"Got a furnace and expansion tank question"
Column #837 03/12/2011
On The Level
Q. I am 71 years old and I have a 95% efficient gas furnace. It is 17 years old now and I am starting to have problems with it. No trouble with it till now. So I guess it would be best to purchase a new one now, instead of waiting down the road when it would cost me more. I was going to get another Carrier, but some friends say I should get a Rheem. Is there a web site that I can go to see which gas furnace would be the best? I went into the company's web site and it looks like both are good. I notice if I get a 80% efficient furnace, the warranty is 20 years instead of 10 years on the heat exchanger of a 90% efficient and the life of a 80% efficient is longer then a 90% efficient model.
My water heater tank was right up next to the furnace and a wall kept the tank from being moved away from it. I knocked down the wall which was of a small closet so I could move the tank away from the furnace. Now I understand I must put in an expansion tank on the water heater tank. Talking to different plumbers, I get different answers on the expansion tank. One tells me that if I have a "check valve" in my system, which I don't, I need the expansion tank. Another tells me I need the expansion tank regardless. One tells me I need the expansion tank, a backflow preventer, a reducing pressure value and a pressure gauge, which I have on my cold water line now which shows my pressure at 55 psi. I am on city water. I am trying to find out if I just need the expansion tank and where I can install it in the cold water line. Does it need to by the water tank or a short distance from it?
A. I’ve never seen a side by side comparison of high efficiency gas furnaces. Both of the brands you mentioned have very good reputations. That’s probably because technologically they are so similar that the real difference is long term performance which you noted seems to decline the higher the efficiency rating gets with respects to the warranty length.
The early models experienced some difficulties because when ignited gas gets almost every bit of energy harvested from it via the heat exchanger, one of the byproducts is condensate water. In early models designers didn’t realize how corrosive that condensate could be and there were premature failures.
The designers went back to the drawing boards and through slight design changes and the use of some different materials problems have been greatly reduced. Remember that manufacturers are business people who don’t like to make bets they might lose. A warranty is a wager between you, the buyer, and the manufacturer that their product will last at least some specified period. With the furnaces you’ve considered the ranges went from 10 to 20 years. Most products will survive well past their warranty periods because manufacturers don’t want to be in the business of giving away expensive products because the ones they’ve sold prematurely died within the warranty period. They know what they are doing. Sure, it happens from time to time but those are the exceptions that prove the rule. So try to figure out where you’re going to be in 10 to 20 years and buy the furnace whose warranty fits your time table.
Real quick story about those expansion tanks you see sitting above all new water heaters installed on municipal water supplies. In the early 1990’s or late ‘80s they started installing pressure regulators on water supplies around here and for good reason. They knew they needed to keep the water pressure to residential users in the 40 to 75 psi range. But after a while many residential water users experienced valve failures in their homes on such things as toilet filler valves, ice makers, humidifiers and the like.
Somebody got the idea to put some gauges inside the house-- past the regulator-- and watch what was happening. Folks would do their normal morning rituals, taking showers and using water at sinks and tubs, then shut everything off and leave the house. As the depleted water from the water heater was replaced by 55ºF line water and was heated up there was slight expansion. The pressure regulators just happened to be one-way valves and where previously the expanded water could push back out the water main, it now could not get past the regulator. Measured pressures were astoundingly high and no wonder there was trouble. The little expansion tank was the cure.
Plumbers mount it where they do-- usually just above the water heater on the cold water side-- for ease of installation and they know the Plumbing Inspector will be looking for it so putting it where they can’t miss it is a bonus for them. The plumbing code as I read it (P2903.4) doesn't dictate where to put it as much as that under the condition of the check valve being present, it too must be there.
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