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We have an oil furnace that heats the house using a two-zone hot-water circuit, and also acts as an on-demand hot water heater. There is a pipe coming off the collection of pipes coming out of it that has a 2-quart drip bucket beneath it. This bucket fills every other day. In other homes I've seen with this arrangement, this bucket never fills, and any water in it evaporates after a day or so. What's causing it to fill so quickly, and is it something worth calling the furnace repair service for?

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A picture might help here. –  Ecnerwal Mar 7 at 17:15
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Is the dripping coming out past a pressure relief valve? Hot water systems usually have an expansion tank. Some have an expansion tank that needs to be periodically recharged (with a bicycle pump) older ones use just a plain tank that needs to be drained every year. If the expansion tanks aren't maintained the extra pressure in the system has no were to go and exits the pressure relief valve. –  OrganicLawnDIY Mar 7 at 18:04
    
The heating technician cut off the drain pipe, and put in a plug, so problem solved? The tech completely botched a hot water repair while he was at it, so now I'm nervous. –  RI Swamp Yankee May 12 at 13:04

2 Answers 2

Depends what it's dripping.

If it's a high-effiency condensing boiler (despite not usually boiling the water these days, a central heating device that heats water is referred to as a boiler, while furnace refers to those that heat air, in the usual vocabulary), it may be dripping condensate (mostly water) from the exhaust. This often has a somewhat purplish coloration (just as a possible diagnostic/disambiguation point) from the non-mostly-water fraction. Those are usually directed to a drain, or to a collection bucket with a condensate pump that pumps to a drain.

It could also be a straightforward leak, (usually) from an overpressure/overtemperature valve - look for that type of valve on the pipe that is dripping. If so either the valve is faulty and needs to be replaced, or the furnace controls are slightly out of whack causing it to actuate.

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The issue could be a faulty overflow valve. These values are typically rated at 30 psi (should be a metal tag with the rated pressure). Watch the pressure gauge when the furnace fires up. The furnace is usually about 12- 15 psi when it hasn't run for a while. If the furnace goes beyond the valve's rated pressure and starts dripping a lot, then the issue is most likely the expansion tank is waterlogged. Shut off the boiler, then the valve that leads from the furnace to the expansion tank. There should be a faucet attachment that you can attach a hose to. Open the faucet and drain the tank entirely ( might take some time if it is full as air has to get in the tank in order for the water to get out. You may have a release screw in the upper part of the tank to speed this along). Once completely drained, pop the lever on the over flow so that the pressure gauge drops below 10 psi. If you opened a release screw, close it. Open the valve to the expansion tank and you should hear the water rushing in. This is normal. You should see the pressure increase as the tank fills. The water should stop flowing into the tank once it reaches the set pressure of the boiler. Turn the boiler back on and watch the pressure as it is running. An increase of about 5 psi should be normal. If it still goes above 30 psi, expect the overflow to release water. The issue could be water temperature.

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