Expansion tank immediately downstream of main shut-off?

Today I was at a friends house to help with a plumbing repair that involved changing the shut-off valve for the dishwasher. Since there was no isolating valve upstream of this leaking valve, we had to shut the water off at the main. I then noticed that there was an expansion tank almost immediately downstream of the main shut-off. The water heater was integrated with the boiler and they were on the opposite side of the basement. Since I wasn’t there to do an inspection, I didn’t look for another expansion tank for the boiler. I just want to know if anyone has seen such a set-up before. It didn’t seem like a proper installation for the boiler, given that it was so far from it. What do you guys think?

Not sue what you mean by “water heater integrated with the boiler”

Was there a water heater or just a loop in the boiler supplying a storage tank for hot water?

The boiler needs an expansion tank.
Was it a closed or open system?
Open systems may have the expansion tank near the top end of the system.

More info if you have it may help us to help you.

Sorry for the poor description. Yes, it was a loop in the boiler supplying a storage tank for hot water. I was aware that it needed an expansion tank but I had never seen one so far from the system and right after the main valve. I thought it seemed a little bizarre since it was in the line that supplied the whole house and not just the boiler. I also didn’t notice any backflow device.


You appear to be describing an expansion tank for the water in the house and not for the boiler.

The boiler water is a seperate system and requires it’s own expansion tank.
And yes there needs to be a backflow preventer on the water feed/make up line to the boiler to prevent cross contamination with potable water.

That’s the way it appeared, but is that unusual? I know I am a pretty new inspector but I have worked in a ton of homes prior becoming a Home Inspector and I had never seen that arrangement, at least not in this area.

Generally I see the expansion tank for the water heater near the water heater but it could be anywhere in the system as it is only there to give the expanding heated water some place to go. :slight_smile:

In your case I will assume there is an expansion tank for the boiler somewhere else.

Thanks for your feedback. I appreciate it. Since I was busy do other things at the time that I noticed the tank, I didn’t spend too much time thinking about it until I was in the car on the way home and it started to bug me!


Are you absolutely sure it was an expansion tank and not some sort of water conditioning equipment? I have seen water softeners, reverse osmosis,a dn filters installed all over the place. It is very common for some water conditioning equipment installs to put a unit right after the main. I will condition the entire house and it is usually in a place to make it easier to service.


This week I saw my first expansion tank on a domestic water supply in a newly constructed townhouse in Wakefield, Massachusetts. The order of things on the new copper water main was: primary shut off, water meter, backflow preventer, small-ish expansion tank.

The plumber showed up while I was there and I asked him about it. He said that in new construction, a backflow preventer is now required (either a separate device or built in to the water meter) and therefore an expansion tank is required.

I have a call into the town plumbing inspector who had been there just ahead of me. BTW, it had nothing to do with a boiler since the heat was a gas fired hot air furnace.

Any plumbers out there with an opinion? If I learn anything else from the plumbing inspector I will post it here.

Donald Bissex
Mystic Home Inspection
Melrose, MA

wish there were pics.

Are you sure this tank was not used as a water hammer arrestor. That happens some times. Is this a well or public?

Ditto for around here. See them on 1 yr warranty inspections.