4300 Gallon Plywood Build (3600+ Take 2)

If you don't get enough temperature drop between the supply and return, you will need to add more heat exchange coils.

Without a significant delta T you will loose heating efficiency massively and possibly over heat the unit as it would be heating preheated water.
 
Wouldn't just an electric element be more efficient? I know tankless water heaters are more effecient then a tank water heater, but that is due to the fact they don't have to keep 40+ gallons of water at temp 24/7. But don't they draw alot of power when they are on?
 
Pavo71 - I don't know what a "significant delta T" is.

nc0gnet0 - it won't be on 24/7. As I said, once the tank it up to temp the heating will only be on intermittently. The fish room is very well insulated, so keeping the tank 78-84 degrees won't be an issue. The top will be covered, so heat loss will be minimal as well. I also live in New Orleans. Our winters are generally mild and short.

And no an electric element wouldn't me more efficient. If I was concerned about its efficiency and/or thought it would be running 24/7, I'd install another natural gas hot water heater like I have for the rest of the house. The gas line goes through the ceiling of the fish room, so it wouldn't be that big a deal to get a supply in for a small gas hot water heater.

frigginchi - I think so, but I haven't researched it that much.
 
Hey pete..
Menards sells an on demand radiant heat system that you can install in the floor of a bathroom, its electric, but it heats and circulates water through a closed system of tubing in the floor. Might be something to look into. If you want I'll check it out and get some info for ya.
 
Nolapete said:
Pavo71 - I don't know what a "significant delta T" is.
Click to expand...

Delta T is the temperature difference between supply and return. It should be >75% of the difference between supply temp and tank temp to be significant and have any semblance of efficiency.

The closer the return temp is to the tank temp the more thermal energy is being transferred to the tank and thus more efficient.

Multiple short paths of small tubes or really long paths of larger tubes is how to increase thermal transfer efficiency.

Furthermore, an on demand water heater is designed to have a low temp input (~60F). A raised return path temperature could have 2 effects depending on the design of the heater.

1. If the heater senses flow only to turn on the heating element, then you could end up with super heated water possibly melting down or softening of the heat exchange tubing in the heater or the plastic tubes. At the very least you will have nusance tripping of the pressure relief valve.

2. If the heater senses input temperature and flow an elevated input temp could "trick" the heating element into not turning on.

Both of these issues can be addressed by increasing the efficiency of the heat exchange coil in the tank.
 
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Ahh I see now and it gives me a few ideas.

The easiest would be to make it an open loop and supply the hot water heater from my regular water supply therefore requiring no pump. It supplies hot water to the coil of pex which would then exit the tank to a drain. I could use the same drain the clothes washer uses in the laundry room since it's next to the fish room. The water supply could come from there too.

In addition, I'll extend the tubing loop to go around the perimeter of the tank or even make a grid that covers the entire bottom for even heating.

Thanks Pavo71 for getting the creative juices flowing on this part of the project. Not having to use a pump to move the water makes this a lot more efficient I think since my water is so cheap. I'll put carbon and sediment filters ahead of the tankless hot water heater to extend its life.

Just had another idea, I could just buy a solenoid driven valve and the digital thermostat and use my existing hot water supply. I don't even need the tankless hot water heater. More things to think about.
 
I've been lurking the whole time (since version 1 of this project) and I finally have something to contribute...

Nolapete, if you put the hot-water exchange coils outside the tank in the filter loop, you'll have a few advantages:
1. It's externally accessible, which prevents the need for SCUBA gear if you need to work on the loop. An algae buildup on the coils could inhibit the heat exchange.
2. You get more tank water passing over the heated loop is the same amount of time because it's forced by the pump that drives the rest of the filtration system. Otherwise you have to rely on the tank's natural circulation and convection to bring new water to the coils.

You could stick the whole thing into one of the blue barrels that you're planning on using for the filtration - one pipe in (and out) for tank water, and another for the PEX tubing in and out. It's a combination water heater / radiator. You could even stick a water-heater insulation blanket on it for bonus efficiency.

The downsides I can see are that you'll get some flow loss due to the extra barrel in the filtration system, as well as the space that the extra barrel would take. I'm sure that there are other downsides that I don't see right now.

Just thought I'd toss in my 2 cents - awesome project and a huge inspiration.
 
Already thought of that too, so you're right on track. I'm not sure which way I'll go with it yet. That'll probably be determined by cost once I research the parts pricing for each option.
 
What I was trying to say is that on demand hot water heaters get there effiency by not having to continously heat a tank like a conventional hot water heater. But in effect, at least with your original idea, you have added back the tank so the on demand system is no longer more effecient than that of the stadard hot water heater, maybe even less so do to the difference in heat transfer through a marginal heat conductor (pvc, etc). If your going to go with a closed loop system, you might just as well use a standard hot water heater set on low with a small tank.

An open loop system were you are actually supplying the tank with hot water would be a different scenario and its effiency would be determined by how often it cycled on and off. Can the output temp of an on demand heater be regulated?

I keep envisioning your tank using a liner, and I now recall that is not going to be the case. If it did, I think two water bed heaters installed under the liner would be the way to go, giving you a much larger heating surface. The drawback would be maintence.............
 
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