Are 'accumulator tanks' the same thing as 'thermal expansion tanks?'

Kevin Warnock · April 17, 2007, 03:42:48 PM

Hello,

I want to put a 2 to 4 gallon accumulator tank into my water system. I see Camping World sells 2 gallon models for $117, but that through hot water heater dealers, one can buy a 4 gallon 'thermal expansion tank' for $52. Are these products accomplish essentially the same thing? I will be using an 'on demand' propane water heater, if that makes any difference. Here are two links... one to the Camping World 'accumulator' product, and one to the 'thermal expansion' tank product.

http://www.campingworld.com/browse/skus/index.cfm?skunum=14747&src=SRQB

http://doitbest.com/DoItBest/Main.aspx?PageID=64&sku=406139&memberid=0129&associate=true

As you can see from the second link, a 2 gallon thermal expansion tank costs only $39, and the 4 gallon model is just $52. I am inclined to get the four gallon tank, as I presume that for many short uses of water, the pump won't need to come on at all... for example, flushing the toilet which takes 16 ounces per flush.

I don't drink the water from my water system, if that matters.

Thanks,

Kevin

Lee Bradley · April 17, 2007, 04:11:14 PM

They both preform the same task; allow the water some where to go when the pressure increases due to heat or a pump. The accumulator tank is probably 'food grade' so you can have portable water in it.

mc8 tin tent · April 17, 2007, 04:13:31 PM

Hello Kevin
And welcome to the board!
Both tanks are a bladder style tank,if you have the room I would go with the larger size.(the only difference is steel vs stainless)I would go with the regular steel tank.THE AIR BLADDER is usually the only part that will fail,except for an shell rust through(which is rare--unless the system is in operation for 8+ YRS.).
The larger tank will keep your water pump cycle time down to a min.
DO WHAT WORKS BEST FOR YOU!!!!
Dwayne MC8 tin tent

Jeremy · April 17, 2007, 04:25:04 PM

I'm no expert on this, but I suspect the two products you have identified work in the same way, but are used for different reasons in different applications. As I expect you know, the basic principle behind their operation is that these small tanks incorporate an 'air chamber', and a 'water chamber', seperated for each other by a diaphram. On the basis that air can be compressed but water cannot, the 'water' side compresses the 'air' side, which in turn pressurizes the water side.

In the case of the 'accumulator tank' this principle is used to allow a pressurised COLD water system, whereby (with the use of a non-return value in the pump), pressure is maintained even when the pump is not running - and as you say, it is beneficial here to choose a accumulator tank with as large a volume as reasonably practical.

The 'thermal expansion tank' is designed for use in the HOT water system, and is there to absorb the expansion of the water that takes place as it is heated.

Just to complicate things, you are probably using a hot water system which itself is pressurized from the cold water system. I'll leave somone else to figure out and explain how the tanks should be configured in this system, as it is 00.22am here at the moment and my brain is half asleep.

Jeremy

Kevin Warnock · April 17, 2007, 04:36:29 PM

According to this PDF, http://www.watts.com/pdf/ES-PLT.pdf, the thermal expansion tank is designed to go before the hot water heater, where the water would always be cold, right? I have to assume the thermal expansion tanks are potable water, as the PDF says these tanks are for potable water in the headline.

Comments?

Kevin

Melbo · April 17, 2007, 04:39:13 PM

The tanks will perform the same function the question I would have is what is the charge on the second tank. Boiler expansion tanks are sometimes only charged to 12 lbs. The camping world tank is charged at 20 psi. The only thing that will affect this how much pressure your pump puts out. Just some random thoughts about your application. Some tanks are not designed for wide differences in pressure. If there is a big change in pressure or the pressure precharge is not what you want you may rupture the bladder quicker.

Melbo

Jeremy · April 17, 2007, 04:51:22 PM

Quote from: 1967_MCI5a on April 17, 2007, 04:36:29 PM
According to this PDF, http://www.watts.com/pdf/ES-PLT.pdf, the thermal expansion tank is designed to go before the hot water heater, where the water would always be cold, right? I have to assume the thermal expansion tanks are potable water, as the PDF says these tanks are for potable water in the headline.

Comments?

Kevin

You are right in that the water would most likely be cold(ish) there, but it is the pressure rather than the temperature that is relevant I think. As the water further up the pipe is heated (ie., inside the water heater), that water expands, so whole system becomes pressurised - it doesn't actually matter where in the system the expansion tank is. There are, incidentally, other ways of acheiving the same objective of accomodating the water expansion - for instance you can get taps that are designed to trip as the pressure builds, or alternatively the chamber of (smaller) water heaters can themselves be made of a material that will expand slightly as required, so a seperate expansion tank isn't required.

Jeremy

Jeremy · April 17, 2007, 04:58:59 PM

Quote from: Melbo on April 17, 2007, 04:39:13 PM
The tanks will perform the same function the question I would have is what is the charge on the second tank. Boiler expansion tanks are sometimes only charged to 12 lbs. The camping world tank is charged at 20 psi. The only thing that will affect this how much pressure your pump puts out. Just some random thoughts about your application. Some tanks are not designed for wide differences in pressure. If there is a big change in pressure or the pressure precharge is not what you want you may rupture the bladder quicker.

Melbo

I know that some expansion tanks incorporate valves (like the valves on bicycle tyres) that allow the end user to adjust the pressure in the air chamber - and I guess to get the whole system in harmony you could probably also fine-tune the pressure switches that control when the pump switches on and off. It would probably be easier though to simply buy an off-the-shelf pre-assembled pump/tank/filter system

Jeremy

Len Silva · April 17, 2007, 05:09:09 PM

You can buy a 2-3 gallon bladder tank at Home Depot. I think they are around $40.

Len

Stan · April 17, 2007, 05:55:43 PM

Tou want to get a bladder tank and not a diaphram tank. The water is completely contained within the bladder so it doesn't matter what the shell is made of, unless you want something shiny. Every plumbing supply sells these tanks from very small to very large and they have a schrader valve in the top to set the pressure on the air above the bladder.