Gravity Hot Water

Daniel Becker

I am planning to add some new radiators to my house. The heating is an old gravity system with cast iron radiators. The system was converted to forced circulation before I bought the house. Any recommendations on pipe sizing for adding radiation? My instincts tell me to size pipe consistent with what's there now rather than how a new forced circulation system would be designed. Also anything special that needs to be done for radiators in the basement below the supply and retrun mains.

Mike T., Swampeast MO

I believe your instincts regarding sizing are dead-on correct. You also can't go wrong by piping in the same general method--ensuring that horizontal branches have a steady even pitch back to the mains.

Rads installed below the mains are notorious for causing air problems I believe. "Think like air" and install valves/bleeders to allow you to: 1) force it out when filling; 2) remove if/when it accumulates and 3) prevent such accumulation as much as possible to begin with. If installing more than one or two in the basement you might want to give SERIOUS consideration to running such in a completely separate zone with its own main piping.

Mike T., Swampeast MO

Sizing new rads in old converted gravity system

Unless you're installing an UTTERLY enormous radiator, I wouldn't use anything larger than 1" for the branch connections however. If you're adding many rads and the original, low, close ones have really big branch lines you might want to consider decreasing them one or two sizes, or (much easier) fashioning restrictor plates to insert in their valves. The restrictor plates I've seen are of thin brass and have a hole about the size of a kindergarten pencil.

Daniel Becker

Do you have any tables showing recommended pipe connection size versus radiator size for a gravity system. The connections in the current system appear to be very generouly sized (typically 1" or 1 1/4" for the larger rads).

Mike T., Swampeast MO

Table

is attached.

IMPORTANT: note how the EDR amount of radiation that can be served by a given size increases with elevation! When sizing for new rads to go into a converted gravity system I would use the column for the highest floor in your structure regardless of the actual placement of the rad.

REMEMBER: The sizing of the mains in a gravity system is integral to its function--generally the mains will be sized to serve the area of the valves still to be supplied by the main--i.e. the mains get smaller as fewer rads are served further down the line.

PROBLEM: Under gravity, the most difficult radiator to supply with heat was the lowest and closest--consequently it usually has the largest tapping on that main pair--often increased by one size over that of the table. When converted to forced flow, this is now the EASIEST rad to serve with the highest farthest becoming the most difficult to supply with heat.

Converted gravity systems are quite tolerant of well-informed "tinkering" BUT if you are trying to add a lot of radiation you may well wind up with balance problems.

Perhaps you have some old capped off connections from rads that were removed in the past? If so, use them FIRST. Pipe nicely and considerately and you shouldn't have a problem making the branch runs from these old connections somewhat longer than original (elevation doesn't really matter in a forced system so if it was able to serve an upper rad quite close to the connections it should be able to serve a new lower rad significantly farther horizontally.)

I know that the ends of the main pairs are going to seem to be the easiest place (by far) to add radiation--remember however that forced circulation turned this into the most difficult place to achieve circulation! Try to add too much at the ends and you won't get much flow as it will favor those branches closer to the circulator with larger branch lines! This is why I mentioned either sizing down the branches to the lowest, closest or adding restriction plates

It sounds like you're doing extensive changes to the system. This IS THE TIME to add TRVs!!!! The pipes are huge, the velocity low and the TRVs will compensate for nearly any conceivable flow balance problem--either existing or one you may well create by extensive modification!!!! The cost is not extreme. They are generally easy to install and require little in the way of modification to the system. The comfort and savings are real!

Mike T., Swampeast MO

That table

Judging by the systems I've seen and worked on that table is only one of MANY different "rules" that were used...

Eric Peterson_2

RE:Gravity Hot Water

There is one other thing to consider with adding radiators in the basement. Since they will be below any other drain point, be sure you provide a means to drain them in case any maintenance is ever required.

I added two radiators to my basement by adding tees off first floor rads. I used 1" for the basement off of the 1.5" pipes used for the first floor. This seemed to work OK. They were piped in such a way that any air rises through to the first floor radiators. I also put shutoff valves in the walls for each basement radiator.

--Eric