I'm new here, as a matter of fact this is my first post, but I will say the forum has helped me in various ways in the past. The lack of info surrounding hydronic dryers is why I joined.....
In February of 2015, my house caught fire on one of the coldest nights of our past winter. The clay and concrete wall collar failed, allowing the heat and fire from a very hot fisher woodstove to make contact with combustible materials in my wall. Insurance company wrote the damages at 29,000. I did all of the work myself, and repaired, and even improved on my house for a little less than 10,000. I had around 17,000 left to do as I pleased with after the deductible and everything else was said and done. I bought an Earth Woodsman 405, and related materials to go with it. I then typically blew the rest on other things like tools, and newer diesel truck, and a really nice used 20' flatbed.
I have a wife, and 5 girls, 4 of which are ours jointly, and one exchange student, so you can imagine, that we wash and dry a lot of clothes, before I had even purchased my furnace, I was looking for ways to utilize it, anything and everything that furnace was capable of doing, I was going to do it. I Have a strong mechanical aptitude, and knowledge of how most things work in the world, I was sure there were kits and plenty of knowledge on how to make a hydronic dryer....I was wrong. I found ONE, that's right, a total of ONE kit to convert an electric dryer, and lots of forum comments stating it wouldn't work, or it would work poorly, and the original poster should just stop being so lazy, and install a clothes line.
Not to be deterred, when I purchased my furnace, I purchased two water to air heat exchangers, one sized to fit snugly inside my propane fired central unit, and the other was the smallest size they offered. I had already done the math on how many btu's an electric dryer uses, and came out with a figure hovering around 10,000 BTUs. The smallest exchanger they offered was a 12X20 rated at 60,000. I figured heck, might as well, if it doesn't work, I'll use it to heat my shop or basement using a scavenged squirrel cage. I had an old sears dryer sitting out in the back of the barn that we retired because it never was great at drying clothes, it had issues from the day it was brand new. Retrieving the old dryer, I set to work re-purposing it for use as a test bed. I set out to call this "new" dryer, my "Water Dryer"
I removed the back cover, and then the heater assembly and related duct. I then test fit the exchanger to see if it would fit inside the cavity, and it did, rather snugly. I then set about adapting it to actually reside there, and fit the air intake to the drum. I used a piece of 1/4" MDF that was slightly larger than the 12X20 exchanger. using a jigsaw I cut a hole for it to fit nicely over the back half of the heater duct. I then attached the MDF to the exchanger. The exchanger and mdf was then fit in the back of the dryer, and I was still able to use the mounting straps that held the original duct assembly into the dryer. The heater duct was a rectangular tube that was made from two pcs of stamped sheet metal that was riveted together. I simply drilled the rivets, and retained the back half of the duct as it was the simplest way to make a good seal for the drum flange, and the rivet holes made it rather easy to attach the mdf using very short sheet metal screws.
From there all I needed to do was convert the remaining electrics to allow the dryer to run. I have repaired, or helped repair a lot of dryers in my past, and I have never seen an electric dryer that doesn't use only one leg of it's incoming power to run the tumbler/blower. Meaning, every dryer I have ever seen, is powered by 240 volts, but the tumble/blower motor only run on 120, or one half of the power supply. That was also the case in this dryer, so I removed the original 30 amp wire, and replaced it with a salvaged cord from some power tool or etc that had failed long ago. With a little wire chasing I could see that one leg of the 240 went directly to the heater, and the remaining leg went to the heater, and everything else. Walla!, hook the common to the original common, and the hot, to the hot side that supplied everything else. Set the timer, and push start, and it runs.
Called upstairs to the wife and told her I was ready for my first test load. Thinking it was a failure from the get go, and hoping to make it a dismal failure, she brought the biggest load of wet clothes I have ever seen over and piled them in. It was a heaping basket full, the big baskets, about 2 feet long, 18 inches wide and deep. An hour later, the dryer was still running, so I checked them, and they were almost dry. BUT, the timer was still at the same setting. Gave them a few more minutes, and called my wife back down. She was pleased! That night we washed everything in sight, and then dried it using my "NEW" dryer. The drying time is about 20 to 50 percent longer than our unmolested dryer depending on the clothes, with towels being the hardest to dry in a reasonable amount of time. That was partly rectified by turning the spin speed up to high on the HE washer. Our last HE washer, a front loader failed catastrophically in the drum. It was a good washer, and it did a really good job at spinning the clothes, it would actually sound like an APU on a jet spooling up when it would spin the clothes out. Although, planned obsolescence, and a really fast spin speed caused the drum to explosively separate one day and killed the washer dead, since a replacement drum was 600, and the washer only cost new 750. We replaced it with a nicer top load HE, but it had a variable spin speed option, and to avoid what happened last time, we choose to set the spin speed on medium. We leave it there now, for everything except towels, and jeans.
Last night I decided to tackle the timer issue because for the last month we either had to check the clothes and manually stop the dryer, or use the unmolested electric dryer if we were not going to be able to check the water dryer for some period, IE we were sleeping, or out of the house. After some poking around with a DMM and tracing wires I determined the timer circuit was tied into various safety and dryness sensors in the dryer, one of which being the over temp sensor on the heater assembly I had removed. I simply re positioned the timer motor wires about the terminals on the back of the timer switch so that the timer motor drew it's power from the same terminal that sent power to the tumble/blower motor. Now, whenever the timer is in a position to send power to the motor, it runs itself as well, and since all settings on the timer send power to the motor it runs as expected with one exception being the auto dry function no longer works. Most dryer auto dry settings use a temperature sensor that only closes above a certain temperature. The way that works is as follows... A wet load of clothes will have a low exhaust temperature, lower than boiling, so in the auto dry setting, the timer cannot run, until the exhaust temperature sensor closes it's set limit. Once the sensor closes the timer will run, and shortly thereafter move to an off position. Unfortunately, that setting is Higher than the temperature the dryer is now capable of outputting and will not function.
Overall we are all pleased, it works moderately well only taking about 30% longer to dry, and after fixing the timer issue, if you set it on the longest run time, the clothes come out nice and dry. I have not measured the tumbler current draw, but I am guessing it is around 500 watts or less, so when compared to the original 4500 or so watts and slightly shorter run time, we are saving about 70% using it over the full electric.
How much wood does it burn?? I don't know exactly, but if I had to make a WAG I would say about 3 lbs or less per load. BTU content of wood is almost the same no matter the species, with that BTU content being around 8600. Cottonwood PER POUND has relatively the same amount of energy as does red oak moisture content not withstanding. I know that when the dryer is running, the furnace does not power the blower any more often than it does just sitting idle, Sitting idle the blower fires about every 30 minutes, mostly due to the 100 or so feet of uninsulated pex running around in the basement ceiling. Makes for a nice cozy warm trail leading all throughout the house. as well as the central unit blower not needing to come on as often as if it would if the underfloor pex was well insulated. Measuring temperature drops in and out of the dryer at my "Estimated" flow rate I come up with a figure of 10,000 BTU's, but it is probably lower, as I'm not calculating my flow rate for my pipe runs or number of joints, but simply using the pumps rated flow at 0 head.