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# Thread: Calculating Propane Usage and BTUs for Tankless Hot Water System

1. ## Calculating Propane Usage and BTUs for Tankless Hot Water System

I hope there are some techies following this forum who may be able to help me do some basic calculations regarding anticipated propane usage.

I'm building a new home this coming year and one of the indulgences I'm allowing myself is a luxury shower with 6 shower heads, each using 2.5 gpm (or 15 gpm total) of hot water. I know how incredibly wasteful this sounds, so I'm utilizing some technology to get my costs down on heating the water. It's also water coming from my own well and going back into my own land.

I'm trying to calculate the amount of propane I'll need to use on a daily basis in two different scenarios--summer and winter.

My basic set-up consists of a TriangleTube Smart 120 indirect hot water heater that is preheated by a boiler loop coming from the wood stove heating the house in the cold months. A GFX Drain Heat Recovery System will also preheat the water going into the on-demand water heaters by means of a copper coil wrapped around the shower drain. The preheated water will then be fed into a pair of Noritz 0841 condensing tankless water heaters. In addition, I'll be using a Puritec MC-14 whole house well water filter and hard water conditioner to filter out any impurities.

In the summer months, my well water is around 56F and my goal is to be able to produce 15 gpm indefinitely at 110F. The GFX coil on my drain will be able to preheat the well water from 56F to 80F, leaving a needed rise in the summer of 30 degrees to get to 110F.

In the winter months, my well water is around 45F. I'll have the benefit in the winter of preheating the water twice, once with the boiler loop between the wood stove and indirect hot water heater, and a second time with the GFX coil on the shower drain. The boiler loop should get the water from 45F to 65F, the GFX from 65F to 89F, leaving a needed rise in the winter of 21 degrees to get it to 110F.

I'm anticipating using the shower at full capacity (15 gpm) for a full hour every day. How do I calculate how much propane I'll need to use on a daily basis, both in the summer and in the winter?

By the way, the pair of on-demand water heaters I'm using are 94% efficient, if that needs to be factored in here. They each have a maximum consumption of 199,900 btu/hr, for a total of 400,000 btu/hr for the pair. Note that I'll be setting the on-demand water heaters to the exact water temperature I want because it won't be mixing with any cold water.

I'm at a loss how to calculate the BTUs needed on a daily basis, and the subsequent amount of propane needed.

Anyone willing to help me calculate this? I'd greatly appreciate it!

John

2. 22K BTU/pound of propane. 400K/22K=18.18pound per hour/4.23pound/gallon = 4.3 gallons. Throw in some efficiency losses and 5 gallons/day, then add a bit more for routine hot water use, and you may have another gallon (probably less). Now, that's at full output, and that is likely to make the water hotter than you need. So...

If you figure roughly 8#/gallon of water and you have 15gpm*60min*8=7200# of water per hour. So, it will take 7200BTU for each degree you need to raise the water temp. WIth these numbers, you can do the rest of the calculations.

22K BTU/pound of propane. 400K/22K=18.18pound per hour/4.23pound/gallon = 4.3 gallons. Throw in some efficiency losses and 5 gallons/day, then add a bit more for routine hot water use, and you may have another gallon (probably less). Now, that's at full output, and that is likely to make the water hotter than you need. So...

If you figure roughly 8#/gallon of water and you have 15gpm*60min*8=7200# of water per hour. So, it will take 7200BTU for each degree you need to raise the water temp. WIth these numbers, you can do the rest of the calculations.
Jim:

I'm still a little confused here. You say I'd be using roughly 5 gallons of propane per day if I used this shower for a full hour at full capacity, and allowed for other incidental hot water usage in the house.

Yet when I follow your other numbers of 7200BTU/each degree, I get:

Summer: 7200*30 = 216,000 BTU/hour
Winter: 7200*21 = 151,200 BTU/hour

And dividing these by 91,500 BTU/gallon of propane, I get:

Summer: 216,000/91,500 = 2.36 gallons propane / hour
Winter: 151,200/91,500 = 1.66 gallons propane / hour

Why is there such a disparity between the 4.3 gallons of propane you're estimating for an hour of showers/day and the ~2 gallons of propane I'm estimating when I work backwards from BTUs/gallon of propane?

I hope it's your calculations that are off, because I can't imagine spending \$12/day just to take 2 indulgent showers. That's \$4,380 a year using significantly preheated water!

John

4. Originally Posted by Diavolicchio
Jim:

I'm still a little confused here. You say I'd be using roughly 5 gallons of propane per day if I used this shower for a full hour at full capacity, and allowed for other incidental hot water usage in the house.

Yet when I follow your other numbers of 7200BTU/each degree, I get:

Summer: 7200*30 = 216,000 BTU/hour
Winter: 7200*21 = 151,200 BTU/hour

And dividing these by 91,500 BTU/gallon of propane, I get:

Summer: 216,000/91,500 = 2.36 gallons propane / hour
Winter: 151,200/91,500 = 1.66 gallons propane / hour

Why is there such a disparity between the 4.3 gallons of propane you're estimating for an hour of showers/day and the ~2 gallons of propane I'm estimating when I work backwards from BTUs/gallon of propane?

I hope it's your calculations that are off, because I can't imagine spending \$12/day just to take 2 indulgent showers. That's \$4,380 a year using significantly preheated water!

John
Jim:

I forgot to point out one thing. Extrapolating from the chart on the Noritz site (see specifically the 841/842 series column) and factoring in the necessary rise in temperature, 400k BTU/hr for the pair of Noritz would give me 23.5 gallons/min of hot water in the summer (with a 30 degree rise) and 28.0 gallons/min of hot water in the winter (with a 21 degree rise). There would need to be a 50 degree rise for the 400k BTU/hr to provide me with no more than the 15 gallons/min I'm after. Make sense?

Let me know, would you? I may have this totally backwards.

John

5. Originally Posted by Diavolicchio
I hope it's your calculations that are off, because I can't imagine spending \$12/day just to take 2 indulgent showers. That's \$4,380 a year using significantly preheated water!
John,

If it was municipal water and sewer like here you could add another ~\$4/day for those.

Is your well going to be able to produce this much water each day and will your septic tank and field lines handle it? I would imagine the size of both is effectively doubled compared to what it would be otherwise.

6. Originally Posted by Runs with bison
John,

If it was municipal water and sewer like here you could add another ~\$4/day for those.

Is your well going to be able to produce this much water each day and will your septic tank and field lines handle it? I would imagine the size of both is effectively doubled compared to what it would be otherwise.
Thanks for the feedback. I'm glad the numbers look to be closer to 2 gallons/day rather than 5!

I lucked out with my well; it produces 20 gallons/minute. Pressure drop with the GFX I'll be using will be around 2.7 psi. You'll find a schematic of the particular unit here.

John

7. Man, that is a lot of flow. An hour at 15 gpm is 900 gallons...that is 9 times as much total water as what my family uses in a day. It is good that you will be able to recover some of the heat through a falling film heat exchanger. However, I do wonder about how much area it will have and how much it can really recover. What length of unit, what tubing size, and what pressure drop do they project for 15 gpm?

Back of the envelope, even with heat recovery it looks like your water heating fuel use will equal or exceed what it takes to heat my home.

Since you say you will heat 900 gallons/day by 30 F in summer and 21 F in winter with a 94% tankless heater I believe the calcs work out as follows:
Winter = 15 gal/min * 60 min/day * 8.33 lb/gal * 1 Btu/lb F * 21 F / 0.94 = 167,500 Btu/day
Summer = 15 * 60 * 8.33 * 1 * 30 / 0.94 = 239,000 Btu/day

Propane's heating value is 91,600 Btu/gal. So you would use 1.8 gal/day in winter and 2.6 gal/day in summer if I did the calcs correctly.

Of course if the preheating falls short you will need more than this. Plus I've not included water heating for other uses: handwashing, dishwashing, clothes washing, etc.

8. Originally Posted by Runs with bison
Back of the envelope, even with heat recovery it looks like your water heating fuel use will equal or exceed what it takes to heat my home.
The funny thing about it is that the house into which this system is going will be so super-insulated, that in the dead of winter here in Maine, it'll take around 12,500 BTU/hr to keep the house at 70F when it's -20F outside. And I'll be heating predominantly with wood.

If it weren't for this, I wouldn't be allowing myself the indulgence of such an over-the-top shower.

John

9. Here's an illustration of the shower set-up. I'm eliminating the transfer valve so that all six shower heads can work simultaneously, and setting the whole system up so custom showers can be pre-programmed using a digital thermostatic valve and interface:

I think it's a pretty safe bet that when you step out of one of these showers in the morning, you're AWAKE.

* * * * *

Here's a photo of a 4-column GFX similar to the one I'll be using, but this one uses PVC manifolds instead of copper. It gives you a good visual of the set-up though:

John

10. ## Gfx

IF you take the money you will save by using the GFX system and add about \$4.45 to it, you MIGHT be able to buy a \$4.50 Starbucks cup of coffee. By the time your "hot water" goes down the drain it will not have enough residual temperature to do much preheating, especially if the water does not gravitate to the pipe walls while flowing down the risers, and if your drain system looks like the one in the picture, you have created a drain nightmare. Especially when it comes to unplugging it, because you will not know it is getting obstructed until ALL four risers are plugged, and then unplugging them all will be somewhat expensive.

11. Originally Posted by hj
IF you take the money you will save by using the GFX system and add about \$4.45 to it, you MIGHT be able to buy a \$4.50 Starbucks cup of coffee. By the time your "hot water" goes down the drain it will not have enough residual temperature to do much preheating, especially if the water does not gravitate to the pipe walls while flowing down the risers, and if your drain system looks like the one in the picture, you have created a drain nightmare. Especially when it comes to unplugging it, because you will not know it is getting obstructed until ALL four risers are plugged, and then unplugging them all will be somewhat expensive.
I appreciate your 2-cents. Seriously. I've done my research though and am content with what I've learned and the technology I'll be utilizing. The water in this particular shower should hit the drain at around 95F and have no problems doing its job clinging to the pipe walls.

My drain system will be a bit different from the one in that photo. I just posted that because it was the best example I could find of a 4-column GFX. They do work though if you size them and set them up properly. I know a number of people who have them and are quite happy with the results.

I will say though that I was saddened to learn you'd waste \$4.50 on a mediocre cup of designer coffee. Isn't that a little wasteful?

Cheers,

John

12. Originally Posted by hj
IF you take the money you will save by using the GFX system and add about \$4.45 to it, you MIGHT be able to buy a \$4.50 Starbucks cup of coffee. By the time your "hot water" goes down the drain it will not have enough residual temperature to do much preheating, especially if the water does not gravitate to the pipe walls while flowing down the risers,
Plumbers should stay away from bold statements about economics, it isn't their strong suit. For the extreme amount of hot water he is using and propane this will easily pay for itself if used as stated.

The blanket assumptions you make don't hold a candle to doing actual calculations and measurments. Economics for my own use in DWHR are marginal because my home uses 1/27th as much hot water for showering as what this fellow is talking about.

I might try one of these systems just out of curiosity. The deal killer in the economics for my home is not so much the cost of the exchanger, it's the additional cost of paying a plumber to install it. That holds true for about anything that requires plumbing or flue changes. And with the serious attitude that plumbers seem to have (not to mention frequent competency problems), it probably makes more sense to cut them out and DIY properly.

Originally Posted by Diavolicchio
I've done my research though and am content with what I've learned and the technology I'll be utilizing. The water in this particular shower should hit the drain at around 95F and have no problems doing its job clinging to the pipe walls.

My drain system will be a bit different from the one in that photo. I just posted that because it was the best example I could find of a 4-column GFX. They do work though if you size them and set them up properly.
If you really shower at about 110 with that high of a flow you shouldn't have any trouble getting 95 F water at the drain. I measured my 1.6 gpm shower temp as 106 F at the head and 97 F at the drain. A much higher flowrate on the same body/space will result in less temperature loss.

Getting even distribution of the water among the drain exchangers could be challenging. The entrances for each need to be very level and uniform with respect to one another to minimize maldistribution. That's why they did the 1 to 2 to 4 split. I've done distillation and scrubber tower distributor design/troubleshooting and subtle differences had large impacts on performance.

With the high flowrates and relatively smaller ratio of soap scum and hair, plugging in the big lines is unlikely.

One concern I have with your earlier numbers is your assumption of winter preheating by a wood stove (at least based on the Btu rate you provided.) It wasn't apparent to me how the stove would provide much preheating after the first 10 minutes of shower. What sort of Btu rate do you see that injecting into the system.

Another thing that had me puzzled about the preheat is wouldn't it make more sense for the boiler preheat to be after the drainwater preheating rather than before? The greater the delta T at the DWHR coils, the greater the total heat flux. I would expect the boiler preheating to be relatively fixed heat flux by comparison. I'm going by what you stated in the initial post about preheating. Process flow diagram wise I believe the arrangement should be reversed for maximum efficiency.

13. Originally Posted by Diavolicchio
I can't believe this "device" is even acceptable in any code jurisdiction.

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

14. 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.

15. Originally Posted by Doherty Plumbing
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?

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