You guys seem to have all the answers . I have a outdoor wood fired hot water heat unit that provides heat for my home. It has a small leak into the fire box. What kind of epoxy or other stuff can be used to seal this?
Weld it if you can.
Assuming this is not an open system and therefore develops pressure Jerry has the right answer.
Any leak in a heated water pressure vessel needs to be investigated thoroughly. I don't know if your unit has fire tubes, but you could have pitted tubes indicating time for new ones to be installed. Typically these are rolled in.
This is not a pressure system. It is just a fire box with a water jacket around it and a pump that circulates the hot water into a radiator in the house and returns to the unit.
I doubt any epoxy will stand the heat of the fire in the fire box. I agree, it should be welded if it is not a pressure system. However, you may find the whole thing is thin and the leak is just where it was the thinnest. If so, a weld is going to be temporary and may indeed show you just how thin it is.
If it is just a pin hole there was available some time ago a self tapping plug used on the old style external hot water tanks. The plugs generally came in two sizes 1/4 & 5/16 and were fitted with a rubber seal that tightened up against tank and would seal the leak. An aged tank may have several plugs in it. They were a common spare in most house holds along side fuses.
Where the stays were welded in the water jacket of my father's boiler there were several places that leaked. We reached in, wire brushed things clean and looked for suspect places, if you can wait a day or so rust will form fast where there is a leak. After draining the system we just MIG welded the places that had problems then re-pressurized the system (~20psi) and waited to see if there was a decrease in pressure. It took a couple of tries to track down the really small pin holes but the boiler works better than ever, less creosote buildup and holds a fire much better. don't be surprised if it takes a while to track them all down or if you need to grind and re-weld, I thought I had some done but then there would be a new pin hole at the end of my weld it was a pain to weld in that fire box, never mind that it is vertical and overhead all of the time...
The bottom of my furnace uses cast iron grates, when they broke up after 30 years of use I tried using steel grate, then a grate of 3/4" stainless bars -both melted and sagged in short order. I had to contact the company and buy original cast iron pieces in the end. So, no I don't think epoxy has a chance of working even if this is a water jacket area the fire side sees some intense heat.
Jim, What brand heater do you have. I have a Hardy wood furnace. Most wood furnaces built in the last 20 or more years are a non pressure system. The Hardy furnaces are all stainless steel and hold up very well. The bulk of the rest of the furnace brands are made of steel and depend on water treatment to stop rust out. If your furnace is steel it may be so pitted inside the water box that any fix you do will only be very temporary. If the hole is on the firebox side, you will have to weld it. Nothing else will hold up. If it is on the outside of the water box then a screw with a seal or some RTV sealant may fix it for a while. If for some reason you have a pressure system, then it is probably "toast" and time for a new furnace. As a side note on steel vs stainless. There was a company named Taylor Furnace that built a very well designed wood furnace. But it was steel and they depended on water treatment to stop rust. I asked the company if they would build me one out of stainless. I was willing to give double the price of their steel furnace. They refused and said "stainless is not needed, and is a waist of their time" So I bought a Hardy furnace built of stainless. My furnace is now 24 years old and is the only heat source for a 4000 square foot house. I have only replaced one cast iron grate, @ 59.00, one fan motor @ 165.00 and one water pump @ 239.00 in 24 years of very hard use. The Taylor brand of furnace outsold the Hardy's in our area 2 to one. There is not one single Taylor furnace still going and every Hardy furnace I knew of is still going. All the Taylor furnaces died of rust out to the water box. Being a Boilermaker for over 30 years, I know the problems involved in water treatment to stop the steel rusting. Big companies will hire water treatment technecians doing daily test of the water and still have problems with rust out. There is no way a amateur home water treatment will ever work. People will not monitor it enough. Good luck with the problem ... Donnie Brown
Just a follow up. We welded a patch and then used 2 epoxys to seal a small damp spot in our weld. We then installed fire brick over the spot. Works good now.