converting a gravity system to a pumped system

robert griggs

that is presently a gravity hot water system. I was wondering if I am better off replacing the system as it is, or putting a circulator in and pumping the water. If I put the pump in, I would need to put ball valves in to balance the system, wouldn't I? The supply comes out of the boiler as 2 1/2", and tees off to 2" to feed one side of the house and 1 1/4" to feed the other. I would have to balance these with ball valves or gate valves right?

I am not too familiar with gravity systems, but there is no air elimination on this system, and no expansion tank, from what I can see (although it may be up in the attic- I will have to take a better look in the morning). The other thing is that there are two thermostats, one on the first floor and one on the second floor. They are tied together and which ever one is set higher overrides the other one.
Could I seperate them, and pipe the system as two zones?

I appreciate any input.

Thanks,

R. Griggs

Unknown

check out Hot Tech Topics at the left, here

Steamhead wrote this piece, about this very subject.

http://www.heatinghelp.com/newsletter.cfm?Id=125

Noel

Mike T., Swampeast MO

Help with some of it.

The tappings in modern boilers (and the quantity of water they hold) are generally WAY too small for them to work under gravity. Dan's book "How Come" shows good ways to add the circulator and pipe for a modern boiler. There's a good discussion of sizing the circulator elsewhere on this site.

You [probably] won't need to add balancing valves in the mains--with such "wide open" spaces, you're pretty well assured of circulation through the entire system. As long as you get sufficient circulation in the highest, farthest rad (and it's not too fast in the lowest, closest) you should be reasonably balanced. If you have circulation problems FIRST look for flow restrictor plates in the upper rads (they're generally on the hand valve side "inside" the union). If found in a rad that's not getting enough heat, remove it (save it for possible use downstairs if you find a rad--usually a short one connected bottom-bottom--that doesn't seem to be heating properly.)

Old-timers sometimes used the small amount of air at the top of rads as the compression "tank." If still working under gravity you should fine an expansion tank in the attic with an overflow to the roof. Disconnect the line to the tank and add a new tank by the boiler. If the new boiler has a "built-in" air separator, they're generally effective for a gravity system.

Rarely is the layout of a gravity system such that you can divide into logical zones. Kind of a strange t-stat setup, but if it works and the customer is pleased with the arrangement... It's possible that the 2nd thermostat was added as the pipes corroded (roughened) a bit and gravity circulation slowed.

If you're not piping primary/secondary you generally have the boiler and circulator come on in unison with a call from the t-stat. It's may not be the best arrangement, but it's what I see most and it seems to work OK. You [generally] don't have to be concerned about condensation issues with simple iron boilers on gravity conversions. BUT the more efficiency that is wrung from a conventional (non-condensing) design, the greater the potential for problems. Flue dampers in particular seem to be notoriously short-lived on such systems.

Steamhead

Thanks Noel

Lots of good suggestions in this thread. You're definitely going to need a circulator, but the job it must do is very small- moving the water thru the new boiler and that's about it. The pipes have so little resistance for the load they must carry that it's too easy to overpump such a system.

I prefer to use a smaller circ on this type of system rather than a throttled-down (using valves or smaller pipes) larger one. It uses less electricity and is easier on the pump.

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

greg_7

How much does the diameter of the supply and return of a boiler set up for gravity operation matter as compared to the size of the pipes to the radiators? I'm wondering whether the heat and pressure of the water being forced up and out will over come the size of the opening.

The low pressure of the large mains is for circualtion to the rads.

Boilerpro_2

I don't agree, Mike

I've looked at alot of standard cast iron natural gas chimney vent boilers in these old gravity systems and they are loaded with rust, some to the point that the passages are becoming blocked. Dunkirks, Weil Mclain EGH and CG.....doesn't matter whose. Had a power vent 82 % Dunkirk that was plugging up every 18 months and a 3/4 inch hole had already rusted through the bottom pan of the combustion chamber in only 4 years..The boiler looked like it had been there 40 years. When I come across the older units, I usually at least install a bypass and on newer or new installs pipe them P/S with an mixing valve on the boiler return or use Variable speed pumping. Recently went back to service some new installs with ESbe Thermix mixing valves (and standing pilot and stack damper)and after 2 and 3 winters of use, there was not a speck of rust under the boilers...just some ash and dust. Also, I find the chimney is usually beginning to fall apart when the boilers don't have return temp protection.

Boilerpro

Mike Kusiak

ESBE mixing valve

I have seen several postings regarding the use of these mixing valves. Where can I find info on them?

Boilerpro_2

Danfoss carries the ESBE valves

So I get mine through a Danfoss supplier. I belive there are other sources too. The valves are made in Sweden.

Boilerpro

robert griggs

Did the job yesterday,

and went back today to put in some hangers and clean up. Everything was working good. All the radiators were heating up. I used 1" coming out of the circulator into the 2 1/2" main. It took a long time for the boiler to shut off on high limit at first. After that everything was good.

As Boilerpro said, the chimney was not in very good shape. He is going to have a liner put in after the snow on the roof melts.

Thanks to everyone for their help.

R. Griggs