But you only want one to run at a time
First off I am only addressing a residential application. This is not the way to pipe multiple boilers when installed at the same time that will work in sync. This application is one boiler or the other and only one at a time. I have been asked about adding a mod/con gas boiler to an existing boiler. Usually the existing cast iron boiler is newer and probably oil. The first question is usually can I pipe it like multiple boiler installation? Yes you could but that calls for a lot of extra re-piping. If you utilize the existing piping and add the second boiler piped off a set of close spaced tees for hydraulic separation it would work fine. This is a situation where you are not staging the boiler to meet the heat loss each boiler is sized for the heat loss. I would not pipe a wood boiler in this way although it would work. This is used more often when a gas and oil boiler are piped together. The owner may have a newer oil boiler and wants to change away from the oil boiler but wants to keep the existing oil boiler in place and choose between the oil or gas whichever is cheaper. This is very easy especially if the cast iron oil boiler has a boiler bypass installed already for boiler protection.
The idea is to have only one boiler running and both will never run at the same time. This should be done electrically so it is impossible to run both at the same time. I have done this before and use a single pole double throw center off switch. When pushed one way the oil boiler runs, the other way the gas boiler runs and in the center position neither boiler runs. If your plan is to stage these boilers on do not use this piping diagram. See multiple boiler installations.
If the existing boiler does not have a boiler bypass loop already installed, it must be installed and pipe the new gas boiler in per the following diagram.
Many old system will have the circulators mounted on the return side of the boiler. This would be a good time to have them moved to the supply side after the expansion tank connection as shown in the last drawing. The tee's the gas boiler (left boiler) is piped into the existing piping with need to be as close as possible.
When operating the gas boiler valve off the oil boiler so the flow goes as shown below. This would also work with zone valves.
If you want the oil boiler to run valve so the flow is as follows.
As I said the circulators for the system should be on the supply side of the boiler pumping away from the expansion tank connection. Take the time and expense to move them now so it looks like this.
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