Gravity Hot Water

MikeJ

Customer wants to convert gravity hot water, there are no pumps now (will be adding a pump) in the system, they want to go to a high efficiency boiler, with indirect water heater. read Dan's book How Come? very informative book. I do have some questions I'm hoping  to get some help on. First one would be on the radiators , there is the incoming water with a angle valve at the bottom, the return on the opposite side at the bottom. The air bleed is at the top on the angle side. Does this air bleed need to be moved or is it O.K where its at?

Thanks

Mike

TonyS

Gravity systems are piped

with a 2 pipe direct return arrangement. This means the first radiator to get water is also the first back to the boiler. Like electricity, water takes the path of least resistance which means you wont be getting much flow to the furthest radiators. There are several ways of dealing with this. First you can replace the radiator valves with thermostatic radiator valves. This is the best option because it gives each room its own setpoint but can be expensive. Second ,you can reverse the returns by following your return from the boiler to the first radiator T, Remove that piping and plug the T, now follow the return to the end and break out the last 90, replace with a T and run copper back to the boiler, copper size is one half diameter then drop one pipe size(2-1/2 =1") now your first radiator to get water will be the last back. This also can be backbreaking and a pain if the returns are in crawl spaces ect. If you can weld, a threadolet can make this easy. I have also seen orfice disk put in each radiator valve, but I have never done this. Your air vents are fine. Hope this helps.

EricAune

Tony is right

Reverse return is a very effective way to get the system balanced.  Hopefully you have access to the piping without major destruction.  I like this method over TRV's because it removes the mechanical failure rate of the TRV's from the system.  

Mark Eatherton

Leave the vents where they are...

Older upright cast iron heating systems are bottom filled, top vented.



As for failure rate of TRV's. I can count the failed TRV's I've had in 35 years on one finger. And I fixed that one with a pair of pliers (stuck needle).



My personal experience and opinion says that you should use TRV's. They allow the heat to be delivered only to those areas that NEED heat, and will automatically balance out the flows to the radiators.



Be careful in the selection of the circulator. You don't need a lot of head, and you are only really wanting to assist the gravity circulation. The smallest circulator you can find will be more than adequate in most cases.



ME

Steamhead

That is

assuming you have a balance problem in the first place. Many of these old systems work perfectly well without reversing the returns.



With TRVs on the rads in rooms you want to keep cooler than the main house (bedrooms, etc) and outdoor reset, you should be good to go.



For a discussion on sizing circs on old gravity systems, go here:



http://www.heatinghelp.com/article/11/Hot-Tech-Tips/238/Sizing-Circulators-for-Hot-Water-Heating-Systems

MikeJ

Thanks

for the help, I like the TRV idea, and not having to do any thing with the air vents. There is no way to do reverse return. Will look for orifices on the 2nd floor. The system I'm looking at  looks just like the one on page 8 the How Come book?





Plan on converting it to a 95% boiler with P/S piping, indirect water heater ( plan on doing it only if I feel comfortable doing it). Mostly have pipe up new systems with pex, indirect water heaters and hot water A/H.



Still confuse about the piping on page 28, if there are any links to more information, or drawings that would be great . Thanks again.

Mike