Drain boiler for long term inactivity

I'm just wondering what the implications are for draining my boiler for an unknown length of time. It's about 15 years old, running perfectly fine, but we heat with wood now and I keep it on just to maintain it's own internal temperature so the gaskets don't shrink. Are there other considerations or complications with draining it completely for an extended period? Will parts rust, shrink, seize, etc?

Thanks

Unless there's a freeze potential, keep the water in it. Fresh water will contain minerals, oxygen, and potentially some organic material. Once it sits inside the boiler for a long time, the oxygen is used up, the minerals tend to deposit themselves, and the water becomes more inert. Replacing it with fresh, starts that all over again. I'd check the pH, and if it's neutral, just leave it alone.

Curious what the pros have to say on this, though.

I couldn't have said it better. Other than to remind our wood enamoured friend to fire up the old boiler once a year for a few hours as he would a car just to give all the parts a bit of exercise and confirm that the backup boiler works. If there is freeze potential (we are often called out for fin-tube baseboard freeze-ups when the new fireplace is left on too long or the "setback" thermostat turns off the boiler and circulator on the coldest night of the year). I costs about $25.00 a year to run the circulator full time and very hard to freeze flowing water.

Thanks to both of you for your replies back in February.

I decided to take your advice and leave the water in it and it does come in handy to run it when the wood stove is inconvenient. My concern now is this: I was once told by a plumber not to turn the power to the furnace off completely because it has to run even in the summer to maintain a minimum temperature or else the gaskets will shrink and water will leak out and rust the furnace. He pointed out the rust already present because I had shut the water off one summer. But even with all thermostats set to 'heat off' (as of last March) and the values to the water heater configured to bypass the furnace, the furnace still comes on surprisingly frequently even in June. We've used a 1/4 tank of oil in the last 3 months just to maintain that temperature. By next September, I figure we'll have used half a tank. That's got to be about $300 or $400, perhaps more. Maybe a little rust isn't so bad. Do you think it's a serious concern to power down from March to October?

Thanks,
Robin

Condensation or very high relative humidity air is what causes the rust on the fire side of the heat exchangers. Assuming you keep the boiler in a basement, this becomes an issue is when the summertime outdoor dew points are high- near to or even higher than the temperature in the basement or if the basement isn't well protected from ground-water moisture wicking through the soil/concrete, keeping the relative humidity in the basement high. If you run the circulators full time the boiler's heat exchangers will never drop below the temperature of the room air upstairs, which would always be above the dew point of the outdoor air. Alternatively (or in addition), running a dehumifier in the basement keeping it at or below 55% relative humidity would keep the oxidizing conditions at the heat exchangers well controlled. The expense of either/both will be in the 10s of dollars rather than hundreds over the course of a summer.

Through-bolts sufficiently stretched to cause leakage & rust at the seams between plates is a separate issue. They can sometimes be re-torqued, but most boilers younger than 30 years would have been designed & built to not have that problem. Many newer oil boilers are designed for cold-starting, and PRESUME that they will be allowed to cool fully for multiple, possibly extended periods. What does the manufacturer say? (Email them, if it's not in the manual.)

When the boiler is eventually completely shot, (or maybe even before), it's worth considering not replacing it, and heating with heat pumps instead. No matter what happens with pipelining the Alberta oil sands or the shale-oil frack to market, the notion that oil production will get ahead of (or even keep pace with) world demand for oil as transportation fuel over the lifespan of a new oil boiler is a dubious proposition at best. Heating oil prices will likely see dips, but will also see spikes, with increasing price volatility over time. Now that there are multiple vendors of cheap high-efficiency ductless air source heat pumps capable of operating at the extremes of Nova Scotia's wintertime temps, it's well worth looking into those as the backup/auxilliary heating for your wood burner. See this recently published policy piece from RMI.

The Fujitsu AOU-xxRLS2-H series is fully rated down to -26C, and keeps on chugging even at lower temps than that. The Mitsubishi MSZ-FExxNA "Hyper Heating" series has an output rating at -25C, but automatically shuts down to self-protect before it hits -30C (but automatically re-starts when temps rise.) Either would cost a bit less than half what it costs to operate than an oil boiler over the course of a season at standard electricity rates. If you have an off-peak discount option you can do even better, if you turn the setpoints down during peak hours. (Peak heating loads typically occur during off-peak power periods.) The all time record low temp for say, Glasgow NS is about -26C, it's a rare, but possible condition. Most winters bottom out at around -20C or warmer, a temp at which several other vendors & series will work, for a modest savings in up front cost. Most 1.5-2 ton units would cost about $4.5-5KUSD (installed) in my neighborhood, but it's not clear if that would be the case in NS, where the air-conditioning market smaller, and contractors handling heat pumps probably fewer. (This is in the range of a DIY project for the technically-minded/gifted though.)