expansion tanks on hw systems

I've noticed some homes in my area have expansion tanks connected to the water heater, while others don't. These homes are not particularly expensive so I'm guessing there must be a logical reason for the tank. Can someone explain this to me?

If you have a well you must have an expansion tank.

As far as hot water heaters and expansion tanks they are needed if your water meter has an anti flow back device on it. So when the water heats then you get expansion; this happens every time you use hot water. The small tanks will help control the pressures that are made by heating the water.

It is not an "upgrade" item. Either you need one or you don't. It seems that newer water heaters, with higher efficiency numbers, seem more likely to cause expansion to the degree that the system cannot absorb it and an exp. tank is needed. Most WH manufacturers recommend it these days, and some municipalities are now requiring it.

got_nailed said:

If you have a well you must have an expansion tank.

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Ok. I had a well at my previous house. Although I didn't have an expansion tank on the water heater, I had one for my incoming line, of course. This must be what you are referring to? In any case, the exp tanks I saw connected directly to water heater had city water ...

got_nailed said:

As far as hot water heaters and expansion tanks they are needed if your water meter has an anti flow back device on it. So when the water heats then you get expansion; this happens every time you use hot water. The small tanks will help control the pressures that are made by heating the water.

Click to expand...

Ok. now it makes sense. There are several municipal water systems in the area. Some may require backflow prevention at the meter hookup, others may not. My current house is on city water, but it doesn't have an expansion tank. All the outdoor hose bibs in my neighborhood have backflow devices on the hose bibs (required by code). This suggests there's no backflow prevention at the meter. Does this sound correct?

Backflow (vacuum breakers) on hose bibs have nothing to do with having one on the water meter. Also, if you have a pressurereduction valve (prv), you need an expansion tank, since those contain a check valve, too.

jadnashua said:

Backflow (vacuum breakers) on hose bibs have nothing to do with having one on the water meter. Also, if you have a pressure reduction valve (prv), you need an expansion tank, since those contain a check valve, too.

Click to expand...

Thanks for your response, Jim. I knew pressure valves prevent backflow (none of the homes around here have prv's). But I'm not sure I understand your point about vacuum breakers on the hose bibs. I mean, what purpose do they serve if not to protect the water supply? And if they *don't* protect the water supply, then what is? Does the fact that I don't have an expansion tank mean there's no backflow device between my house and the main?

What about the pressure relief valve on the water heater. A pipe drops down the side of the tank near a drain tile. Wouldn't that effectively serve the same purpose? As I understand it, all water heaters have these.

David

The vacuum breaker prevents water from being sucked up into the hose and contaminating the water supply if there is a momentary hiccup or pressure drop in the supply and the hose is potentially sitting in a puddle.

The T&P valve on the water heater is a totally different thing - it prevents the tank from blowing up if the pressure or temperature gets too high.

If you don't have a check valve or prv (which contains one) in your water supply, then you don't need an expansion tank. When cold water is introduced into the tank and then heated, it expands. WIthout a check valve, it actually expands back into the supply out in the street. Some municipalities don't want that possibility to pollute the main supply, thus they install a check valve. WIth a check valve, you have a closed system, and heating water needs somewhere to expand into. Thus, the expansion tank.