Calculating Propane Usage and BTUs for Tankless Hot Water System

[jadnashua]

IF you needed all of the tankless output to produce the hot water you need, it would be the values indicated, and I also said you probably wouldn't need all of it. So, since one pound of water raised 1 degree is one BTU, and knowing the temperature rise you need and the pounds needed, you ran those numbers. My first calc was using all 400K BTU running full out for an hour. My goal was to give you the info you needed to run the calculations with an example, not do your homework for your!

That's still pretty indulgent, but if you've got it and don't care about your carbon footprint, it's your choice. You'll need a MUCH bigger septic system to accommodate the flow, too. Hopefully, you have enough land, or you'll have a swamp after a bit. Well, maybe not if it is really sandy.

[Diavolicchio]

IF you needed all of the tankless output to produce the hot water you need, it would be the values indicated, and I also said you probably wouldn't need all of it. So, since one pound of water raised 1 degree is one BTU, and knowing the temperature rise you need and the pounds needed, you ran those numbers. My first calc was using all 400K BTU running full out for an hour. My goal was to give you the info you needed to run the calculations with an example, not do your homework for your!

No worries. Bison understood what I needed. I appreciate you responding.

That's still pretty indulgent, but if you've got it and don't care about your carbon footprint, it's your choice.

For the record though, I do care about my net carbon footprint. That's why I'm building a house that only requires 12,500 BTU/hr in the dead of a Maine winter to keep it at 70F. Even with the indulgent shower, I'm guessing that my net carbon footprint is still considerably less than most people. Keep in mind that I'm only using propane for a delta T of 20 to 30 degrees. Everything else is produced by renewable energy (wood) and free recovered energy (GFX on the shower drain). Even the water for the shower is coming from my land and going directly back into it.

Don't be so quick to judge.


John

[Runs with bison]

And Bison us plumbers should maybe stay away from economics but you should stay away from math.

Have you got a basis, or are you just pulling this out of your wazoo the same as you did your declaration that smaller piping produces less pressure drop?

[Runs with bison]

The preheating by the loop in the wood stove was based on nothing more than feedback from a plumber friend of mine. Your suggestion about reversing the order in which the coils get heated makes perfect sense. This system hasn't yet been fully designed, nor have I broken ground yet for the house, so I'm open to any suggestions you have that may make things more efficient. Do I need a 119 gallon tank? Would two 80 gallon tanks hooked up to the boiler loop make more sense? Would a completely different option make better sense? I simply don't know.

I've not ever had a home boiler and radiator set up, so I don't know what sort of arrangement is feasible. (The boilers I've worked with were industrial high pressure in the 1200 psig range.) Others here have had more experience with domestic boilers.

What I can tell you about the boiler loop is that it's "a 16,000 BTU boiler capable of running a single radiator as well as providing domestic hot water." I plan on harnessing this extra energy in the winter both for the shower water pre-heat as well as for a radiator on the 2nd floor of the house. I'm completely open to your suggestions as to how best to get that 20+ degree pre-heat for the shower system.

The problem I see is the 16,000 BTU (Btu/hr?) rate. If I'm interpreting that correctly at 15 gpm that will give a rise of only 16,000 / (15*8.33*60*1) = 2.1 degrees F. By comparison a typical natural gas tank water heater runs ~40,000 Btu/hr input.

I do think you will want a large tank like you were thinking of to get things rolling for showers, but I have no experience with the controls on such a system in the home.

[Diavolicchio]

The problem I see is the 16,000 BTU (Btu/hr?) rate. If I'm interpreting that correctly at 15 gpm that will give a rise of only 16,000 / (15*8.33*60*1) = 2.1 degrees F. By comparison a typical natural gas tank water heater runs ~40,000 Btu/hr input.

I do think you will want a large tank like you were thinking of to get things rolling for showers, but I have no experience with the controls on such a system in the home.

Bison:

Maybe someone else in this Forum will have some constructive suggestions. I'm planning to give a call to the folks at Heat Transfer Products (SuperStor), TriangleTube and ESSE Wood Stoves on Monday to run this by them directly. I like the sounds of TriangleTube's Smart 120 indirect water heater even better than the SuperStor Ultra, but I'll need to have someone calculate just what affect the boiler loop on the ESSE may have in getting a Smart 120 to continuously pre-heat water at least 10 to 12 degrees.

There's got to be a way of making this system work year-round without exceeding a delta T of 30 degrees with the Noritz's and without requiring any additional energy in the winter beyond the boiler loop from the wood stove. I'll post my findings.

Thanks for all of the feedback.


John

[Terry]

I don't know how efficient some of the exchangers are for reclaiming energy,
I did install a few in the 70's, some would reclaim from commercial washers to preheat the incoming.
A lot of it was done from sketches on paper by hand and was left to me to figure out how to do it.

That was a long time ago though.

I've always liked the idea of air heat exchangers.

[Runs with bison]

The key thing that the GFX hit on was the falling film design. It makes perfect sense for resolving the problems of a gravity drain with venting concerns and the like. This maximizes the heat transfer coefficient on the drainwater side while minimizing the potential for plugging.

I haven't had the opportunity to see one of these units in action. I'm skeptical of manufacturer's claims and would like to see their real world performance, especially since short ultra low flow showers and piping lag and lost volumes would hammer the overall performance.

[Diavolicchio]

The problem I see is the 16,000 BTU (Btu/hr?) rate. If I'm interpreting that correctly at 15 gpm that will give a rise of only 16,000 / (15*8.33*60*1) = 2.1 degrees F. By comparison a typical natural gas tank water heater runs ~40,000 Btu/hr input.

Bison:

I've found a great woodburning boiler stove for the living room (here's a better photo) that will easily heat the whole house as well as provide a 50,000 BTU/hr boiler (as opposed to the 16,000 Btu/hr boiler with the ESSE) to pre-heat the water in the storage tank. I'm just not sure what kind of a bump-up in temperature this will give me in the hot water tank if I'm starting with a full tank that's already hot from the boiler loop and bleeding the tank at the rate of 15 gallons/minute for 30 minutes at a time.

I'll talk to the folks at TriangleTube on Monday and have them do some calculations for me. If I'm able to get the cold water that's replenishing that tank at 15 gallons/minute to warm up by at least 10 degrees before leaving the tank, I'm happy with that. A 20 degree increase would be optimal though. I just don't know what 50,000 Btu/hr would be able to do using the technology behind this particular tank. From what the TriangleTube website says, these tank-in-tank systems heat up MUCH faster than the ones with a coil-based heat exchanger. Check out this quick Flash demonstration for the TriangleTube tank and let me know what you think, would ya? It's an intriguing set-up.

If I'm able in the cold months to get the additional 20 degree increase that I'm after from the boiler, two 30 minute/450 gallon showers in a given day will cost an average of $2.40/each in propane (which is the current cost per gallon for propane in my part of the country.) Personally, I think they'll be worth every damn nickel.

Incidentally, without the GFX and wood boiler pre-heat they'd cost over $7.00/each--nearly 3X as much.


John

[Doherty Plumbing]

Have you got a basis, or are you just pulling this out of your wazoo the same as you did your declaration that smaller piping produces less pressure drop?

Yep. I'm a ticketed gas fitter not just some guy who searches google to come up with answer about a field he knows NOTHING about. Your math was wrong plain and simple. Not by a lot but still wrong none the less.

[Diavolicchio]

Yep. I'm a ticketed gas fitter not just some guy who searches google to come up with answer about a field he knows NOTHING about. Your math was wrong plain and simple. Not by a lot but still wrong none the less.

I'll give it 10 minutes and this guy is going to start pulling someone's hair and calling him a doodyface.


John

[jadnashua]

IF you're going to have multiple heating sources for an indirect (and even just one), it's good to put a tempering valve on the output to limit the temperature of the output so you don't get scalded. The only thing you don't want to do is ensure you don't get the water so hot you could start to get steam..that could be catastrophic.

[Diavolicchio]

IF you're going to have multiple heating sources for an indirect (and even just one), it's good to put a tempering valve on the output to limit the temperature of the output so you don't get scalded. The only thing you don't want to do is ensure you don't get the water so hot you could start to get steam..that could be catastrophic.

Thanks. I really appreciate the advice.


John

[Runs with bison]

Yep. I'm a ticketed gas fitter not just some guy who searches google to come up with answer about a field he knows NOTHING about.

Your lack of even the most basic knowledge on piping losses indicates the exact opposite: you comment on things you know NOTHING about.

You did it again with your assinine comment about that parallel GFX set up arrangement.

Your math was wrong plain and simple. Not by a lot but still wrong none the less.

Excuse me if I'm unimpressed at your attempt at a pissing contest. I'm just a chemical engineer accustomed to operating on a much larger scale, chemical plants and refineries. I've designed, modeled, troubleshot and operated various equipment, reactors, exchangers, distillation vessels. etc. I'm accustomed to using natural gas, propane, ethylene, propylene, ethane, hydrogen, carbon monoxide, O2, etc. as reactor feedstock...as well as for simple combustion.

Now, if you want to point out where my "math was wrong", please go ahead. I don't claim to be perfect. (You might also be taking something out of context...that's my guess...I'll wait and see if there is any substance to what you say.) If I have gotten something substantially incorrect then it should be corrected.

[Runs with bison]

IF you're going to have multiple heating sources for an indirect (and even just one), it's good to put a tempering valve on the output to limit the temperature of the output so you don't get scalded. The only thing you don't want to do is ensure you don't get the water so hot you could start to get steam..that could be catastrophic.

I agree.

Diavolicchio

I looked at the flash presentation of the tank and it still isn't clear how this is going to work into the preheating. You will start with a very hot tank so you would have to do a mix control valve to achieve some specific preheat target I suppose. Otherwise it's hot water would be used first, then the big tankless burners would kick in several minutes later. Residential boiler's are an area I don't have experience with. The tankless folks might have some experience working these in.

Even at 50,000 Btu/hr the steady state rise (once the tank was depleted) would be ~7 deg. F.

The project is far from the beaten path and that makes it interesting.

[Diavolicchio]

Diavolicchio

I looked at the flash presentation of the tank and it still isn't clear how this is going to work into the preheating. You will start with a very hot tank so you would have to do a mix control valve to achieve some specific preheat target I suppose. Otherwise it's hot water would be used first, then the big tankless burners would kick in several minutes later. Residential boiler's are an area I don't have experience with. The tankless folks might have some experience working these in.

Even at 50,000 Btu/hr the steady state rise (once the tank was depleted) would be ~7 deg. F.

The project is far from the beaten path and that makes it interesting.

I was just intrigued with the particular slide (the one entitled "tank-in-tank: the advantages") that showed how the tank-in-tank system preheated the same amount of water as the coil-based cylinder tank but in 1/3 the time because of the exceptionally large heating surface area inside the TriangleTube tank.

If each shower requires 450 gallons of 110F water, and the first 100 gallons/shower are already at the proper temp inside the tank, I'm trying to identify the quickest solution for preheating the remaining 350 gallons still to enter the tank using energy solely from the wood boiler. The GFX should then give it an additional 24 degree boost prior to it hitting the pair of tankless water heaters. I guess I'm hoping that the TriangleTube technology will have some kind of edge over the tanks with the traditional coil heat-exchangers to allow me at least an extra 10 degree of preheating before the water hits the GFX.

I'll run this all by the folks at TriangleTube tomorrow and will let you know what they have to say.


John