Sidearm Heat Exchanger Install

I'm in the process of installing a sidearm heat exchanger on the water heater and have a question. Does the hot water supply from the boiler connect to the top or bottom of the heat exchanger?

Some might disagree, it won't make much difference. Plumb it whichever was is simpler.

Does it run off the circulator pump or is it thermosiphon?

Nearly all sidearms are thermosiphon on DHW side and pump on the exchanger (boiler side). Mine feeds from the bottom which is easier to remove any air from the system. Lots of folks have trouble with sidearms, I've never touched mine in 6 years and never run out of hot water.

I'm not an hvac guy but I think the supply would go in the bottom since that is where the coldest water will be.

It does matter, the hot water from the boiler should be connected to the top of the side arm heater.

Reason being, hot water rises to the top of the boiler, exits the boiler, goes to the side arm heater where it cools, becomes heavier and falls back to the lower, cooler, area of the boiler.

It becomes a continuous process, no pump needed.

Yes, anything that I have seen called a sidearm heater was always thermosiphon, making it imperative of course that it be plumbed in the correct direction.

This is a typical sidearm for a hotwater heater as is used with a woodfired boiler. You can buy it already made or buy the fittings and make your own wqhich is what I did.
The diagram shows how I installed one on my son's electric hot water heater when we installed an outdoor wood boiler.
If your heating water is above 130 degrees (his wood boiler runs 175 to 180 degrees) you should (better) install a tempering (anti-scalding) valve or someone could get seriously burned!
It simply makes sense that the bottom of the sidearm connects to the bottom of the water heater as shown.

I reread your question; the connection of the hot water line from the boiler is not so critical but it will work best if plumbed as shown in the diagram.

The movement of the domestic (heated) hot water is by thermosyphon so the sidearm has to be hooked as shown. Most heating water is moved by circulator(s) and if it is, that part will work either way.

The boiler water should enter the top of the sidearm because the hottest boiler water should be in contact with the DHW just before it enters the DHW plumbing or enters back into the storage tank. At the bottom of the sidearm, the cooled boiler water will still be sufficient to warm the cold DHW from the tank. This applies to just about every heat exchanger theory.

I came across your layout of Side Arm heater for DHW H and connecting from boiler. I would like to resolve my issue and get mine set up back on line. First, when I purchased this home, I was not aware of the purpose of the existing Side Arm heater (which looks exactly the same as the one in the picture from your post). I never really utilized it until I started using my wood boiler in exterior building to heat the main house. The boiler does a Great job of heating so I decided to re-initiate the SAHeater. I opened the ball valves plumbed into the top of the tank, but nothing happens other than the hot water runs out in a day (after shutting off my EWHeater). Takeing a closer look, I found the bottom of the SAHeater is tied into Nothing. The Tee at the bottom was capped off. Evidently, the EWH from the previous owner was replaced and they eliminated the SAHeater. I was a little fooled at looking at the SAHeater at first as the bottom of the SAHeater Tee also had a small extension pipe to ACT as a leg to stand on the floor. I did not see the cap on the pipe and thought the line was cut off and from the lateral tee and replaced with the capped off pipe (appearing to go down through the floor). Obviously, that is why the SAHeater is not working. So, I can tie the SAHeater back into the bottom Drain of the EWH as indicated for proper installation. Also, noted my boiler water is being circulated underground in the loop by a pump. I have only ONE Question (after I make the change). According to the picture posted, the top of the SAHeater is plumbed into an independent opening on top of the EWH to make the circulation. Mine is not plumbed like that, RATHER - it comes directly out of the SAHeater and ties directly back to the top of the EWH via a TEE at the HOT Discharge of the TANK. There is a Ball valve which can be opened to allow that access. The Tee also continues out to the Domestic side but is routed first through the protective scalding valve. This appears to be OK, but I just wanted to run it pass you as based on your success using the SAHeater. IF so, it will be and easy change over to tie the bottom back into the EHWHeater. Sorry for the long drawn out comment. Tom

(quoted from post at 18:04:56 02/26/12) The boiler water should enter the top of the sidearm because the hottest boiler water should be in contact with the DHW just before it enters the DHW plumbing or enters back into the storage tank. At the bottom of the sidearm, the cooled boiler water will still be sufficient to warm the cold DHW from the tank. This applies to just about every heat exchanger theory.

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This is correct, hot water goes in the TOP of the side arm and comes out the bottom. Water from the tank comes in the bottom, gets warmed up and goes out the top on its way back to the tank. The diagram posted is wrong.

It was not mentioned but it might be noted that the tank water goes on the inner pipe and the boiler water goes on the outside part of the side arm. Its a better heat transfer to the inner pipe, it is sourounded by warm water rather than warm water on one side and cold outside air on the other. The inner pipe also has less flow resistance so it is easier for it to thermosyphon to and from the tank.

Fixitright and others with the same answer are correct to obtain max btu exchange for the hx surface area.