Did I Make a Mistake?

Swisher · March 2012

Hello Everyone,

I've just installed an outdoor wood boiler, and I'm using part of the heat to take over from electricity on a 60 gallon DHW tank in a basement. I've plumbed a Grundfos 15-58 into a very short loop of galvanized pipe outside the heater to run tank water through a heat exchanger. The 15-58 is an iron circulator, and I now realize I should have gone bronze. Is it worth buying a new pump and changing it? After all, the DHW tank is steel (with a fresh anode installed). Also, jet pumps and piston pumps for well systems are iron or steel, and they seem okay.

I like to do things right, but how wrong is my current set-up?

Thanks a million everyone!

Bye,

Steve

Ironman · March 2012

Why do You Need a Circ. on the Domestic Side?

I can't see any reason for a circ. on the potable side of the heat ex. I install wood boilers regularly and when using a plate heat ex. we place it right at the tank and take the cold inlet and run it through one side of the heat ex. This way, when any faucet is opened, water is automatically drawn through the heat ex. and heated, then dumped back into the inlet of the tank.

If you're trying to keep the tank elements from ever coming on, that's not necessary. The amount they will run is minimal.

I've had guys do it your way after I told them not to and the results have always been the same: disastrous. You'll scale up the heat ex. and pump from the constant circulation of minerals and use more energy than if it was done as I described above. I'm not saying you won't have scaling doing it my way, but it will be 10 times worse the other way. Put a set of isolation / purge valves on the domestic side like those used on a tankless so you can de-scale when necessary.

Regarding using an iron circ. on an open system: it's a very bad idea. A picture is worth a thousand words. Look at these pics from a recent thread of a CLOSED loop that got O2 in it:

http://www.heatinghelp.com/forum-thread/140883/Anyone-seen-this

Swisher · March 2012

Thanks Bob!

Thanks Bob. I'd heard of plumbing the flat plate into the incoming cold line, but it seemed to me that it wouldn't keep the tank hot unless lots of hot water was used. But your experience convinces me not to worry. I'll go with your option.

Bye, and thanks.

Steve

Ironman · March 2012

Heat Ex. Piping

I don't know your line sizes, gpm or heat ex. size, but here are some rough guidelines:

1. I always use a 20 plate 5" x 12" heat ex. With proper flow rate on the boiler side, it will give you about 6 gpm @ a 77* temp rise. That's enough for three simultaneous showers.

2. If you have 1" lines (one set) coming in from the boiler, then run all the flow through the plate heat ex for the domestic first, then on to the space heating. This will give priority to the domestic which is the correct strategy. If you have two sets (four pipes) of incoming lines, then only run one set in series through the domestic heat ex.

3. If you have two heat exchangers and say 100' or so of piping between the boiler and the appliances, you'll need at least a Grundfos UPS26-99 circ. or equivalent. A lot of variables apply with pump sizing, but this is ballpark with the above parameters.

4. If you size with the above guidelines, the domestic heat ex. will pre-heat the water going into the tank to about 160 - 175* under most circumstances. The tank will dilute the heat if there's not a long draw and if the electric elements due come on it will only be after there has been no draw for many hours. When the do come on, they will only have to make up about 10 - 12*, not 75*. Therefore, the electric usage will be minimal.

5. With this setup, the longer the draw, the hotter the tank will get. You may want to install a tempering valve if you have small children or elderly folks in the home. This is highly recommended.

6. After the wood boiler is up to temp, open a hot water faucet and let about 50 gal. of water run out. This will fill the tank with hot water so the elements shouldn't come on unless there is a long idle period. A lot of my customers have told me they leave the circuit breaker turned off unless they're going away for several days and they never lack hot water.

Hope this helps.

Best wishes.

Swisher · March 2012

Much Appreciated!

Hello Bob,

Thanks very much for your generous explanation of a DHW system that works. I'm going to tear out what I have, then build what you recommend. Before I do, I'd like to explain a little more about what I did, then ask a couple of questions (just to make sure there's no more tearing out to be done).

I didn't explain in my original post that I'm not using a flat plate heat exchanger, but rather a sidearm. My thinking was that a sidearm would be more resistant to scaling up. Rather than letting it work by thermosiphon, I put the 15-58 on top to speed water flow and boost hot water production. (My wife and I have 5 kids, so we go through a fair amount of hot water).

The long and the short of it is that the sidearm isn't producing significant hot water. I've fooled with flow direction (first counterflow, then unidirection), with differences in flow rates (I rigged it so I can vary DHW flow rate with a valve, beyond the variation possible on the pump itself), and a few other things. I've monitored water temperature in and out of the sidearm using a thermocouple on my Milwaukee infrared thermometer, and in the tank itself. I can leave the sidearm working all night long, yet only see a 5º or 10º rise in temperature in the 60 gallons. At both slow and fast flow rates, the sidearm only causes a 3º or 4º temperature rise. I'm at a loss as to why the sidearm doesn't work for me, since sidearms do seem to work for other people. The only thing I can think of is that my basement is unheated (about 50ºF) right now. My plan was to insulate the lines and sidearm, but I'm not confident enough that this would make a difference, so I won't bother. The sidearm is a stainless steel double-walled unit. Perhaps a single wall sidearm would work better. Do you have any experience with these?

So, as I said, I'll be tearing the whole sidearm/circ pump out and replacing it with a flat plate exchanger in series with the cold water intake into the DHW tank. You mentioned a 5x12 20 plate. Will my brand new 3x8 40 plate Bell & Gossett work? (I imagine so, but want to be sure I'm not missing things. It's rated at 225K btus/hr).

My plan is to plumb the flat plate in with unions right at the exchanger outlets, allowing me to take it off conveniently for treatment with vinegar as needed.

Are my ideas sound?

Thanks a million, Bob!

Bye,

Steve

Ironman · March 2012

Side Arm

Side arms work O.K. if piped correctly and supplied with 160* + water on the boiler side. They won't give the output or have the recovery that a plate has, but if sized right with a large enough tank, they'll work.

If you're only getting a 5* rise, then your piping may be incorrect. A side arm must be placed as low as possible and Tee'd in at the drain connection of the tank. The outlet must connect back to the cold inlet no higher than 4" above the tank. Remember, it works by percolation (like a coffee pot) so it has to be at the bottom with all potable piping pitched upward. Connect the hot boiler supply at the top with the return at the bottom and it should work fine. See the attached pic. (Not one of my installs).

Due to the size of your family (I have six girls and a boy) and tank, I think you'll be better served using the plate heat ex. I'd use the Webstone ball/purge valves advertised at the bottom of this page instead of unions. You should only need them on the potable side.

Swisher · March 2012

Thanks Bob!

Good Morning Bob,

Thanks a lot for pointing me in the right direction. Those Webstone valves look terrific. I've got an order ready to go.

Bye,

Steve

Did I Make a Mistake?

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