Garn preheating?

boiler heat said:

Next week we are having our 3 year old WHS-2000 drained, piping flushed, and cleaned for the first time.

I'm planning on running my 150,000 btu waste oil burner (OMNI OWB-15) to reheat the Garn.

I'm thinking I need to preheat the Garn to 80*F before I light a fire in it to prevent condensation in the flue piping.

Well water is 50* and will need to raise about 2,000 gallons 30*F. So am I looking at about 200 hours before losses and DHW usage? I don't remember how we preheated it last time, but 9 days does not sound right to me.

Click to expand...

You have an extra factor in there.

2000 gallons x 30 degrees x 8.33 lb/ gallon. Then divide out. Should be around 3.5 hours.

You would probably have some condensation in the flue when first firing, but so what? With all the hot air thats going to be coming from the fire box through those pipes, it will dry itself out in no time.

While raising the water temp to 100* or so will eliminate any flue gas condensation for sure, we've started more than a few up with water temps in the low 50's and even high 40* range.
Flue gas condensation is a function of a few things, temperature obviously, but often overlooked is the most important variable which is moisture content of the fuel.
A well designed pellet boiler for instance will not condensate even at flue temps in the 170-180* range.
Why?....because the fuel is only 6-8% moisture content.

That being a fact, we have started more than a few Garns with cold tanks using very dry wood and monitoring the flue gas temp + fuel quantity very closely.
Using extremely dry wood which is finely split, we'll get a fire going and add enough fuel to get the flue temp up in the 300*+ range quickly. (not very difficult) .
We keep adding fuel a little at a time to avoid overloading with the finely split wood (or it will puff) and maintain that 300-350* flue temp until the water temp hits the 100* range than just load normally sized fuel to finish off the tank to normal operating temps. We've observed little to no condensation from the flue using this method on at least a half dozen start ups.
The primary prerequisite is very dry wood in splits no larger than a couple inches square. This gets all the fuel "involved" very quickly and creates super hot conditions in the fire box and heat exchanger tubes. What you are doing is creating enough heat to keep the flue warm all the way to the outlet even though water temps are low and there is tremendous heat transfer going on.

I keep a few pails of wood charcoal on hand to bring a cold boiler back up to temp before adding regular fuel. Wood charcoal doesn't produce any water vapor when burning so there is no condensation in the boiler.