anti-freeze in closed loop?

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Old 08-14-14, 09:41 AM
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anti-freeze in closed loop?

I plan to use a direct vent tankless water heater in a closed loop to heat a large aquarium. A heat exchanger will be in the aquarium's sump. The smallest units I've seen have more than enough BTUs to easily do the job.

Thankfully it's unusual, but sometimes it gets down to -20F here in NE Ohio. The problem is, only the very expensive units are freeze resistant. As I read reviews on the low budget tankless units there were a few complaints that would not matter in my application and a consistent complaint of freezing.

If I used a check valve between the closed loop and the house water, would it be safe to add anti-freeze to the closed loop?

Joe
 
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Old 08-14-14, 10:38 AM
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Hi Joe,

First, you need to verify that the anti-freeze you use is acceptable to the heater manufacturer.

You do NOT under any circumstances want to use automotive type (ethelyne glycol) in ANY boiler system.

You will be looking for a PROPYLENE glycol type. These are relatively non-toxic types of AF that are made for heating systems.

Yes, a check valve, but more specifically a proper BACKFLOW PREVENTER that is acceptable to your local building code jurisdiction.

Backflow preventers have 'atmospheric vents' on them, and several layers of check valves.

A common one that is used with boiler systems is the " WATTS 9D " but there are others.
 
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Old 08-14-14, 06:08 PM
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Thank you NJ Trooper! That's outstanding information-I appreciate it a lot.

Would the vent for the WATTS 9D be the same as one used on an air scoop?

If I use propylene glycol with a corrosion inhibitor, would a cast iron pump be a good choice?
 

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Old 08-15-14, 07:52 AM
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Would the vent for the WATTS 9D be the same as one used on an air scoop?

If I use propylene glycol with a corrosion inhibitor, would a cast iron pump be a good choice?
A backflow preventer, such as the 9D, is not the same as an air eliminator vent. Google Watts 9D for info.

Cast iron pumps are customarily used in hot-water systems whether there is a corrosion inhibitor or not. Corrosion inhibitors are not necessary if there is an effective air eliminator for the system.
 
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Old 08-15-14, 12:13 PM
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Thanks gilmorrie.

I did look at the watts 9d online. The specs look ideal for my application. The word "vent" is stamped into the 9d beside what appears to be female NPT threads. Which led me to believe a vent needs to be added to the 9d just like a vent needs to be added to an air scoop. So now I'm not sure if the 9d needs to have a vent added or not. In any case you guys are really moving me along the right track-thanks!

It's good to know using an air eliminator, what I've been calling an air scoop, helps prevent corrosion.
 
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Old 08-15-14, 12:30 PM
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Hi Joe,

The 'vent' on the 9D can either be left open if water discharge from that vent won't cause property damage, or can be piped to an appropriate area where water can be discharged in the event that the backflow preventer operates.

Best practice is to not install the backflow preventer where anything that can be damaged by water is below it... for example, don't install over boiler controls, etc.

Backflow preventers work on differential pressure principles. If the pressure on the inlet side drops below the pressure on the outlet side, multiple check valves prevent backflow, and the 'vent' will dump water in some cases...
 
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Old 08-15-14, 04:27 PM
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I'm intrigued by your heating an aquarium with a water heater. Is the water heater electric or fueled by gas or oil? It seems like electric would be far simpler.
 
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Old 08-15-14, 05:31 PM
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When using Propylene glycol in a heating system it should not have a fresh water hook up , instead a low water cut off switch should be used to protect the boiler. This of course requires a tight system to keep system on. This is only my opinion based on my experience.
 
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Old 08-15-14, 06:15 PM
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Thanks NJ Trooper for that explanation. I like not having to get an additional vent. (:

gilmorrie, the tankless will be gas fired. The aquarium and sump are about 800 gal total. Sometimes I'll need to maintain water temp 20 degrees warmer than ambient temp. It's a chunk of change upfront, but gas will be considerably less expensive than electricity. There will be electric heaters set below the normal water temp in case the closed loop system fails.

saves, what's the problem with using a fresh water hookup on a closed loop with propylene glycol?

Thanks guys.
 
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Old 08-15-14, 06:49 PM
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salty joe the problem is diluting the glycol to the point it all has to be replaced . If it is not diluted only the leak has to be found and fixed and the lost glycol replaced , much cheaper .
 
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Old 08-16-14, 04:45 AM
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Is the aquarium in a separate building, isolated from the heat source(s) and with the feed and return lines traveling to and from underground ?

It just seemed to me that all this concern with propylene glycol is for naught if the power goes out and you come to rely on the freeze protection of the glycol, you'll have even bigger problems for the sealife inside the aquarium anyway.
 
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Old 08-16-14, 11:34 AM
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The aquarium is in a dedicated "fishroom". It's an inwall job and is viewed from the front room. The closed loop will be in the basement. The anti-freeze is to protect the direct vent tankless water heater.

Can I use the same PEX that's used for drinking water?
 
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Old 08-16-14, 01:49 PM
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For chemically treated water I would strongly recommend a Reduced Pressure Backflow Preventer. This is a much more complex device than the Watts 9D and priced accordingly.

No, you may NOT use regular water piping PEX but must use a PEX with an oxygen barrier to prevent internal corrosion.

I would also suggest an inhibited polyethylene glycol to further protect the metallic surfaces of the water heater, the pump and the heat exchanger.

You might need a heat exchanger that has a polyethylene coating to prevent any interaction with the aquarium water. Copper is highly toxic to marine life.
 
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Old 08-16-14, 04:16 PM
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OK, I'll use PEX with an oxygen barrier. Thanks Furd.

There will be a 100' coil of PEX in the aquarium's sump as the heat exchanger.

When I was looking at polyethylene glycol I saw the type with corrosion inhibitor. I liked it too. Any downside to using it?

It's my understanding that the risk of backflow is greatly diminished when the closed loop has lower pressure than the feed line. The tankless water heater I'm looking at runs fine at 30 PSI. My house is set at 60 PSI. I plan to install a pressure regulator right next to and downstream from the 9d and set the closed loop pressure at 35-40PSI. Is that a sensible plan?

Edit; On a side note, I buried 500' of 1/2" regular PEX along my footer a few years ago to see if I could get a little geothermal cooling for the aquarium. I had no clue about O2 barrier. Could a brass pump be used there in conjunction with a corrosion inhibitor? It won't be connected to the closed loop for heating, just thought I'd ask.
 
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Old 08-16-14, 05:32 PM
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It's my understanding that the risk of backflow is greatly diminished when the closed loop has lower pressure than the feed line. The tankless water heater I'm looking at runs fine at 30 PSI. My house is set at 60 PSI.
/That sounds good 99.9% of the time, but there are ways that the city water pressure can drop below the heating system. That's why outdoor hose bibs now need to have back-flow preventers. My city water utility requires backflow preventers in a boiler fill line - they ask me, in writing, every year or so the model of my backflow preventer, and they have the right to inspect.

A brass (or more likely, bronze) pump is fine with a corrosion inhibitor. But your 500' of Pex without an oxygen barrier is not OK if there are any ferrous materials in the loop. I suspect that there are some.
 
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Old 08-17-14, 07:37 AM
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OK, I'm looking at reduced pressure backflow preventers. How long do these units typically last? How do you know when one has failed? The reason I ask is I was looking at used as well as new.

The tankless WH will be about 60' from the heat exchanger. I am considering a Taco 007 f5 pump. I am not sure if I should use 1/2" or 3/4" PEX. I like 1/2" because it's easier to work with and there would be less cold water in that 120' PEX between tankless WH and the heat exchanger. Based on others experience, 100' 1/2" PEX is sufficient for the heat exchanger. The tankless WH has 1/2" connections, everything else I've looked at has at minimum 3/4". Any suggestions, corrections, ideas all very much appreciated.

That 500' 1/2" regular PEX in the ground is a continuous coil, so with a bronze pump I might be OK? It sure would be nice to get some cooling for the energy to run a small pump.

Thanks guys.
 
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Old 08-17-14, 04:07 PM
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That 500' 1/2" regular PEX in the ground is a continuous coil, so with a bronze pump I might be OK?
The pump is probably OK, but are there any ferrous materials in the loop? Valves? Heat exchanger? etc.
 
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Old 08-17-14, 07:14 PM
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I am looking at PEX brand ball valves. They have a brass body and Teflon seats, hopefully the ball is a decent grade stainless. I should have an answer in a day or two as to whether they are OK to use with regular PEX.

The air scoops I've seen online appear to be cast iron, but it seems using a 1" (or a larger diameter than the rest of closed loop) copper T with a copper or brass plate or maybe even a quarter soldered on top with threads to accept an air vent ought to work. I could do the same sort of thing for the expansion tank.

The tankless WH will be bypassed in the summer.

The heat exchanger will be a coil of PEX.

That leaves the backflow preventer, float vent and expansion tank. Am I asking for trouble using these parts with regular PEX?
 
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Old 08-18-14, 12:27 PM
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The seller said there is no ferrous metal in the PEX brass valves.

It would be simple enough to bypass the tankless WH and run the water through the 500' buried regular PEX, but don't want to do that if it's going to destroy the backflow preventer, float vent or expansion tank.
 
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