Notes from Windward: #58

Part II

more stuff all the time

Slab heating components
Well, stuff just keeps on coming. When I stopped by the hardware store/salvage shop that sometimes seems like my second home, Eric had something set aside that was just too good to go for scrap. This strange assembly of stuff is the heart of a slab heating system and consists of four critical items.

To review, we'll be installing a propane-fired on-demand water heater in the dining hall. Most water heaters are of the type in which water is heated and kept hot in an insulated tank. When you draw some of that hot water out, cold water is drawn into the tank and heated in preparation for next time. Water heaters of this design are cheap to build and buy, but they steadily consume energy in the process of keeping all that water hot.

The second problem with a "reserve" system is that it can only supply a limited amount of hot water, and then you have to wait for the heater to heat the newly introduced cold water. That can take anywhere from fifteen minutes to an hour.

2 gallon expansion tank
The "on demand" system doesn't keep water hot. Instead it senses a demand for hot water, and fires it's burners to heat the water as it passes through the unit. This means that we could be running our commercial dishwasher, using hot water in the sink, and have someone taking a hot shower, and never run out of hot water. It also means that when no one is using hot water, we're not spending money keeping water hot. An on demand system costs more in the short run, but it pays for itself fairly quickly.

We're including a bathroom and shower room in the dining hall, and since they'll have cement/tile floors, heating those spaces will be difficult. None of the traditional forms of heat seem to have any effect on a cement floor, with the result that in winter, people often suffer from cold feet. Older folk are especially sensitive to this.

circulation motor
Just about the only way to whip the cold concrete problem is to include tubing in the floor when it's poured, and to then run hot water through those tubes. That warms the floor, and since heat rises, it also warms the room.

As you're probably guessing, the on-demand water heater is the source for the hot water that heats the floor. To make that system work, there are some key components we'll need, and Eric's found most of them for us.

The large red thing is a diaphragm equipped expansion tank. When water is heated, it expands in volume and you have to have some part of the system store that expansion. The purpose of the diaphragm is to keep the air in the tank from being adsorbed into the water over time. In effect, there's a balloon inside the tank. Wholesale cost - $53.

bronze flanges
The purpose of the motor is to draw water from the heater, and to then circulate it though the system. This particular model is designed to work with hot water, and is a heavier version of the pump we use on our wood-fired hot tub. Wholesale cost - $163.

The pump is bolted in place using two cast bronze flanges. These form the interface between the cast iron body of the pump and the copper tubing that's used to carry the hot water. Wholesale cost - $23.

The fourth component is the pressure relief valve. If the water expands to the point where it exceeds the capacity of the reserve tank, something has to give. The plan is for this valve to be that "something." All water heating systems are required by law (and good sense) to have them, so they're stock items. This one is different because it's adjustable. It's a specialty item, and my Grainger catalog doesn't list it, but you can trust me on this one, such valves don't come cheap.

adjustable pressure
relief valve
So, not even counting the fancy valve, the replacement costs of these items runs around $250 wholesale. Eric let us have the assembly as scrap for $20.

From his point of view, it is scrap. No commercial contractor would install used components in a new system. For one, he'd lose his usual markup from wholesale to retail, and most people don't want used components anyway. It's certainly not worth his time to fiddle with them and risk having to go back and do a service call because a used component failed prematurely. While it's entirely possible to have a problem with a new component, in that case the manufacturer takes the heat, not the installer. From his point of view, why should he take that risk just to save you some money?

So this is a good example of quality components which will meet our needs, but would otherwise have gone to the scrap pile. I always feel a thrill of victory when I can save a good piece of gear from going to the landfill.

It's important to remember that this was a bargain because these components are right for a job we're planning on doing. It isn't a bargain if it isn't the right component, and you don't want to let chance make your choices for you. Still, when it's real sweet when it works out.

It was unlikely that this particular item would come along before we needed to buy it, but if you keep your eyes open and are patient, such things do happen. The trick is to work your plans out far enough in advance, and in sufficient detail, that you'll have a solid idea of what you'll need, and will be able to grab it if it should turn up along the way.

Index for Notes Issue # 58 ---- The Windward Home Page