water that the flue gasses are in the condensing region, isn't that an efficient (if undesirable for the boiler & flues health) way to run? I still don't quite get it...
Nathan

A.J.

Bioler bypasses

are a great bandaid for protecting the boiler from flue gas condensation. Todays smaller more efficent boiler often hold less water than just one of the radiators they are heating.When ever you are firing your boiler below the condensing tempeture of those gases you are condensing water vapor, mix it with CO and you have got an acid I forget witch one.The wet sections not only wear faster but the carbon also sicks to it and blocks up the boiler faster.To stop this bypass some of the boiler water and let it come up in tempeture faster. Now sure your combustion efficency is going to suffer alittle becuase your delta T isn't as much as it would be with out the bypass but your out to protect the boiler not mention mantain constant tempeture for your tankless coil if that is the case.

Kupetz's Plb & Htg

Mike T., Swampeast MO

Played With That Before

Using the adjustment in a differential pressure bypass valve on a gravity conversion with reset, constant circulation and TRVs.

Amount of bypass does influence the total burn time in a given period of steady outdoor temperature. EXTREMELY sensitive adjustment. Too much bypass and the burns become shorter and more frequent--too little and the burns become longer more than they become less frequent.

Interesting thing was that for a given bypass setting that burn time would remain constant regardless of outdoor temp (provided of course that TRV settings were unchanged).

A condensing modulating boiler makes the point moot--the only bypass needed is to prevent dead-heading the pump.