Time for my first Gravity to forced circulation conversion - DVW

David Van Wickler

"Q: Does the bypass serve any other purpose?
A: It allows the boiler to come up to high-limit temperature and shut off. Without the bypass, the large volume of water moving through the boiler often keeps the temperature low and prevent the boiler from reaching high-limit. This does a good job of increasing the fuel bill. "


A system bypass allows hot water from the boiler supply to mix with the cool return water. In essence the pump is diverting flow FROM the system and should keep the boiler from condensing. As soon as all the cold water is purged from the pipes the boiler will most likely reach it's limit.

But why would no bypass increase fuel. I agree it would most likely shock the boiler and condense but without the bypass the pump will show it's full flow to the system and in some ways I think the system would come up to temp sooner?

Either way the system is a real old one - full of steel tube heaters wth 100% outside air.

Gonna pipe it P/S with copper fin boilers - a minimum of zones to keep the load high on the boilers and install secondary 005ZCP pumps instead of ZV's. The ZCP's have a built in relay and end switch if needed.

Gonna' be a fun one.......

DVW

[Deleted User]

A recommendation

go with a condensing boiler and reduce your fuel consumption by 50%. They LOVE cold returns.

ME

[Deleted User]

Also,

if it's amultiple story building, it might be worth you rwhile to pull the top floor radiators adn look for restrictor orifice plate. If they're there, take them out and put them into the radiators on the bottom floor.

Let us know how it works out.

ME

Josh Ormsby

So if you use a ...

new condensing boiler, like for example a Burnham Opus, is the system bypass a good idea? Does the bypass with two pumps make more sense?

This is for an old gravity fed, hot water system, that the homeowner wants to update with a new gas boiler and indirect water heater.

Thanks,
Josh

[Deleted User]

System bypass...

If you pipe it as a primary/secondary system you will have a system bypass inherently in the piping. I think it would still be a good idea to pipe it P/S even with a condensing boiler. This will allow the space heating loop to be a continuous circ loop, with the boiler set up as a secondary input loop to the main. There isn't any reason that you COULDN'T pipe a condensing boiler in series with the loop, other than possibly causing flywheel effect overshoot. Just remember that some condensing boilers have an inherently high pressure drop through their water side.

As for DVW's question, and this is just my take, so if anyone disagrees, go for it, I think Dan was addressing the use of non condensing high mass heat sources. As far as reducing fuel consumpotion is concerned, if the full output of the boiler was allowed to dump 100% of its output into the loop, it would probably cause an overshoot condition, which is a waste. By trickling heat into the loop with the system bypass, you are avoiding overshoot conditions and saving the boiler from condesing and running excessively.

I don't believe Dan's recommendation was considering some of the more prevelant (today) technologies, like continuous circ with outdoor reset. That said, I think his assertion was correct for the application.

JM$0.02W

ME

Mark J Strawcutter

bypass

The Q&A in "How Come" is confusing, at least the excerpt over on the Q&A sectio of the wall is. The diagram shows a system bypass, but the narrative describes boiler bypass.

My take on the fuel consumption issue is that an older cast iron boiler is most efficient at higher operating temp. A bypass will let it get there.

With a single boiler and primary/secondary you can have the boiler loop be either the primary or the secondary. Depending on the boiler, you may still want a bypass in the boiler loop itself.

Mark

Paul Pollets

Gravity systems

usually have cast iron radiators attached. I believe matching the type of emmitter to the heat source. Cast iron or biferral boilers work best with cast iron rads. CIRads have colder returns, but are considered medium or high temperature applications. Wouldn't you agree that condensing boilers work most efficiently when the heat emitter is low temp? Then there's the issue of "casting lag". CFT boilers have very little or none. Why not take advantage of a larger mass boiler to heat the rads without the burner cycling?

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Mark J Strawcutter

over-radiated and over-boilered

The gravity conversion systems I've seen (granted, not a lot compared to some of you here) tend to be both over-boilered and over-radiated.

My own system, for example, never goes above 140F even when recovering from setback on a design-temp morning.

Size the boiler to the building's heat loss, and the oversized radiators are going to give up the btus as fast as the boiler can supply them. Return temps stay low, perfect match with a modulating, condensing boiler.

Mark

Mike T., Swampeast MO

Overshoot OR extreme short-cycling

Gravity conversions are just plain strange.

Even with boiler way oversized to the heat loss, the radiation can generally liberate WAY more than the full output of the boiler.

As you said, if 100% of the boiler output goes to the system, the volume and mass of the system let the boiler run quite a while before you see much increase in the return temperature. The heat isn't "lost" of course (except what goes up the flue) but the system can effectively deliver more output that the boiler can possibly produce.

So, if the boiler runs a while you get overshoot. If you try to prevent the overshoot you wind up with extremely short burn time and rapid cycles.

The bypass with a high-mass (but low water content compared to old at least) boiler lets some of the heat return immediately to the boiler. It seems to try to find the "middle ground" between overshoot & short-cycling.

I REALLY can't figure out the efficiency part of the equation. Simple logic would tend to tell you that if your input (the boiler) is used essentially immediately (little increase in return temperature) that you're operating at very high efficiency as the heat is going just where you want it--the system.

BUT atmospheric cast iron boilers {seem} to work more efficiently when supplying water hotter (and raising the temperature more quickly) than needed by most gravity conversions. Not to mention my suspicion that the closer the boiler is to the true heat loss of the structure the more you have to concern yourself with condensation.