Repairing the tankless propane boiler for my solar augmented heating system

4 solar collectors shown at right

When I moved to Maine in 2001 I converted the open barn on my property to a well insulated workshop.  I also designed and built a heating system that uses solar and propane to heat the radiant concrete slab on the ground floor and baseboard radiators on the 2nd floor office/lab spaces.

The system relies heavily on the Bosch Aquastar BS124 tankless propane boiler that has performed flawlessly since it was installed with only token maintenance.  (You can see a system diagram and live performance charts on my web site.)  Unfortunately it recently failed dramatically due to a prolonged period of extreme cold. 

In the images above you can see the storage tank that accumulates solar heated water during the sunny hours of the day.  Above left is the boiler that automatically brings the water temperature up to 140F when it is lower than that.  In the dead of winter I am lucky to get 140F in the tank so the propane boiler runs a lot.  To reduce the propane consumption, I use my wood stove.  The drawback to the wood stove is that when lit it pulls air into the building and that comes through the powered exhaust vent above the boiler.  Originally, the boiler vent had a flapper valve on the outside to prevent a back draft, but I had foolishly removed it.  So what happened this winter is we got a prolonged period of very low temperatures with night time temps dropping below zero a lot (see the red line below).

(lost data on Dec 28 for some reason)

So on December 30th when the wood stove was running and the heater was off - the cold back draft got down to -15F and eventually a pipe burst in the top of the heat exchanger:

Heater running AFTER repair - arrow shows location of the burst pipe

The arrow points to the section of pipe that blew open.  I immediately researched replacement parts and found a source for a complete replacement copper heat exchanger and in a panic - ordered one online for $350.  But then after calming down, I realized that the damage really wasn't that bad, and that I probably had the plumbing skills to repair it since I had plumbed the whole system myself!

Here is what the burst pipe looked like when I looked closely at it.  It was fairly simple for me to gently hammer the lips of the burst out section back together after draining the system down below the leak.  Then I carefully sanded the whole area around the cut to expose clean copper.  I wiped on a lot of paste flux and warmed it all up with a blow torch.  By gently applying solder, I was able to fill the crack and build a layer of solder over the whole area.  Problem solved and I canceled my order for a new heat exchanger!

There is no running water in the building so in order to re-fill and pressurise the system I had to drag a garden hose across the yard from the house.  This was challenging with 8" of snow on the ground and day time temperature peaking at 10F!  I had to pre-warm the 2 - 50ft lengths of hose in front of the wood stove to loosen them up first!  But it all worked out and it has been holding 50psi pressure for over a week now.  And of-course I made a new back draft flapper for the vent to prevent frigid air from being drawn in again.

Problem solved!  And I saved $350 in replacement parts!  Incidentally the whole building only uses about $200 gallons of propane a year at a cost of about $400 in a good year.  This is very inexpensive and due to the great insulation, interior storm windows and about 1 cord of firewood that I cut for free on my property.