gravity hot water replacement

Mike T., Swampeast MO

Certainly no need to re-pipe. Essentially you remove the old open expansion tank (usually in the attic), calculate the system volume and install either a plain tank (uncommon) or likely a bank of diaphragm tanks. Try to remove the piping to the old attic tank back to some connection that's always filled with water. If you don't, the piping to the tank will be filled with air and it might migrate through the system.

You won't find any new boiler with a sufficient number of large enough tappings to work the system under gravity. Most gravity systems have two pairs of mains. Bring the supply and return mains together near the boiler using a simple rule of thumb for the new piping. Half the size of the largest pipe minus one pipe size. For instance if there are 3" mains, you use 1¼" for the new piping.

Over in "Hot Tech Topics" at this site, you'll find a good article for sizing the circulator in gravity conversions. A B&G 100 or Taco 007 generally fits the bill for residential systems unless it's a small cottage or a mansion.

Do a thorough heat loss calculation to size the replacement boiler. DO NOT size the boiler based on the radiation or the existing boiler--if you do the boiler will almost certainly be hideously oversized.

If you use a conventional boiler, cast iron is generally better than copper tube. With a properly sized boiler conventional boiler you will need to use primary/secondary or thermostatic bypass to avoid extremely low return temps and condensation problems. Many replacement boilers in such systems are terribly oversized--while this seems to make gravity conversion systems nearly immune to condensation it takes a TERRIBLE toll on efficiency.

MUCH better to use a condensing/modulating boiler. Most gravity systems can operate at very low temperatures even in extremely cold weather. They LOVE being fed by a condensing/modulating boiler!

If the homeowners want zoning (or if you want to do a truly first-class conversion), install TRVs (thermostatic radiator valves on all radiators). The piping arrangement of most gravity systems does not tend to make for easy zoning and the TRVs will be MUCH less expensive that a complete (and unnecessary) re-pipe.

The system should be flushed and treated for corrosion protection. Rhomar (and others) make very good products. A side-arm style filter with cleanable filter cartridge is not a bad idea. It will trap small rust particles that [may] cause problems with some heat exchangers (particularly the stainless steel used in some condensing boilers). The sidearm filters only filter a small portion of the total flow--when the filter clogs there won't be any flow problem for the system as a whole--it just has to be cleaned--quite frequently after initial treatment. For aluminum heat exchangers, consult the manufacturer for suggested treatment. Flushing/treatment/filtering isn't as critical with cast iron boilers but still probably not a bad idea as the flow velocity in a gravity system is VERY low. Once a circulator is added, particles that would never have been able to go into suspension may find their way to the boiler. At a minimum, I'd add a wye strainer in the new common return piping. If you reduce the mains down while they're horizontal and use normal concentric bushings, the huge mains themselves will act as rather effective and huge debris collection areas.

Systems balance [usually] isn't too much of a problem after conversion. If rooms(s) above the ground floor underheat, remove the offending radiators and look for flow restrictors (thin metal disc with a hole in the center--usually found at the hand valve connection but sometimes at the angle union. If you find them remove them and save them. Provided they're the same size you can place them in lower radiators that might overheat.

Provided the system hasn't had poor/improper modifications (unlikely if it's still working reasonably under gravity) TRVs will completely eliminate any sort of imbalance problems. Each and every room will be able to be kept at its' own temperature with no wiring, no additional zone valves or circulators required. If you go truly first-class with a Vitodens and TRVs, you can likely drive the entire system with a single, very small, variable-speed circulator that's usually built into the boiler. Operated in this manner (single variable-speed circulator, no primary/secondary or low-loss header and TRVs on all radiators) flow through the system will actually be very similar to what it was under gravity--low, slow and variable with just a very small amount of electricity used to circulate the water.

schiller

gravity replacement

It looks like a steamer, but its not!!! but where is the pump?? no pump!! so under the thick coveing of absotos is a gravity boiler (it was coal, it was oil, now it N.G.) The home owner wants to replace it (+ $900.00 in N.G each of the last 2 Months). so.... what do we have to look out for? do we repipe the whole house? any advice would be helpful.

Ron Schroeder

Schiller

mp off the Wall and feel free to look around lots of good stuff out there, try this.
http://www.heatinghelp.com/heating_howcome1.cfm

Constantin

Great Reply, Mike.

I agree with everything you laid out so well. A modulating/condensing boiler, combined with outdoor reset and (perhaps) TRVs will have these homeowners singing the praises of the heating contractor for years to come.

However, another thing to consider is nudging these folk to insulate first, then size the boiler to meet the lower need. What is not lost does not have to be replaced.