Sidearm Circulator- Aquastat

[jdeere5220]

This is my first post here so apologies in advance if I mess it up.

My question has to do with how to regulate the temp of my hot water heater. I've built a sidearm heat exchanger which I'm running off of my outdoor corn burner. If I just leave it wide open, convection flow works well enough to heat the tank (80 gal) over a few hours to really hot. But the problems are 1) The water temp is stratified, i.e. too hot at the top of the tank and too cool at the bottom and 2) The recovery rate is quite slow.

So I've installed a small (Taco 003) circulator pump on the potable side of the DHW. This should mix the water in the tank and give us 80 gal of evenly hot water and also speed the recovery rate by forcing water through my sidearm. My problem is, no matter where I put the Aquastat I can't get nice temp regulation.

I installed the circulator so that it pulls water from the top of the tank, runs it through the sidearm, and injects the hot water back into the bottom of the tank. I did this to eliminate the "hot on top and cold on the bottom" issue. So where to put the aquastat? If I put the aquastat on the top, it takes to long to realize when cold water is flowing into the tank. If I put the aquatat on the bottom, it continuously short cycles because it senses the cold water at the bottom, but as soon as the circulator turns on it gets hot water exiting the sidearm and turns right back off.

Maybe just leave the circ pump running all the time? That would give me good temp regulation but would waste electricity..... Any ideas on how I can fix this up?

 

[stee6043]

First off it sounds like you don't have a mixing valve on your potable water line? Yes? You really should have a mixing valve installed that prevents scalding hot water from reaching your plumbing fixtures. This should help with your recovery time as well. With a mixing valve installed you can heat the water in the DHW tank as high as possible (say 160 degrees or more) and you're only actually outputting 120 degrees. This will save you hot water.

As far as the stratification issue I haven't ever heard of anyone complaining about that in the DHW tanks. Perhaps addressing the mixing valve issue will solve all of your problems??

 

[jdeere5220]

Yeah, I guess you nailed it. I was trying to hide the fact that I didn't install a mixing value. I don't like them because they tend to rust-up over time with well water so you have to replace them every so often and they are also pretty expensive ( > $100 ??) and I wonder if they adversly affect flow rate?

So we are basically mixing at the faucet right now. I know it's not safe so please don't holler at me. Regardless we are running out of hot water (3 women!) so I need to improve the recovery. I thought by installing a circ pump and aquastat I might kill two birds with one stone.

 

[Eric Johnson]

You can get mixing valves these days for around $15, I think. Last time I saw one (a Taco, I believe) at Home Depot, that was the sale price. Not much to it, actually. They don't sense anything--just mix the water to whatever you set it at, so you're still going to get variable temps at the beginning of the draw.

Recovery time shouldn't be all that bad if things are working correctly. It won't be as fast as electric or gas, but you should be able to get hot water at the top of the tank in about a half hour from a cold start. Overnight, the tank will heat completely if you have hot enough water going into the heat exchanger.

One thing to watch: you might want to drain off 3-5 gallons once a week from the bottom drain on the tank if you have hard water. I have a water softener (city water), and I still get enough crap accumulating at the bottom of the tank to stop convection if I don't give it a good flush ever couple of weeks. You shouldn't have that problem with a pump, however. And you should get faster recovery.

 

[Jim Post]

I have my sidearm piped with ball valves so I can bypass it when we don't need domestic hot water....We can open the flow to the sidearm and have hot water about 20 minutes later... we haven't run it out of hot water yet...I do have a mixing valve and depending on how long the sidearm has been open the shower temp will be fine with just hot on at the tap. It can take a couple of hours for the sidearm to fully heat the water heater tank from top to bottom...it will do it though.

jp

 

[Sting]

Yeah, I guess you nailed it.

So we are basically mixing at the faucet right now.

I know it's not safe so please don't holler at me..

Who would do that??? That's reserved for other places on the public internet

 

[Duetech]

jdeere5220,
My dhw tank is smaller so it heats faster I guess. But the big thing I have found is the water from my boiler is much hotter than what the water heater is used to and mineral deposits in the heater tend to block the inner pipe of the sidearm and slow the heating process and need to partially flush the tank annually. We have a metering valve and have not experienced a flow rate problem. The amount we have saved not using propane to heat water has paid for the valve more than once in the time we have had it. It has never failed though we have soft water. In principle there is no way you should have sustained stratification in your dhw tank if you have consistent boiler flow and once again the amount of energy that a circ./pump uses is far less than the gas/elec. you would use. We heat dhw with the boiler year around and that means the boiler circ. and the secondary circ. are basically running all the time. If you could plum so that your dhw is part of the primary circ then you could drop the secondary (if that is the way it is plumbed right now) and only have one circulator active. My primary is "boiler only" and the secondary is dhw and the air/water heat exchanger in the oil furnace. In the summer we only have short term daily fires for dhw (no storage yet) so it is not as wood consuming as it sounds.

 

[Vtgent49]

First, the mixer will save a lot of hot water, as the hot water left standing in your pipes will be mixed, not pure hot.

Second, if 1/2 of your tank is above 105, then you'd have 40 gals at least, more like 50 mixed gals. That's plenty.

Third, your circ set-up is backwards. Injecting hot into the bottom will just stir your tank, causing all the water to become cool.

I suspect your original set-up was plumbed a bit incorrectly. It could be that the Cold-in pipe was either dumping cold at the top, or letting it short circuit it's way in. We've seen a few pics of sidearm set-ups that have this problem.

Stratification is the best way to assure that the hottest is available.

 

[jdeere5220]

First, the mixer will save a lot of hot water, as the hot water left standing in your pipes will be mixed, not pure hot.

That is an excellent point, thanks!

Third, your circ set-up is backwards. Injecting hot into the bottom will just stir your tank, causing all the water to become cool.

I can reverse it. I did it this way because when the tank is running on propane, it heats the bottom, so I figured my sidearm should probably work the same way. Otherwise I'll end up with hot water on the top and cold water on the bottom, right?

Here's a pic. I used 1.5" copper for the outer tube and 3/4" copper for the inner tube. I've insulated the whole thing since this was taken. That's a taco zone value in the lower right, to which I've since connected a Honeywell thermostat.

Don't get me wrong, the system works, I'm just tinkering trying to make it work a little better.

 

[Vtgent49]

Looks good, a couple of ideas:

What is water temp from boiler as it reaches and leaves the sidearm. Then is the tank top getting near this temp?

What logic is used to open the zone valve?

The stack/flue inside the gas-fired tank is an ideal chimney for cooling... Which is why I have electric tank.

You may have reverse thermosiphon when feed water is cool. I stuck a check valve in mine, right next to the drain cock. Just a swing gate, not spring loaded.

You might use the circulator to encourage the flow, but only if the boiler return water is still hot as it leaves. I wouldn't reverse the flow though.

 

[jdeere5220]

Looks good, a couple of ideas:

What is water temp from boiler as it reaches and leaves the sidearm. Then is the tank top getting near this temp?

I don't have a thermostat to check this accurately. Can you recommend a simple, portable strap-on type that I could use to monitor things like this?

But I do know that the tank is getting near the boiler temp. If put the aquastat on the top (where the TP valve is), set to 150F, it gets there. The boiler is generally 165-173F, depending on if my furnace is pulling it down.

What logic is used to open the zone valve?

Well that's sort of where I started. I first put the open-on-rise Honeywell strap-on aquastat on top, set to 150 with 5 deg differential. It works, but if there is a water drain like someone takes a shower it takes a LONG time to close (aquastat close = opens the zone valve) or kick on. Cold water comes into the bottom of the tank, so the aquastat mounted on top doesn't react quickly.

I also tried putting it on the bottom, set to 100F with 20F differential. It works better that way, except with the reverse circulator flow it short-cycles.

The stack/flue inside the gas-fired tank is an ideal chimney for cooling... Which is why I have electric tank.

Yes, I noticed Bradford White is making units that have an internal HX now, so when this tank goes I'll upgrade to one of those.

You might use the circulator to encourage the flow, but only if the boiler return water is still hot as it leaves. I wouldn't reverse the flow though.

OK, got it, circulator needs to go the same direction as convection flow. Thanks for all of the input!

 

[in hot water]

heat exchangers work best with counter flow. if the hot boiler water goes in the bottom and out the top, the DHW should flow the opposite. Two pumped flows will transfer energy better than thermosiphon, but a stratified tank will provide more hot water than a mixed tank. Although 80 gallons of 180F water should be plenty for any household.

You need flow protection on BOTH sides of the side arm HX to prevent two way flow in a single pipe, ghost flow. Even with the zone valve closed you will get enough flow to overheat the tank. A spring check on the other side of the sidearm HX would prevent ghost flow.

The aquastat should be about 12" up from the bottom of the tank, typically where the burner aquastat or electric element and control are located.

I'd highly recommend a listed 1017 mixing valve on the outlet of that heater! Install it with a thermal trap as per instructions and isolation valves to service and delime it if you have hard water.

hr

 

[Sting]

OK, got it, circulator needs to go the same direction as convection flow. Thanks for all of the input!

yes

BUT

when you first asked me-- you were not going to incorporate a thermostatic mixing valve to protect the family

that's why I told you to do it incorrectly on the other site - it WAS the most safest possible solution for your wishes because you would have been building tank of 120 degree DHW and not stratifying a tank that was over temp at the top -- near the point of use.

Glad your getting it worked out now

Kind Regards
Sting