Gravity Fed Boiler Room Piping Conversion

Nick Salido · February 2003

> First find your largest size pipe in the mains, 
> cut it in half and drop one size. This will be a 
> good size for the near-boiler piping. i.e. 3" / 
> 2 = 1½" - 1 size = 1¼". 
> 
> This is possibly 
> unnecessary, but I try to use the same number and 
> type of fittings when connecting the main pairs. 
> I bring these to a tee installed "bullhead" 
> fashion--combined flow through the branch, main 
> pairs at the straight portion. These tees should 
> be the size determined above. 
> 
> Older cast iron 
> boilers often have a built-in air separator. 
> You'll see an "extra" small tapping coming from 
> the top. Often these are unused, but they are 
> effective in a gravity system. Connect your 
> expansion tank to this tapping if you find it. 
> There's no need to replace your tank with a 
> bladder model if you use a Bell & Gossett 
> Air-Trol fitting. Inexpensive AND effective. 
> Once all the air makes it where it should be (the 
> tank) you can pretty well forget about air 
> problems. This can take quite a while given the 
> very large water content. 
> 
> Pumping away 
> certainly can't hurt, but don't expect to be able 
> to "power purge" the system--you still have to 
> bleed each and every radiator when filling. The 
> pressure differential across the circulator in a 
> gravity system is extremely slight so the chances 
> of dipping below atmospheric pressure aren't 
> good. There is certainly no reason not to pump 
> away and if you're redoing the near-boiler piping 
> anyway... 
> 
> Simple cast iron boilers on gravity 
> conversions seem to last decades with zero 
> concern for low return temps and no bypass BUT 
> the bypass does seem to increase efficiency a 
> bit. A differential pressure bypass valve 
> (Braukman/Honeywell & Danfoss are 2 mfgrs I know 
> of) isn't particularly expensive and a good 
> alternative to a fixed bypass line. Be aware 
> though that the hefty boiler oversizing quite 
> common in older systems may well play a 
> significant role in the lack of condensation 
> damage. 
> 
> By pumping away and installing the 
> differential pressure bypass you can add 
> thermostatic radiator valves (TRVs), reset, 
> constant circulation, warm-weather shutdown 
> without any further piping changes. 
> 
> IMO simple 
> atmospheric standing pilot cast iron and fully 
> condensing boilers of the finest quality are the 
> best boiler choices for gravity conversions. The 
> greatly increased efficiency of the condensing 
> boiler should pay back the cost difference quite 
> rapidly but no-one really knows if they will have 
> the long trouble-free life of simple cast iron. 
> It's that unknown that makes me say "of the 
> finest quality."

Nick Salido · February 2003

Conversion Job

I own a home with a "forced hot water" hydronic heating system. Originally, the system was an open upfeed gravity system, but at some point the previous owners closed the system and added a circulator and compression tank. The existing system has two separate infeed and outfeed lines, but only one circulator. Additionally, based on my reading of Dan's books, the compression tank and circulator are located incorrectly, and there's not an air separator or low water cut-off. Air is undoubtedly a big problem with the way it's currently operating.

My question however, has to do with how to re-pipe the boiler-room. Does anyone have a recommendation regarding how I should connect the two supply and return lines so that I can use just one circulator?

At a minimum, I'm trying to add an air separator and low water cut-off, but need to know how to re-pipe the boiler to accommodate one supply and one return with the desire of placing the circulator pumping away from the point of no pressure.

I've also attached a diagram of my existing system. Any help that can be offered would greatly be appreciated. Here in Kansas City, I can't seem to find anyone, who really understands my system, so I'm looking for this Group's expertise.

Thanks for your time and for Dan's good books.

Mike T., Swampeast MO · February 2003

Gravity conversion near boiler

First find your largest size pipe in the mains, cut it in half and drop one size. This will be a good size for the near-boiler piping. i.e. 3" / 2 = 1½" - 1 size = 1¼".

This is possibly unnecessary, but I try to use the same number and type of fittings when connecting the main pairs. I bring these to a tee installed "bullhead" fashion--combined flow through the branch, main pairs at the straight portion. These tees should be the size determined above.

Older cast iron boilers often have a built-in air separator. You'll see an "extra" small tapping coming from the top. Often these are unused, but they are effective in a gravity system. Connect your expansion tank to this tapping if you find it. There's no need to replace your tank with a bladder model if you use a Bell & Gossett Air-Trol fitting. Inexpensive AND effective. Once all the air makes it where it should be (the tank) you can pretty well forget about air problems. This can take quite a while given the very large water content.

Pumping away certainly can't hurt, but don't expect to be able to "power purge" the system--you still have to bleed each and every radiator when filling. The pressure differential across the circulator in a gravity system is extremely slight so the chances of dipping below atmospheric pressure aren't good. There is certainly no reason not to pump away and if you're redoing the near-boiler piping anyway...

Simple cast iron boilers on gravity conversions seem to last decades with zero concern for low return temps and no bypass BUT the bypass does seem to increase efficiency a bit. A differential pressure bypass valve (Braukman/Honeywell & Danfoss are 2 mfgrs I know of) isn't particularly expensive and a good alternative to a fixed bypass line. Be aware though that the hefty boiler oversizing quite common in older systems may well play a significant role in the lack of condensation damage.

By pumping away and installing the differential pressure bypass you can add thermostatic radiator valves (TRVs), reset, constant circulation, warm-weather shutdown without any further piping changes.

IMO simple atmospheric standing pilot cast iron and fully condensing boilers of the finest quality are the best boiler choices for gravity conversions. The greatly increased efficiency of the condensing boiler should pay back the cost difference quite rapidly but no-one really knows if they will have the long trouble-free life of simple cast iron. It's that unknown that makes me say "of the finest quality."

Gravity Fed Boiler Room Piping Conversion

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