Replacing a very old gas boiler

Old 09-20-07, 07:19 AM
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Replacing a very old gas boiler

I purchased my first home a year ago - it was built in 1893. For the first winter I endured the pain (on my wallet) of having a ~50 year old gas boiler. I knew it had to be replaced for the coming winter, and I've pretty much locked myself into the project by removing the old one. Here are some pictures showing the old setup and removal:

The system has 4 pumps, but used to have 5, hence the manifold with 5 spots (the 3rd floor heating was removed at some point). Also attached to the lower manifold is the fill line preceeded by the green pressure regulator valve. Two of the pumps are just a few years old. The pumps pulled hot water back into the lower manifold, where it entered the boiler and exited reheated at the top, where you can see the big green drum expansion tank gets piped in. Going further up, there's another manifold just below the cieling piping the hot water to the various loops. The first floor is all on the same zone, and the 3 rooms on the 2nd floor each have their own zone. Each zone is controlled by a thermostat which simply turns on/off it's corresponding pump.

Not knowing anything about boiler heating, my initial thought was: yank out the old one, put in the new one, and connect the water supply, gas, and the input/output just like it was before.

After doing some reading, it seems that all the new boilers need a pump dedicated just to circulating water within the boiler? So I guess that's a component I have to add. Is this pump necessary in all systems, and does it run continuously?

What about kicking out the gas water heater and using my boiler to make hot water? Would be nice to have everything running on one efficient system (not to mention that the water heater has venting problems).

I feel very confident in doing the installation myself, I just need some help figuring out the layout. What parts of the system can I keep? Can I stick with the manifolds I have? Ideally, I'd like to replace the boiler, expansion tank, pressure regulator valve, venting, and 2 zone pumps. At some point in the future I'll add a 5th pump to heat the 3rd floor.

Some photos of example setups would be imensely helpful, especially where a new boiler was retro-fitted into a very old system.

I've been perusing boilers and it seems that DESCO energy is very DIY friendly and the Trinity boilers they sell seem like a good deal.

Thanks for any help you can give me!

Old 09-20-07, 01:02 PM
Join Date: Mar 2006
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Although my experience with hot-water heating systems (design, installation and operation) is mostly on a commercial and industrial scale I can offer a few suggestions.

Make a new manifold for your pumps and install isolation valves for all the pumps. Ideally you will want to use the same size pumps for all the zones (if possible) to allow easy replacement when necessary.

You want to be "pumping away" from the expansion tank. This means that the connection for the expansion tank must be on the suction side of the circulation pump(s). Since the expansion tank is often connected directly to the boiler this means that the pumps are connected to the boiler outlet and pump towards the heat emitters.

You will definitely want a new expansion tank. You want a diaphragm-type tank and be sure to not undersize it. You cannot oversize the expansion tank and the only problem with a larger-than-necessary expansion tank is the room to install it and the additional cost to purchase it. Unless your local plumbing or mechanical code prohibits it I strongly suggest that a ball valve be installed prior to the expansion tank to allow it to be changed without draining the entire system. The ball valve should have the handle locked (baling wire or nylon tie works) in the open position except when actually needed to change the tank. If you have space restraints you can use two or more smaller expansion tanks piped in parallel.

When you install the make-up water pressure regulating valve be sure to use isolation valves on each side of it and at least one union to allow for it to be replaced without draining the entire system.

Adding an indirect heater for domestic hot water is an excellent idea. You will probably need an independent circulator pump for it and possibly a priority control connected in conjunction with your space heating zones.

A circulation pump for the boiler alone is not always needed but because modern boilers contain such a small amount of water it is often a good idea. This pump would not necessarily run continuously but would be controlled by the burner firing system. With their small amount of water most boilers heat up quite quickly and also would be firing anytime there is a zone calling for heat.

I would look seriously at condensing, modulating boilers for replacement. Look also for outside temperature reset for the system. These can save you some serious dollars in operating cost.

I strongly suggest that you get several bids for the boiler replacement. It has been over thirty years since I did any residential work and as I understand it is almost impossible for a non-professional to purchase a boiler and still get any warranty coverage from the manufacturer.

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