Adding a Boiler to an Existing Boiler

But you only want one to run at a time

I have been asked about adding a boiler to an existing boiler. The first question I usually get asked is, "Can I pipe it like a multiple boiler installation?" Yes, you could, but that calls for a lot of extra piping. If you utilize the existing piping and add the second boiler piped to a set of closely spaced tees for hydraulic separation, it would work fine. This is a situation where you are not staging the boiler to meet the building's heat loss, but instead, one boiler is designed to run as a stand-alone unit. I have used this in situations where there was an old oil boiler, and the customer wants a new gas boiler. He also wants to keep the oil boiler as a backup boiler or to use during price spikes.
I would not do this with a wood boiler, as the second boiler should be electrically disconnected from the first boiler. Therefore, boiler two does not take over when the wood fire goes out. The idea is to wire it so that it's not possible to run both boilers simultaneously. The manual switch can only send voltage to one boiler at a time.

We are going to combine primary/secondary piping with standard cast iron manifold piping. This is very easy, especially if the cast iron boiler has a boiler bypass installed already for boiler protection

. This would require some staging control, and both boilers would be required to heat the building on the coldest days.

If the existing boiler does not have a boiler bypass loop already installed, it must be installed and the new gas boiler piped in according to the following diagram, utilizing closely spaced tees.

Many old systems will have the circulators mounted on the return side of the boiler. This would be a good time to move them. The circulators work best on the supply side of the boiler, pumping away from the expansion tank. This is shown in the last drawing. The tee connects the gas boiler (left boiler) to the existing piping, which needs to be as close as possible.

When operating the gas boiler valve off the oil boiler, the flow goes as shown below. This would also work with zone valves instead of zone pumps.

If you want the oil boiler to run, adjust the valve so that the flow is as follows.

You will probably be cutting into the piping to add bypass pipe and valves, so take the time and expense to move the circulators now so it looks like the diagram above.

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