I think this will end up being the last of the keropunk series for now - though I reserve the right to write more of them in the future! This week’s post is going into a bit more detail about the kerosene heaters, doing my usual thermal imaging, and looking at some of the economics of various heat sources.

Regular Maintenance: Dry Burning

If you read up on heater maintenance, you’ll see regular references to something called “dry burning” - which is just letting the heater “burn dry” - run it out of fuel while burning. This helps remove tars and other crud from the wick, and more or less resets the wick to the flexible, even, nicely flowing thing it should be from the stiff, carboned up thing it turns into after a while of use.

Get in the habit of feeling the wick every now and then (while the heater is cool!), and you’ll get a sense for when it starts hardening up. You want the wick soft, flexible, and generally “fluffy.” If it’s starting to be “not those things,” which will happen after a while, it’s worth a dry burn to reset things. And if your heater is “acting weird” with regards to how it starts or how it burns, go ahead and dry burn it before bothering with any more troubleshooting.

The process is really simple: Drain as much fuel out as you can from the tank (or just wait until the tank is nearly empty). Then set the heater outside in a calm area (or a shop, or something out of the wind), light it, and let it burn out. Depending on your font size (the thing feeding the wick, not the size of your letters!), it may take an hour, or it may take longer, but the end of the process will stink as the kerosene isn’t burning fully and various tars and deposits are burning off - so you really, really don’t want to do this inside - and if you must, leave some windows pretty well open.

After you’re done, inspect the wick. You should see a fairly white wick, free from a lot of deposits, and it should feel generally fluffy along the top! If that isn’t the case… add some fuel and try again.

For comparison, when I started, the top of the wick was quite black and just “hard and crunchy” to the touch.

If the wick remains stiff and crunchy feeling, or if it’s uneven and ragged after dry burning, it’s time to replace the wick.

But what is this process doing, and how does it avoid destroying the wick? For a kerosene lantern, “letting it burn dry” is a good way to char the wick and require, at the minimum, a good trim. Why doesn’t it damage heater wicks? Easy - the top part of kerosene heater wicks are fiberglass, and that tolerates the heat! The lower bits are cotton (which can clog easily with water - keep your kerosene clean), interfacing with the fiberglass about halfway up. So as long as the heat remains concentrated in the fiberglass, it doesn’t damage anything!

On a brand new wick, the top is ferrety-soft and smooth. The lower part feels exactly like the cotton strands they are.

Inside, you can see how the fiberglass bundles together and is interleaved with the cotton. If you remember the teardown, you’ll recall that the interface between cotton and fiberglass is far enough down the wick that it’s being cooled by metal from both sides - so despite the high temperature of the burner, the wick remains cool enough to keep the cotton intact.

Normally, once the heater is burning, the wick is simply serving to bring kerosene up and deliver it into the airstream. The catalytic cylinder is doing the actual burning, and so the wick, despite being in a very hot environment, doesn’t actually have a flame against it. As the kerosene evaporates, impurities get left behind, and the heat helps turn them into a range of hard, tarry substances.

When the wick is dry burned, eventually the combustion process drops back down so the flame is actually burning at the end of the wick, and this allows the impurities to burn off, resetting the wick for yet more heating duty!

You should probably dry burn your heater at the end of the season, and then just keep an eye on it during the heating season to see if it needs the maintenance process. I don’t have a great feel for exactly how often my heaters need it yet.

Fuel Storage

While I’ve gone into some great detail before about how kerosene has a high flash point and generally isn’t particularly hazardous to keep around, neither is it the best idea to keep 50 gallons of the stuff in your house. It’s probably fine, but if you’ve got better options, you probably should store it somewhere outside.

However, storing it somewhere that will be 130F during the summer also isn’t a good idea. You want to keep it as cool as reasonably possible, while also having it somewhere safe enough. A shaded shed is a great option here, and I tend to keep assorted fuels in my garden shed. It’s in the shade (mostly), but also has low air intakes pulling from the shaded carport area, a ridgeline vent to exhaust heat, and radiant barrier roofing. So even in the summer, it stays fairly cool - and is far cooler than the shipping container, which is broadside to the sun. I should do something about that at some point…

As far as containers, metal is better than plastic. I’ll put some kerosene in plastic containers for short term use, but for longer term storage I prefer the 5 gallon metal drums one gets when buying kerosene from your typical hardware store. Yes, it’s more expensive than from a pump, but you get a nice storage container in the deal you can refill from your local kerosene pump. You can get blue metal jerry cans that will work well for kerosene storage as well.

If you don’t know where your local kerosene pump is, you should try to find out (asking your local “works on a lot of cars in their garage in the winter” sort is probably a good way to find out). It’s going to be far cheaper than buying from your hardware store - though you might want to burn a bit of it first to see how it smells. Sometimes the pump kerosene is higher sulfur than the hardware store stuff. Tradeoffs, and all that. I burn heaters with a window open and a bit of airflow, so it doesn’t generally bother me - or outside, where it simply doesn’t matter.

Thermal Images

Kerosene heaters qualify as a heat generating device, so of course you can expect thermal images for it!

This shouldn’t surprise anyone, but a kerosene heater, in operation, gets hot. Seriously hot. Depending on where I’m measuring from, the hottest point shows in the mid to upper 600F range - and given the shiny metal throughout, I’m far from certain my thermal imager is measuring particularly accurately with regards to the hottest points. Recall that the hottest point on most of my big kerosene lanterns was around 400F, with the tiny Comet’s lid showing over 500F. The heater runs quite a bit hotter - which explains the commonly made observation that “a kerosene heater will burn fuel that stinks in a lantern without any real odor.” It’s just burning a lot hotter.

In thermal, you can see the heater purring silently away, with a warm spot in the carpet in front of it. I don’t really like the thermal scale here - the carpet is not several hundred degrees, but but it’s noticeably warmer than the surroundings in front of the heater, and the shape of the warm spot is about right.

From the side, it’s clear that the top of the heater is getting warm - and I wouldn’t suggest touching it in operation. But neither is the side of the cabinet running hot enough to be hazardous. One of the design features of radiant heaters is that they can be operated “fairly close to walls” without being a fire risk, and the cabinet temperatures back this up. After several hours of operation, the front of the heater is quite hot, but the sides just aren’t that warm.

The back of the heater is an interesting one to image, because it has vents - and through these vents is visible some of the very hot metal spreading heat around. Yet the back actually isn’t that hot - there’s some warm airflow through the vents, but the actual cabinet of the heater cabinet is far lower than 500 degrees. You can just see through it to the really hot bits.

From straight on… all I can say is that the parabolic reflector works. Because I can’t distinguish between the central catalytic heater column and the back reflector here. This says very good things about the efficiency of the reflector. Were I able to see a big difference between the front of the catalytic cylinder and that which was reflected off, it would mean the reflector wasn’t being particularly efficient.

The thermal camera agrees with the experience of putting a hand in front of the heater. Simply, “Wow, that’s hot!” But, yet, the cabinet doesn’t get particularly warm!

Economics of Heating

In the last post, I touched a bit on the cost of heating with kerosene, but I since realized that not many people have a good sense for what BTUs or kWh of heat actually look like. I have a pretty good feel for it, since I heat on those in my office, but if you’re only used to paying a power or natural gas bill, it won’t mean much.

Heating in the modern world is typically either done with electricity, wood, or natural gas. In some places (Japan comes to mind), kerosene is still used. And propane is still used often enough in various places. So, in late 2022, how do the prices of all these solutions compare?

Depending on the energy source, you’ll find prices either per BTU (British Thermal Unit, heating 1 pound of water 1 degree F), or per kWh (1000W of electricity, sustained for an hour). The two are directly convertible - 1 kWh is 3412 BTU.

Natural gas, over in Boise, is currently $0.734 per therm. A therm is 100,000 BTU - and I’ll be nice and assume an 85% efficient furnace. This is typical for a “random furnace” in a house. Math it out, and you get $8.63 per million BTU. It’s properly cheap, at least out here, right now. Of course, I don’t have access to it.

How about electricity run through a heat pump? I pay… oh, let’s call it $0.0886 per kWh in the winter. Actually, I don’t, it’s “free” (“prepaid”?), because solar, but if you pay retail prices, there you go. And I’ll give a heat pump an average 250% efficiency - it’s higher in the warmer weather, worse when it’s cold and the ice fog has clogged the coils. Converting from kWh to BTU, and factoring in heat pump efficiency, you get $10.38 per million BTU.

What if you’re not using a heat pump? If you’re using a pure resistive heater (small fan driven heater, computer, cryptocurrency miner, radiant heat panel, or anything else of that sort), you have a nice even 100% efficiency - every kWh into the heater ends up in the house. Do that, at my power rates, you have $25.97 per million BTU.

How about propane? Around here, it’s about $4/gal right now, for 91,500 BTU. That works out to about $43.71 per million BTU.

I can get kerosene locally, from a pump, for $5.68/gal. Kerosene has 131,890 BTU per gallon, and at curent prices and 100% efficiency for a ventless heater, it works out to $43.07 per million BTU. Of course, if you’re paying hardware store prices at $12/gal, the numbers aren’t as nice at $90.98 per million BTU. Try not to do that - though even there, I really do prefer kerosene to propane for indoor use.

What about heating with firewood? A modern wood stove is about 85% efficient (they need some heat left in the exhaust to draft), and a cord of Douglas Fir around here is about $400, for 20.7 million BTU. Math it out, and you’re at $22.73 per million BTU.

But… what are you heating, exactly, with those BTU?

Is it Efficient?

Given the numbers above, it’s clear that kerosene isn’t the cheapest option for “Bulk, whole-house BTUs.” And that’s fine… if your goal is to heat the entire house to an even temperature. But, to borrow a turn of phrase I quite like, “Efficient in terms of what outputs for what inputs?” Is your actual goal a warm house, or is your actual goal a warm human?

If the actual goal is a warm human as opposed to a warm unused room, then heating humans is far better than heating all the air in a house - and radiant heat is particularly good at heating humans. A kerosene radiant heater, in particular, is really good at heating a human. So is a radiant propane heater, and an electric radiant panel accomplishes about the same thing - though rather “less so,” given typical wall current limits in a modern American house.

If you’re curled up watching a movie, a bit of radiant heat in front of you will solve most of your “I’d like to be warm without being bundled up in several layers of wool sweater” problems. The blast of radiant heat coming off one of these is quite powerful, and will absolutely keep you comfortable without a sweater in a cold room.

A “10k BTU” heater (remember, 10k BTU/hr - BTU is a measure of total heat energy) is about 2.93 kW. Not all that heat flows out the radiant side, but an awful lot of it does, and I’ll tell you from experience that a 10k BTU kerosene heater puts out far more radiant heat than an 800W radiant heat panel. On pump kerosene, it costs about $0.45/hr to run a radiant kerosene heater in the evening. I don’t care what your internal air temperature is, you’ll be nice and warm in front of this. And if that’s a bit expensive, you can always lower the wick a bit and produce less heat - though I’d suggest against lowering it too much. Kerosene heaters do drop out of their efficient realm if the wick is too low. They’re best run at or near rated thermal output.

How Are They to Use?

So, that’s a lot of technical data. But what does a kerosene heater feel like? This is a harder one to describe, but I’ll certainly try.

A kerosene radiant heater is a very different type of heat from most other things I’ve used in the past - to include smaller propane radiant heaters, convective kerosene heaters, and my convective propane office heater. You need to sit back - it’ll roast you up close. But at the proper distance, the heat is just… really nice. It’s warm, and you adjust your position and distance to optimize the heat for what you want to feel. It’ll warm clothing up in a hurry, and it also is quite nice on bare skin. Up close, the heat is very aggressive - it’s a harsh heat. But back up, and it’s simply a powerful “hot spot” to move yourself around in.

As far as the exhaust indoors, I typically leave some windows open a half inch or so for airflow, but the smell isn’t too bad. It’s a nicer smell than propane, too - at least to me. Kerosene has a “warm” exhaust smell, sort of a faint rich hydrocarbon mixture. Propane just smells metallic to me - very industrial. It’s fine in my office, but I don’t like it. It would be very out of place in the house, and that’s before you get into the whole “pressurized cylinders of propane in the house” thing I’d rather not deal with.

It’s very, very much a radiant technology, though. The right spot to be, in a cold room, is “directly in front of the heater.” That solves your problems. Elsewhere in the room? There’s still some heat, but it’s just not the same.

Should you get one? Again, this depends on how pessimistic you are, but I will argue, strongly, that “having and being familiar with kerosene heating and lighting technology” is a wise little side hobby to have in 2022.