inject into a gravity system

Adk1guy

I have a 3 story 4 unit apt house with the original gravity system, original boiler, and expansion/over flow tank in the attic. Because I can buy propane for 55% of oil per million btu, I'd like to put in a condensing propane boiler and inject hot water into the gravity boiler. What are the pitfalls and should i put hot water into the top and draw from the bottom? As an aside, I had a realtor tell me my boiler was on it's last legs because his boiler cracked after 15 years. Mine has to be 80 or 90 years old. They don't make them like they used to.

hot_rod

It would be best to parallel pipe the boilers so either or could run without flowing through the off line boiler.

If the LP boiler flows through the oil boiler a portion of you heat goes up that flue as the cast block warms. You might waste more than you gain from the LP?

Depending on the type of high efficiency boiler, you may need to do a primary secondary piping. That would involve 3 circulators. The building, the oil boiler and the LP boiler.

EdTheHeaterMan

You could inject the hot water into the gravity system, but that makes oil fired boiler hot. That heated boiler will absorb heat like a radiator in order to have the gravity design work. The vent connection to the chimney will act as a vacuum cleaner sucking the heat from that big radiator (THE BOILER) up the chimney unnecessarily.

@hot_rod has the best idea. Turn that old gravity system into a forced hot water boiler. Add circulator and flow check valves as needed. The large capacity cast iron pipes and radiators will actually be a good fit for the condensing exhaust LP boiler you are proposing.

mattmia2

Unless you really want the dual fuel option, you could also replace the oil burner with a propane power burner. It is very questionable if any of these options will pay back what it costs you to implement them.

EBEBRATT-Ed

I would double check the fuel prices. Propane at 55% of oil doesn't sound rigt but maybe in these crazy times it is.

Adk1guy

I would gleefully cap the flue after temporarily disabling the burner because I am aware of how the chimney is sucking the heat out of that gravity oil boiler right now as I type.

I don't want to convert the gravity system to forced hot water for several reasons:
1. Its already heating season. I can set up a wall hung gas boiler and tie in without shutting the heat off.
2. Speed. Wall hung, low loss header, magnetic filter, two circulators.
3.Avoid the wait for asbestos abatement.
4. The building might get torn down for repurposing the lot. Why sink a mint into the conversion? The asbestos will be abated if demolished but I won't have to reinsulate as I would have to if abated now.

I am fairly confident on fuel pricing. I owned fuel oil and propane biz 34 years , plus I currently pay bills for several commercial properties.

Writing this jogged my memory and now I remember doing this same thing with coal boilers in the 80's. We injected into the existing boilers to serve as a mixing vessel and they already had the distribution system set up. They all worked fine including a gravity job. I think we injected and returned at the bottom of the boilers.

I'm almost done with my 1st conversion at my residence. My masonry chimney has always been very warm to the touch all winter and I am looking forward to capping the flue.

EdTheHeaterMan

Sounds like you have already made the correct decision.

You could inject the hot water into the gravity system, but that makes oil fired boiler hot. That heated boiler will absorb heat like a radiator in order to have the gravity design work.

then cap off the chimney

Adk1guy

converting an oil fired boiler to gas fired is a heart breaker. Boilers and furnaces designed to burn gas have much smaller passages than oil fired. Especially with an 80 year old boiler. When they went from coal to oil they used the same boiler and just put a different dry base with a combustion chamber under it. They have big clean out doors that reveals big passages with lots of room for fly ash to accumulate which of course isn't a problem when burning oil. We used to stack bricks or pipe nipples with caps inn those spaces to force the gases closer to the surfaces to increase efficiency. Installing a gas burner on one of these big open passage boilers would only make the gas provider happy.

jumper

Adk1guy said:

converting an oil fired boiler to gas fired is a heart breaker. Boilers and furnaces designed to burn gas have much smaller passages than oil fired. Especially with an 80 year old boiler. When they went from coal to oil they used the same boiler and just put a different dry base with a combustion chamber under it. They have big clean out doors that reveals big passages with lots of room for fly ash to accumulate which of course isn't a problem when burning oil. We used to stack bricks or pipe nipples with caps inn those spaces to force the gases closer to the surfaces to increase efficiency. Installing a gas burner on one of these big open passage boilers would only make the gas provider happy.

I agree with the above so I replaced big boilers converted to gas with nice little gas boilers. Gas bills decreased but not dramatically. Gas was less expensive then so now savings are worthwhile.