Update:Todd asked for a schematic, I drew this 2D basic system diagram (click for hi-res picture):
Perhaps I was a little optimistic to think that I would get all the plumbing work done in one weekend… I did make some good headway in the basement, but I am nowhere near done.
First, I build a little stand for the drainback tank to sit on. This allows the solar loop pump to be installed below the drainback tank, which I think is a good idea. I used the legs of the stand to mount the two pumps on. I would rather have the pumps attached to something rather than be supported by 3/4 inch or 1/2 inch copper pipe.
This is a double pumped system which are much more efficient at moving heat around than a single pump/convection system. Basically, there are two plumbing loops; the solar loop which runs up to the roof and through the solar collectors then back down to the drainback tank. The hot water tank loop which runs out of the bottom of the hot water tank through a heat exchanger in the bottom of the drainback tank and back into the top of the hot water tank.
The drainback tank (RADCO DBHX 6029-GC) holds eight gallons. This should be enough as each collector holds 1.6 gallons of water when operating. The total pipe run is going to be about 60 feet of 3/4 inch copper. Type K copper pipe has 0.0227 gallons per foot, so the total water in the pipe will be 1.4 gallons, which leaves a minimum of 3.4 gallons of water in the drainback tank. For a full chart of copper pipe technical data, including gallons per linear foot, check out the Copper Pipe and Tubing Cheat Sheet. This should be enough to keep the heat exchanger submerged and keep the system operating at peak efficiency.
The solar loop uses a TACO 009F pump because of the high static head of the system. At startup, all of the water in the solar loop is in the drainback tank and has to be pumped up to the top of the solar collector, where gravity will then return it to the drainback tank. The top of the solar collector is 24 feet above the drainback tank. According to the pump spec sheet, the flow rate should be 6 gallons per minute. Taking into consideration other factors such as resistance from the pipe itself and things like elbows, temperature gauges, flow meters, etc, the flow rate will more likely be about 5 gallons per minute. Each collector is looking for 0.5 to 1.8 gallons per minute. The are plumbed in parallel, so my maximum flow rate should be no greater than 3.6 gallons per minute. I will likely have to throttle the pump down a little to attain this.
The tank side plumbing loop will use a TACO 003B circulator pump. This flow rate will be about 3 gallons per minute, just enough to keep the heat exchanger working well while not breaking the stratification of the hot water storage tank. This is a potabe water loop, so a pump with a brass casing must be used.
Drain back tank with flow gauge and thermometers for collector send and returns. Flow gauge acts as a sight glass to check drainback tank water level when system is not operating. The solar loop pump is the green unit on the lower left, the hot water tank loop pump is the brown unit on the right.
Paul,
Looking good….have you drawn any schematics of the system? I’d love to see how the whole thing is put together.
I’m confused. What exactly are you trying to accomplish? If it is energy management, what is the cost of the pumps running all the time compared to any possible temperature gain you may obtain from the collectors?
I drew a schematic several month ago, it is likely in my desktop computer, I’ll have to dig it out and post it because it is quite interesting. I am trying to get my wife to shoot some video of the work in progress, we will see. Otherwise I’ll keep posting updates until it is done.
Fred, That is a good question. The two pumps in question draw much less current than the heating element in the hot water tank. TACO lists the F009 pump drawing 1.4 amps at 120 volts, which is 168 watts. The BF003 pump draws 0.45 Amps at 120 volts, which is 54 watts. Total, the solar system will draw 222 watts when operating vs. 3800 watts for the electric element in current hot water tank. Our average annual insolation here is 4.5 hours per day, so I anticipate the pumps running for that period of time or slightly longer.
I anticipate during 7 month out of the year, the solar system will generate more hot water than we need, for two months it should generate just enough, and for the remain three month we will have to rely on the back up heating element (although the solar system will heat the water to some degree). My calculations show a savings of about 3,500 kWh per year, I will have to let you know how accurate that is next year.
Paul,
Looks pretty simple, at least simpler than I had envisioned in my head. I’m assuming that you decide when to run this loop? Or is it controlled somehow by the temperature being generated in the collectors?
Todd, the controller is a differential temperature controller. It measures the temperature on the solar tank (via 10K ohm termistors) and the temperature on top of the solar collector. When the solar collector is 20 degrees warmer than the solar water tank, it turns on the pumps. It turns off the pumps when the temperature is within 5 degrees.
Paul, that’s pretty slick! I’m really curious to see how the system works for you and what you think the payback time period will be once you start running it.
Yes, I can’t wait to see it in action myself. As far as the payback, right now I am about $4,200 into it, there may be another $100.00 or so of misc plumbing parts before it is all over with. The federal government is giving a 30 percent tax rebate for qualified solar systems and NY state is give a 25 percent tax rebate. So I should receive back $2,365 in tax rebates, making the final cost $1,935. If my electric calculations are correct, should save around $470.00 per year making the payback 4.1 years. (If electrical rates don’t go up, which they are supposed to on January 1, 2008 for us). Then, there is the added value to the house itself. That is a little more difficult to quantify, but the environmental issues are huge around here, so I am thinking that a house with solar heated hot water will be a selling point, if we ever decide to move.
hey paul, im wondering where i might find the federal credit info. do you have a link?
Jake,
You can find all of the incentive information at the DSIRE website.
dsireusa.org
Looks nice. I’m going to start installing my system soon. Do you know if a rheostat will work on a Taco circulator to vary the flow?
We just moved into a house with a drainback system like this installed (looks about 10 years old) but the controller is shot. Based on your research, where is the best place to get info about the system (no documentation with it, but the plumbing seems to be in good shape, the panels are good, and the pumps look good) and a new controller?
Thanks!
Max
Paul,
Did you ever do a cost savings for the year you have this system running?
Just curious. As you know from other comments I’ve made, I am still trying to determine the benefits of a geothermal system vs anything else. If my calculations are correct then it would cost me about 14 years to get the money back from the geothermal. That is factoring in the tax credits and such.
I came back to look at this part of your website after thinking about you on Monday when my hot water heater ruptured.
Fred, I think I calculated the electrical savings at about 3,400 kWh per year. Our electric rates are $0.149 per kWh therefore last year’s savings was $506.00. I think our payback time is going to be around 3 years or so.