Now that I have had a chance to gain some experience with operating a wood furnace, there are a few things that need improving. Some things are not ideal but it's not worth changing right now.
If you're planning a new installation, I recommend that you understand how the various components are sized, both from a physical and a thermal point of view. I would also have a plan for how the piping gets hooked up and discuss this with the installer before the work starts. Your installer might have ideas you haven't considered and you might have ideas your installer hasn't considered.
|Main Heating||Central Boiler||PN 106
|Garage||Central Boiler||PN 287
|Hot Water Heater||Advanced Industrial
The wood furnace dealer installed a PN 106 (100k BTU, 19"x16½") heat exchanger in the duct work between the gas furnace and the AC coil, which I would say is undersized. The PN 107 (126k BTU, 20"x17¾") would have fit the 20"x18½" outlet duct of the gas furnace better and it would have been better for the furnace to heat the 3400 ft² house at a rate that is faster than ~0.8°C per hour. If you're planning a furnace upgrade and have some flexibility in duct sizing, I recommend that you size your furnace discharge ducting to fit the largest practical heat exchanger. Central Boiler's largest is PN 111, which is a 220k BTU, 26"x24". Especially now that electrical Time-of-Use Billing now in place, a faster warm-up time would work better with a programmable thermostat.
To keep the hydronic system simpler, no zone valve was installed to control water flow to the heat exchanger in the gas furnace ductwork. My system uses two thermostats: one for the gas furnace and air conditioning and on for the wood furnace. The wood furnace thermostat only causes the furnace fan to come on when it calls for heat. In cold weather, this isn't too much on an issue but the natural convection in the ductwork for a 2 story house causes the upper floor often to be too warm. The only way to circulate air through out the house to even-out the temperatures between the floors would be to put that heat exchanger's manual valves into bypass mode. A better solution would have been to use a 3-way zone valve to control hot water flow through the heat exchanger and this is on my to-do list.
Hot Water Heating
To further reduce our need for natural gas, the dealer installed a brazed plate heat exchanger for our hot water system. The piping arrangement was that our hot water would be heated either by the plate heat exchanger or by our gas-fired hot water heater. The problem we found with this set-up was that, with poor wood and/or need for furnace maintenance, the furnace would die overnight and the hydronic system would be too cold to adequately heat the hot water so we had several cold shower mornings - and a very unhappy wife. If you know what's good for you, your wife must never have to shower in ice-cold water. It would have been better to use the heat exchanger as a preheater for the gas-fired hot water heater which would ensure that gas use was always minimized while ensuring a constant supply of hot water. Another job for my to-do list.
The dealer installed the heat exchanger for con-current flow rather than counter-current flow, which produces lower temperature hot water than the counter-current arrangement. To ensure that the hot water was discharged to the house at a safe temperature, he also installed a tempering valve (Watts Series 70A Hot Water Extender). I have it set at the highest setting (160°F) rather than the lowest 120°F setting to prevent dilution with cold water.
The pool water heat exchanger is piped up with a bypass valve arrangement. Although this heat exchanger looks very compact, its rating (250,000 BTU/hr) is higher than the maximum output of the furnace (209,316 BTU/hr), which results in low pressure drops for both the heating water and the pool water. Even so, the furnace will still cycle on & off while heating the pool with the crappy wood I was using up over the summer. With good firewood (like oak or hickory), I would expect the cycling to be even more pronounced. For faster heating and to minimize creostote build-up in the primary air passages, it would be better to have a much higher capacity pool water heat exchanger and my recommendation would be to have closer to 50% greater rating (like a 300,000 BTU/hr) rather than the next size larger. This would absorb all of the heat generated by the boiler by preventing the boiler from reaching its 185°F upper set point.
I originally thought it would be good to have a zone valve controlling pool water temperature but I've found that, with the use the pool's solar cover, the pool remains fairly warm during the summer without supplemental heat. If the pool needs heating, its 103,500 L (27,300 gallon) volume requires that the furnace must run flat-out for easily 24 hours to raise its temperature 10°F (5.6°C). It is a good idea to use a solar blanket to prevent evaporative cooling, especially when it's windy.
Outdoor Furnace Installation
We built the pad for the furnace according to the recommendations of the installation manual. After having removed the chimney a few times for cleaning, it would have been better to extend the pad behind the furnace a couple of feet so a step ladder could rest on concrete instead of half-on concrete and half-on grass. The pad was also right under the drip edge of the carport. Without an eavestrough, the pad heaved because of the ground around the pad was saturated. Adding an eavestrough to channel the water away from the pad has minimized heaving and has kept my back dry in rainy weather.
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