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[mlappin]

Working on getting a waste oil boiler built for the shop, shouldn't take much longer, waiting on the stainless tubing to show up for the HX in the boiler.

Plan is to tie the waste oil boiler to the wood boiler with a 50 plate HX and during the shoulder seasons use just the waste oil instead of letting the wood boiler idle most of the time, then when winter hits run both for a quick warmup in the shop and to reduce wood consumption while working in the shop. During the major heating season when I'm not running the waste oil it will act as a storage unit, should still hold a little better than 400 gallons of water.

I picked a Delta T pump up last winter and installed on the wood boiler for the house, I like the the ideal of increased efficiency from only pumping as much water as required for the load. Problem I see is this, no matter which boiler I place a Delta T for the HX between the two, if lets say the Delta T is on the wood boiler and I'm only heating with waste oil, the Delta T won't work correctly as the return to the wood stove will be warmer instead of cooler. Exactly the same would happen if I placed the Delta T on the waste oil boiler but was heating all the water with wood.

Am I totally overthinking this?

A few other plans of attack is to just get two three speed pumps and manually switch em. Also though about finding a controller that would monitor the inlet/outlet temps and using a relay switch the sensors leads back and forth, a simple dpdt relay should handle that.

[slimjim]

I believe you are overthinking it, Temp gauges on the plate exchanger, 3 speed circs and an aqua stat in the supply side of the heat exchannger wired to break on rise, use it to interupt B-1 on the waste oil unit. Simple, inexpensiv and effective.

[mlappin]

B-1? Using the HECO heater I bought years ago, a kit that can be installed in anything, simply taking it out of what used to be a forced air wood burner and placing it in a tank in a tank boiler. All manual, rheostat control for the pump with a little squirrel cage fan with a baffle to control air. Light it, set it and it's all manual, was going to place an aqua stat in that boiler so once it hits 190 or even 195 it kills power to the pump then a off delay will shut the fan off five minutes later after the burn pot and oil supply pipe has had a chance to cool.

Waste oil kit for those interested: http://www.wasteoilheater.net

I have another 600 gallons lined up for this winter, might as well get some heat from it at 140,000 BTU a gallon.