I own a condo with a terribly cramped water heater closet (it sits underneath the air handler with about 6 inches of clearance). I need an expansion tank, but it really will not fit in that closet (not to mention I really don't want to have to touch that again after having just changed the water heater). Is it reasonable to tee the expansion tank into the washing machine's cold water valve? My laundry room has a lot more space than where the water heater is, so there's plenty of room to support it, and easy access if I need to change it out.
So long as you do not have any check valves between the hot and cold supply, an expansion tank can be located anywhere in the system.
Thus, if you have a check valve or PRV (which acts effectively the same as a check valve from the system point of view) on the main inlet to the house, you can put the expansion tank wherever it is convenient if there are no other check valves in the system. All the water (hot and cold) experiences the same pressure throughout the system, and the expansion tank will work wherever you put it on the system.
This appears to be a widely misunderstood concept, as can be seen in the comments.
Note that if you put it on the far side of a valve, (i.e. not a check valve) it will not work when the valve is closed (but that will also make servicing it easier, if needed.)
Reasons for expansion tanks on a domestic water systems:
- Required by code/bylaw. Some municipalities may require one be installed so that the municipality's water system does not receive back pressure caused by the expansion of water when heated.
- Use of backflow prevention devises. When using a backflow devise, as may be required by some codes/bylaws, the pressure caused by the expansion of water when heated may cause the temperature and pressure relieve valve on the tank to open. This is not always the case as it depends on multiple variables.
Location of expansion tanks on domestic systems:
When required, the expansion tank is located on the cold feed to the water heater after the shutoff valve. However in your case, installing it somewhere else on the cold side should be fine so long as there are not isolation valves in-between.