Derrick (adw3345) wrote,


As oil prices shot up in the past few years, and my oil furnace was well past its expiration date, I had wondered about alternate ways to heat the house. As I live next to a stream with felled trees and available wood for the taking, wouldn't it be nice to heat the house with wood? One such furnace available was a wood-burning central heating system, with an oil burner backup. Finding any information about this system was rather difficult to find, and detailed reviews were non-existent. So, to remedy this, I bought one and now I will tell you all about the HS Tarm Excel 2200 Multi-Fuel central heating furnace.

The HS Tarm Excel 2200 is a giant behemoth of a furnace, mostly due to the size of its wood firebox and the necessary heat-exchange tubes. It is rated to provide a maximum of 140,000 BTUs on either fuel source. The blue box in the above picture is the furnace itself. It can burn either wood or oil, depending on the mode it's been set to. Under normal operation, the furnace will first burn wood put into its wood firebox, using a squirrel-cage fan to feed oxygen to boost the combustion. If the wood burns out and no more is added because you completely forgot or are snowed in, the oil burner will start up and maintain the temperature you've set your thermostat.

The Tarm is a real pain in the butt to start up when starting from a cold firebox. Not only do you need to recall what you've learned from your Camping Merit Badge, you have a 15 minute limit to get the flue exhaust temperature up to 250 degrees F, or the Tarm decides that it's not worth the effort and shuts down. After resetting the fan and attempting to re-light several times, I'm sorely tempted to douse the inside with gasoline or lighter fluid, both expressly forbidden in the user manual because of damage it might do to the firebrick lining. Eventually I found the best way to keep the Tarm from going cold is to split the day's load to a morning and evening feeding, and to dial down the air feed from maximum to about mid-way. This allowed enough hot coals to ignite the next batch, and still keep the house toasty warm. This seemed to be counter-intuitive at first, with the dialing down of the air feed actually making the house warmer. Eventually I rationalized it as being analogous to flooring the accelerator of a car; going all out doesn't mean you use the fuel efficiently.

So far, I've been using a little less than a fourth of a cord of wood a week (a cord being a stack of wood 4 feet wide and high, 8 feet long). If I were to buy wood, a cord is $160 in Stamford, meaning it would cost me a around $40 a week to heat the house. The Winter has been mild so far here in Southern Connecticut, so I expect I will be using more wood when the weather gets really cold. Sadly, as I had secretly hoped, it isn't really practical to run the Tarm on junk mail. Loading even a moderate amount of junk mail tends to clog up the airways (sending smoke out the intake vents), generates a large amount of ash, and makes no perceptible contribution to the boiler temperature gauge. However, it's still a great way to dispose of newspapers, bank statements, and other paper, provided you just throw a sheaf of paper on top of the wood stack, rather than into an empty firebox. This picture shows the firebox loaded with wood, and just now I see that I should have scrubbed off some of the sealant that melted when the stove was first turned on:

The wood furnace is surprisingly efficient. From a giant load of wood, very little ash remains, and you scrape it out from the refractor box (the bottom door). A week's worth of wood results in a box about two cubic feet of ash. One interesting observation is that when the Tarm is going at full blast, very little visible smoke can be seen from the chimney. There is slight wisps of perhaps steam, and you can see wavy air from the heat, but nothing like a typical fire in the fireplace. From the cut-away diagrams in the manual, there is no chimney on top of the firebox as one might expect, all the exhaust and smoke is pulled downwards from the firebox through a slot into the refractory chamber, which consists of a u-shaped trough, which ignites any flammable gas before feeding the exhaust through a series of heat-exchange tubes, before venting it out the flue. A heating coil rests on top of the firebox, and the entire assembly is surrounded by a 72 gallon water jacket. If the water jacket is cold, it takes enormous amounts of time to get it up to an operating temperature of 160 degrees F, for this reason the manual states to just let the oil burner take care of that part, and then use the wood to take it from there. You can see the oil burner in the back in this shot:

The silver tank next to the furnace is a giant 822 gallon water tank, designed to buffer the output of the furnace and the demands on the heating system. The tank is not pressurized like a normal hot water heater, but uses heat-exchange coils to transfer heat for domestic hot water and the radiator heating system. This picture was taken while it was being assembled:

The hot water feed for the showers and dishwasher and so on are fed through the copper coils, going in cold and coming out hot. The hottest I can make it is 118 degrees F when measured from the hot water tap, much to the disappointment of WifeBot Zhanna, who likes her showers inhumanely scalding. However, there is stupid amounts of hot water available; for the sake of science I tried to determine the longest hot shower I could take before noticing a drop in the water temperature, but had to give up after the hour and 20 minute mark, as I was getting wrinkly and bored.

There is about 3 inches of insulation in the tank on all sides. It holds the temperature of the water really well. From the thermometers stuck on it, it only loses about 0.2 degrees F overnight if no heat is drawn out of it. I suspect the heat loss is mostly through the copper pipes that connect to the tank. We do have a conventional hot water tank, left over from when we were in-between functional furnaces. Currently it's turned off and not being used, but we probably will use it in the Summer when it's too warm to use the heat, even though you can certainly just use the tank for hot water and set the thermostat all the way down to prevent the radiator circulators from being turned on.

The downside to the Tarm is the amount of space it requires. Our old furnace, shown below, didn't take up a whole lot of room:

Conveniently, the space surrounding it was cleared out because in the last few months of its life, the relief valve kept blowing, dumping huge amounts of water on the floor. It was not very efficient, either, and I found myself spending on average $600 a month in oil to feed it. I really wish I knew the efficiency of that furnace, but couldn't find any information about that model. A friend who sort of knew furnaces guessed it could have been 65% efficient, and lacking any better authority, I find it quite plausible. The Tarm has an 86% efficient backup oil burner, which is not in par with the best boilers which can be 92% efficient, but even if I just ran the furnace on oil, I'd probably have significant savings just on the efficiency alone.

Speaking of efficiency, was the Tarm a rational choice? Maybe it is, maybe it's not. The Tarm was an emotional decision for me; I bought it while seething with rage over spending a lot of money on oil while still having a chilly house, and seeing piles of wood out in the backyard otherwise going to waste. For those with cooler heads, I will break down the numbers for you to shake your heads in disbelief (or the vsnishing few who might nod in approval). The Tarm is not going to be for everyone, especially not for anyone who does not have access to cheap and plentiful wood. As I have this shot lying around, take a look at this symbolic picture as we segue into the cost analysis of wood vs. oil:

The Tarm Excel 2200 cost me $16,874, delivered to my garage, and installation extra. I was given a quote of $2,500 from an installer who later turned out to be completely flakey, and after we'd given up on him, the cheapest quote elsewhere was for $7,500 - after a bout of depression we decided to throw good money after bad and hope for skyrocketing oil prices to comfort us in the future. So all told, we'd already spent $24,734 even before loading in the first few logs. That is about the cost of a mid-priced automobile, or a year's college tuition for one of the smaller, not as prestigious schools. To rationalize it, the Tarm has a 25 year warranty, meaning that it's expected to have a 25 year lifetime before it explodes or something catastrophic happens. So if we divided it by 25 years, it's less than a thousand bucks a year, ignoring the effects of inflation. An automobile would have an expected lifetime of less than 10 years, and who really remembers their years in college anyway? However, it's still a lot more than a quote to get a 93% efficient conventional oil furnace installed, complete, for $6,500. (This is in Stamford, Connecticut, where prices are distorted by the cost of living). So in order for the Tarm to make any sense, we have to save $18,234 (the premium we paid beyond the cost of a top rated conventional oil furnace) in fuel costs over the years.

Heating is measured in BTUs, the British Thermal Unit, the amount of energy needed to raise the temperature of one pound of water one degree Fahrenheit. For example, to raise the temperature of the 72 gallons within the water jacket of the furnace from a cold 40 degrees F to an operating temperature of 160 degrees F is 72,057 BTU. A gallon of oil represents 134,500 btu, while a cord of wood's BTU depends on the species of wood, and if the wood is seasoned. You can buy 'bone dry' firewood, meaning it's been through an oven to remove every molecule of water, but if you're like me, you leave your wood in a pile in the backyard on 2x4's and a tarp over it. This represents 'air dry' wood, with a moisture content of 20% and a significantly lower btu rating. A figure of 20 million btu per cord of air dry wood would be a representative number; some species of wood have pitiful energy content, and other stellar, but from the mix of species in the backyard, I think 20 million btu is relatively accurate as a 'middle of the pack' number. Thus, to oversimplify, a cord of my wood represents 166 gallons of oil. Sort of - furnaces aren't 100% efficient, meaning that I'm not going to extract every single btu from either wood or oil, so the efficiencies have to be taken into account.

The Tarm, according to the manual, is "over 80% efficient", meaning that it could be anywhere between 80.1% to 100% efficient, so using the cynical value of 80%, the net energy value of my cord of wood is 16 million BTU. This is the equivalent of 129 gallons burnt by a 92% efficient oil furnace. As the Tarm is dizzyingly expensive compared to the price of the best oil furnace money can buy, it's only fair to compare the Tarm against the best of class for oil furnaces.

Right now, unless oil is free, too, burning wood is a complete win for me, as I've obtained my wood through cutting down trees and donations from the neighbors in exchange for cutting fallen trees for them. Oil is now $3.12 a gallon, so each cord of wood is $402 in equivalent energy costs in oil burnt by an 92% efficient oil furnace vs. my 80% efficient wood furnace. So to get my $18,234 break-even point, where the amount of fuel costs I saved by not buying oil, I'd need to burn 45 cords of 'free' wood (ignoring the cost of my time cutting, splitting, and transporting wood, and daily time messing around with loading the Tarm), which is not entirely impossible, my current rate of 1/4 a cord per week means I'd need 181 weeks to reach my break even point - or about 7.5 years if I use the furnace 6 months out of a year. This assumes that oil remains a steady $3.14 a gallon, something that the manual doesn't cover, but from my dim awareness of current events, an implausible assumption. If oil doubles in price, my break-even point is that much closer.

Can I obtain that much free wood? I think so, I've had no problem getting wood ready for the Winter, as long as I made collecting wood my hobby, which is just as well, as I have no other exercise plan. Let's say that I'm careless and a tree falls and breaks my legs, leaving me incapable of cutting wood - wood is still cheaper than $402 a cord, although the break-even point now stretches even further into the mists of distant time, way past the Hillary Clinton administration.

So there is my review about the HS Tarm Excel 2200, and I hope I did not make any embarrassing mistakes in my math that proves that the payback time will be much greater than the expected time of the heat death of the universe. However, as expensive as the Tarm may be, one priceless result is that I can now leave my fireplace unlit so as not to deprive Baby Nannette from her favorite hiding place:

And that's it!


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