Tankless water heater instead of conventional heaters for larger system.

[cilyjr]

I have a 320 gallon display, 75 gallon sump, 30 gallon frag system all plumbed together. I dose based on 400 gallons of actual water.

Electricity has become increasingly expensive in the state I live in and solar is less economically feasible. This is all different discussion, but in an effort to remove some of the reliance on electricity.

I started discussing this in another thread but in an effort to not completely hijack that, I'm posting this idea here.

The idea would be to use a small tankless water heater, specifically the NR501-OD-NG.

The advantage of a tankless water heater to heat the fish tank would be that it is as on demand same as an electrode heater. Meaning it only fires when the pump calls for circulation through the system. The water heater I listed will run as low as .5 GPM (which is 30 gph) and as high as 5 gpm with a 100° temperature rise at 2 GPM which should be easy enough to do with a 200 gph pump. Not to mention the majority of the water that will circulate which would be fresh water would primarily be sitting in the " heat exchanger" made out of PEX In the sump. So if it is off for any amount of time, the water should hold at roughly 78°. When the pump calling for heat turns on it would be circulating 78° water through a 100° temperature rise.

Will pex do the trick for the heat exchanger because it's really only happy at 200° or less and we are getting close to that threshold in this system?

I'd love to hear some thoughts on the basic idea.

 

[tzabor10]

How is it working out with the idea?

 

[cilyjr]

How is it working out with the idea?

I haven't installed it yet, I'm curious what people think.

 

[BeanAnimal]

It is 120,000 BTU 83% efficiency.
So it (in theory) can impart ~99,000 BTU (per hour) of energy into the water.

That is 1,650 BTUs per minute.
You have 3300 pounds of water to heat.
It takes 1 BTU to raise 1 pound of water 1 degree F.
You set your controller to 1F of hysteresis.
That means you need 3,300 BTUs to raise the whole tank by 1F

That is (in theory) a two minutes heating cycle if it were heating the water directly.
You are going to use a heat exchanger. It will drop efficiency a but (can be overcome, but introduces more latency)

Thoughts -

You are going to need (MUST HAVE) a decent sized expansion tank on the heat loop.
The min setting on most is 100F. So you water bath is going to run pretty hot.

f you turn it off off the tank loop pump it will convection cycle until the water bath reaches tank temperature, as well as grow nasties in the on/off closed loop that has all of that residual heat.

If you leave it running, then of course all of the residual heat will find its way into the aquarium. So the size of the water bath becomes important. The smaller the water bath, the less the latent heat but the higher the Delta T putting HOT water into the tank. A larger bath (given a reasonably short cycle time) will put cooler water into the tank but due to latency have a MUCH larger overshoot.

So this will likely need to be controlled with a pretty sophisticated PID and possibly even more logic than that.

Can it work? Maybe.

I would (without question) put a Kill-a-watt on the current heaters and figure out exactly what the nominal daily operating cost is before trying to solve for it.

 

[cilyjr]

At 8.5 amps in 20 minute cycles 6 to 9 times daily, with an off peak rate of .46 cents I'm looking at about 500 annually. It would only dip into peak at $0.50 in late January through early March.

Winter daily high temps are usually low 60s with summer mid 70s
The night temp is 40 ish winter to 55 summer.

I cannot look up natural gas costs as the site is in maintenance now

 

[cilyjr]

You are going to need (MUST HAVE) a decent sized expansion tank on the heat loop.

Yup.

as well as grow nasties in the on/off closed loop that has all of that residual heat.

I've debated this in my head. I've thought about using something other than water but should a leak occur. I don't want that in my tank obviously.
I'm wondering how much would really grow if the closed loop was filled with RO water that is consistently being heated to 170°.

 

[BeanAnimal]

You actually may get better value and control out of a heat pump style setup. They operate at a much lower temperature and would have a more controllable latency I would think. I would have to research it a bit to see the bet way to set it up.

 

[BeanAnimal]

Yup.

I've debated this in my head. I've thought about using something other than water but should a leak occur. I don't want that in my tank obviously.
I'm wondering how much would really grow if the closed loop was filled with RO water that is consistently being heated to 170°.

I am not worried about the closed loop. It is the tank loop that will be the issue.

The water bath is either going to have to be HUGE to prevent sharp cycles and a tone of latent heat drifting into the tank or the smaller bath is going to remain very hot and the tank loop turned on/off to regulate temp. This means water stuck in the loop is going to get nasty.

I think your system is too small for the BTU output of the heater to make it work well.

 

[VintageReefer]

I switched my house to tankless water heater and what I’ve noticed is it’s very consistent if everything is consistent.

If I have a shower running at 110 degrees or will keep putting out water at 100 degrees forever.

If you turn on a second shower, the temp will drop for a few seconds and the unit adjusts to needing to output 110 to two showers. If the second shower turns off, the remaining one will output very hot water for a few seconds while the unit adjusts back to the lower demand and the remaining hot water in the plumbing gets used and replaced with the new temp water

 

[cilyjr]

I think your system is too small for the BTU output of the heater to make it work well.

Maybe.

So this will likely need to be controlled with a pretty sophisticated PID and possibly even more logic than that.

I'm going to talk to the noritz tech rep Monday and get his thoughts. The units control board should be able to handle a range of conditions. And the apex can handle cycling the system.

I'm not sure we need Skynet to run the thing.

I know a guy back in Rhode Island who was using a pot on his pellet stove with a maxi jet and a flex PVC " heat exchanger". Sort of the 49 Ford version of what I'm thinking of. I just pinged him and I'm hoping he can add some insight to this thread.

At the end of the day it is literally just whether or not it will save money.

The unit costs $600 and I can get it at a 20% discount. There is also a $200 rebate from the state. In order to make this work, I would have to run about 20 ft of gas flex and then build pex exchanger, in addition to that I would need a tankless flush kit and a expansion tank. All in all I feel like it would cost me about 800 bucks to build.

If it reduces my electric bill by $42 a month and adds $10 to the gas bill. I see a return in 25 months.

 

[cilyjr]

I forgot to mention there's extra cost savings for me in this scenario as my wife pays the gas bill and I pay the electric bill....
Lolol

 

[cilyjr]

I just saw an older post about someone who has been doing exactly this for several years. I hope he chimes in here.
He's using a TACO pump and a titanium pool heat exchanger.

 

[theatrus]

You actually may get better value and control out of a heat pump style setup. They operate at a much lower temperature and would have a more controllable latency I would think. I would have to research it a bit to see the bet way to set it up.

This. You can buy ready made heat pumps with titanium heat exchangers.

I wouldn’t try to use pex as the heat exchange loop - you can buy some titanium coils from AliExpress pretty reasonably.

 

[OrionN]

Did not read your post throughly enough.

 

[cilyjr]

This. You can buy ready made heat pumps with titanium heat exchangers.

I wouldn’t try to use pex as the heat exchange loop - you can buy some titanium coils from AliExpress pretty reasonably.

Maybe but the goal is to get get off of electricity.

 

[Jimbo327]

I don't think PEX is a good heat exchanger. Like others have said, use a better material (titanium) with better heat conductivity.

 

[cilyjr]

I don't think PEX is a good heat exchanger. Like others have said, use a better material (titanium) with better heat conductivity.

I agree that the PEX would not be as efficient. I sorta just learned about the titanium pool heat exchangers.

 

[iReefer12]

Hi, not the same but similar.

I heat my 700g using a titanium heat exchanger and water from my gas boiler.

Heated 700 gallons of water from 68F to 78F in roughly 3 hours IRC. Now it turns on for about 60 mins 4 times a day to keep the water at 78F. Keeps the water temp within 1F.

It’s very important to have redundancy and failsafes in place. I have the pump that circulates water from the boiler & ball valve connected to a Ranco temp controller, this is the main control. Then the Ranco is plugged into an EB832 outlet and Apex is the failsafe to shut the whole system down if needed. Then the pump that circulates water from the aquarium also is controlled by and EB832 outlet and will shut off if the temp ever reaches 80F.

I packaged the whole thing onto a compact board, and I’ve been happy with the results. Someone on here did a YouTube video and I just showed that to a plumber to have him connect the unit up to the boiler the right way.

 

[iReefer12]

This is not the exact one I bought, but this is very similar.

55k BTU Titanium Tube and Shell Heat Exchanger for Saltwater Pools/Spas ss | eBay

55,000 BTU Titanium Tube and Shell. - Swimming pools, spas heating. BTU rated with 180F Boiler and 80F Pool. Port to Port -7 1/2". - Hot tube heating. Nominal values are based on 60°C temperature between incoming heating and heated water.

www.ebay.com

 

[iReefer12]

Hi, not the same but similar.

I heat my 700g using a titanium heat exchanger and water from my gas boiler.

Heated 700 gallons of water from 68F to 78F in roughly 3 hours IRC. Now it turns on for about 60 mins 4 times a day to keep the water at 78F. Keeps the water temp within 1F.

It’s very important to have redundancy and failsafes in place. I have the pump that circulates water from the boiler & ball valve connected to a Ranco temp controller, this is the main control. Then the Ranco is plugged into an EB832 outlet and Apex is the failsafe to shut the whole system down if needed. Then the pump that circulates water from the aquarium also is controlled by and EB832 outlet and will shut off if the temp ever reaches 80F.

I packaged the whole thing onto a compact board, and I’ve been happy with the results. Someone on here did a YouTube video and I just showed that to a plumber to have him connect the unit up to the boiler the right way.

Here’s some pics.

Laying the pieces out on a board.


Mounted on the wall. Just missing the connection to the tank lines. (I can post an updated pic tomorrow if that will help).


The apex log of temp & the heat pump turning on/off over 24hrs.