Sidearm heat exchanger recovery time....

[Pat32rf]

Going to tie a sidearm heat exchanger into or Cascade 40 electric hater and I am wondering about recovery time. Normally you would heat the top 10-15 gallons with the top 3kw element, and then flip down to the bottom 3kw element to heat the rest over a longer time period. As long as the water stratifies it works great, but with a gravity feed sidearm off my owb, will it take longer to heat the whole 40 gallons? Will there be any stratification?
I will be cutting the flow from the owb when the water tank hits 140F....

 

[maple1]

Quite sure there will be stratification. Sidearm will heat slower than electric. Where will the 140 be measured at? Bottom is likely better than top. My temp probe is on the pipe out of tank to bottom of sidearm, in as close to the tank as I could get.

 

[Pat32rf]

I am planning to use the bottom T'stat on the tank to close a ball valve between the sidearm and the cascade 40. I have a few Honeywell motors sitting around that we used to use to drive 2" three way ball valves on larger systems.

 

[maple1]

OK, I used a water heater stat for a while too. A totally separate one I picked up. I tied it directly onto the bottom outlet from dhw tank to sidearm, as close to the tank as I could jam it, under some pipe insulation. Some heat sink grease under it likely would have helped. I ran 24v through it, to a zone valve on the boiler supply side of the sidearm. The end switch in the zv was tied to my load circ. The dhw side just convected naturally. That worked fine when I was heating the house. Not so much when not heating the house & just trying to heat dhw from storage.

 

[Eric Johnson]

In my experience (with hard water), you need to periodically flush out the bottom of the tank, or it will accumulate scale and other crap and eventually restrict the convection flow. Then, you'll hear those dreaded words, "Honey, there's no hot water." I just open the valve and drain 5 or 10 gallons off the bottom from time to time, and it keeps everything clear.

 

[woodsmaster]

If you circulate the domestic side also it will recover faster. I have an 80 gallon electric and as long as I have a good supply temp I rarely run out of hot water. I don't circulate the domestic side , but with only 40 gallon it may help.

 

[woodsmaster]

I'm still using the sidearm I got from you ! works great. When I had more people in the house I had to keep supply temp up pretty good to meet demand but with 3 in the house now it works really good.

 

[Medman]

With continuous flow from your owb, you could raise the temp in the DHW tank much higher than 140, as I do. I then mix the output of the DHW tank down to 125 with a caleffi mixing valve before it goes to the house. This way you always have a lot of heat available. On a continuous flow day in winter, with little usage, like a weekday I can get the bottom of my DHW tank to 170 with 185 degree water on the boiler side.

 

[Eric Johnson]

If you circulate the domestic side also it will recover faster. I have an 80 gallon electric and as long as I have a good supply temp I rarely run out of hot water. I don't circulate the domestic side , but with only 40 gallon it may help.

It will also eliminate or minimize the need to drain off the bottom of the water heater, as I recommended in an earlier post. The convection process is a lot slower than circulated water, making it more prone to obstructions.