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Author Topic: Why not use Tankless water heater in bus?  (Read 3018 times)
pvcces
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« Reply #15 on: August 08, 2007, 10:12:05 PM »

One caution about tankless heaters; they usually require a larger flue in order to give any appreciable gpm heating capability. If you overlook this, it's easy to wind up in a bind.

Tom Caffrey
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DavidInWilmNC
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« Reply #16 on: August 09, 2007, 06:39:18 AM »

Another issue with electric water heaters is when operating on a 30 amp service (site), there's not a lot left for running things like A/C's, microwaves, etc.  A propane water heater would relieve this issue.  Another nice feature about standard (tank type) RV water heaters is when one has a propane / electric model, the recovery rate is much higher (when using both together).  The main problem I have with RV types, whether tankless or not, is the big cut-out I'd have to make in the side of my bus.  I'd rather have the water heater in a bay, but then venting it becomes an issue.  I could install it in a cabinet or something in the bus and mount the access door below the windows (in the painted aluminum), but I'm not sure where I'd want to give up that much interior space.  So, for now, I have a 6 gallon electric home water heater.  I haven't installed the shower yet, so it's not really an issue... yet.  When the time comes for a bigger water heater, I'll have to figure out what I want to do in terms of type (tankless vs. tank type), energy source (propane, electric, or diesel), and installation location.
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« Reply #17 on: August 09, 2007, 07:51:36 AM »

The trick to using electric water heaters when hooked to the power pole is to heat them in the evening when usually you only might need one A/C to be running.  I first heat one water heater, then the other water heater.  Since I can't tell if they are running, just keep them on for an hour each.  If I heat them at night, I have more than enough hot water for the next 24 hours, then just repeat the process again the next evening.  If I'm in the mountains with only 20 amp service, can still heat the water, but can't run anything else at the same time-but 20 amp campgrounds are very rare anymore.  Course when running the generator, I can light off everything at the same time.  Good Luck, TomC
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« Reply #18 on: August 09, 2007, 09:12:21 AM »

TomC,

One thing I have, but haven't installed, is a NC 15 amp contactor/relay with a 120V coil.  The coil will be wired to the compressor through a switch.  Close the switch, and the water heater is disconnected when the compressor kicks on.  Open the switch, and everything can run together when on 50 amp or generator.  It's sort of a primitive 'power management' system, I guess.  This will allow me to use the water heater and A/C (not simultaneously) on a 20 amp service without worrying about tripping the breaker.  I had a bit of a hard time finding a relay like this for a reasonable price, but here it is if anybody is interested in one:  www.surpluscenter.com/item.asp?UID=2007080910563673&catname=&qty=1&item=11-1986.  Surplus Center has been very useful in all kinds of items for the bus, so far.  There are often lots of 24 volt items (fans, motors, etc) for those who need 'em.  I'm thinking of adding another relay like this to the second A/C.  This way, one can run at a time but will switch back to the one without 'priority' when the first has cycled the compressor off.

David
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« Reply #19 on: August 09, 2007, 10:48:47 AM »

I will be FULL TIME RVing. The tankless unit in my home is only about 8 x 14 x 20 and it heats water for 3 bd rm home and 2 baths.  Hell, i might just take it with me..  Grin

In any case, Thanks for ALL the advice from those that know and Thimk they know  Grin Wink

Will probably use whatever comes with Bus when i get it for a while anyway.

Paul...
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« Reply #20 on: August 09, 2007, 02:01:57 PM »

Paul,

Here is a link to the Precision Temp RV500 if you want to learn more about it.  I plan to include the tankless RV water heater in my bus project.  I was told the house-type tankless w.h. would not work in a bus due to venting, size, etc.  As they say...."Do it your way"! Wink

http://www.houseneeds.com/shop/HeatingProducts/WaterHeating/rv/rv500waterheatermainpage.htm

Good luck,
Darrin
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Darrin Bruner
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« Reply #21 on: August 09, 2007, 04:55:46 PM »

If you spend most of your time plugged in to shore power, then using LP to heat your water, whether that's a tankless system or a conventional one, is inefficient and expensive.


I fail to see the logic in this statement. Could you expand on it for me?


Jay,

Kyle covered most of my answer -- if you're paying for campsites with electric hookups, then the electricity you use to heat your water is included already, so you might as well take advantage of it.  If you have an LP-only system, you'll be paying for the LP to heat your water, with no discount on your site.

That being said, even if you have an electric meter, electric heat is usually more cost-effective than LP.

There are roughly 86 K-BTU's in a gallon of LPG (slightly more for pure propane, and more still for butane).  That translates directly to about 25 KWh of electricity.  Since LP runs about $3/gallon (national average), and residential electric power runs about $0.10/KWh, you are already looking at a savings of $0.50 using electricity against a gallon of LP.

But it's not that simple.  With gas-fired heaters, some of those BTU's go right up the chimney, whereas almost all of the electricity consumed by a heater goes into making heat.  The numbers are just under 70% efficient for the best gas units, vs. 95% efficient for electric units.

So, unless you are in a place where the local cost of electricity is relatively high and the local cost of LP is relatively low, electric hot water will be more cost-effective.

Now, bear in mind, this is an apples-to-apples comparison.  In other words, a conventional storage-tank electric water heater will be more cost effective than a storage-tank LP water heater of the same size, or a tankless electric water heater will be more cost-effective than a tankless LP water heater.  There is an inherent energy loss associated with the storage tank, that tankless units avoid.  (Adding additional insulation around your tank can help reduce this loss, BTW.)

We opted for the storage-tank solution because we wanted to take advantage of the cheapest heating option of all -- free.  The hydronic loop in our 12-gallon model allows us to use waste heat from the  engine  to make hot water.  12 gallons of ~175° water last the two of us all night, and there is usually enough heat left in it in the morning to do the breakfast dishes.

When we are parked (away from electric power), we make heat from diesel fuel, which contains roughly 139 KBTU/gallon, 60% more energy than a gallon of LP for about the same price.  FWIW.

-Sean
http://http://OurOdyssey.BlogSpot.com



Sean,

Thanks for taking the time to explain your earlier post to me. However, when I read your reply, several figures didn't sound exactly right to me, so I did a bit of detailed research before posting this.

Unlike some here on the board, I'm a full timer. I'm parked on an 80 acre spot on the Eastern/Central Colorado high plains at 6900' MSL, where I'm under cover, and buy power from the local electric co-op, run into a 100 amp pedestal.
I usually only travel 2-4 times a year, and don't ever plan to have hookups when I do go somewhere. However, if they're some there when I arrive, that's OK too.
I also subscribe to the "Do it your own way", and that is why I made the choices I did, based on the information I collected during my two year research period prior to purchasing a shell.

You wrote that there are roughly 86,000 btu per gallon of LPG. Every source I find rates propane at 91,400 btu per gallon.
You also wrote the national average price for propane is $3.00. According to every source I found, the national average price per gallon for LPG is $1.85, with the regional low/high prices listed as being $1.63 in the Midwest and $2.10 in the Northeast, although I'm sure it would be easy enough to get gouged for $3.00 a gallon out on the road somewhere.
I pay $1.75 per gallon to have propane delivered, and loaded into my onboard 86 gallon tank. I can also stop at my local distributor and fill my tank for the same price.
You list diesel fuel as running $3.00 per gallon, and here in my part of Colorado it's just under that at $2.95 per gallon.
My electric rate is presently 0.099 cents per KWH, slightly higher than average, I suppose because of the remote area I live in. I also have a $13.50 'facility' charge on my bill each month that must be factored into my per KWH cost.

Your numbers put the efficiency for gas fired heaters at 70%. Not even the government sites list them quite that low. The heater manufacturers boast around 89%, while the real number for a modern unit is likely closer to 80%, and what I used for my calculations.
Your figure for the electric units seemed a bit too low as well, listing 95% efficiency, when my research finds them to actually be closer to 100%.

Because I live in a place where the cost of electricity is higher, and the cost of LP is just below the average, I hope you can see my confusion with your first post.
I heated all of my hot water for three months on $11 worth of LP...had as much hot water as I wanted, anytime I wanted, with no waiting.
When you're usually parked like I am, I don't understand how you can heat, and keep heating 12 gallons of water for three months, 24/7, on $11 worth of electricity.

Using these numbers...

Propane......91,400 btu per gallon and cost $1.75
#2 Diesel...140,000 btu per gallon and cost $2.95
Electricity......3,212 btu per KWH and cost $0.099

...means at $1.75 per gallon for LPG, I'm getting 52,228 btu's per dollar.
When heating with diesel at $2.95 per gallon, you get 47,457 btu's per $.
When factoring in 80% efficiency for a fuel fired device (diesel or LP), that drops the LP figure to 41,828 btu's per $, and the diesel down to 37,965 btu's per $.
In the event you get some crappy propane that indeed only contains 86,000 btu per gallon, at $1.75 per gallon, you will still get 39,314 btu's per $.
At 0.099 per KWH for electricity, I get 34,474 btu's per $.
Even at 100% efficiency, that is not the value that LP, crappy LP, or even diesel will give me per dollar, given present pricing.

If you're running down the road, it's moot as you're getting that rejected heat transfer for free.
But if you're parked as I usually am, it's a bit different.

Jay
87 SaftLiner

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« Reply #22 on: August 09, 2007, 05:35:32 PM »

It is very unusual to get propane for $1.75 a gallon especially for an RV.  Flying J is mostly around $1.99 to $2.19 with a few places higher or lower.  The cheapest I have found locally here in Minneapolis is $2.25 to fill an RV.  You have an unusually large tank so I'm sure you get good rates.

My comparison of LP to electric was based on closer to $2 a gallon for delivered LP and around 8 cents a KW for electricity. 

I can't imagine the inexpensive RV water heaters are all that efficient compared to home models.  RV makers want everything on the cheap and don't care if the buyers spend more on LP.  Unless you fulltime, one of the gas/electric RV water heaters is probably the best bet.  A few extra bucks for LP is a small cost on most bus trips.
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Sean
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« Reply #23 on: August 09, 2007, 06:04:05 PM »

...
You wrote that there are roughly 86,000 btu per gallon of LPG. Every source I find rates propane at 91,400 btu per gallon.


Well, "LPG" can mean many different things.  The number you quote, 91,400 BTU/gal, is correct for pure Propane (remember, I said "slightly more for propane").  Butane is even higher at 103,000 BTU/gal, and both Propane and Butane are considered part of "Liquefied Petroleum Gas" or LPG.  That being said, most of what is sold as LPG is a mixture, which contains, in addition to high-energy fractions such as Propane, lower-energy fractions such as Hexane, Heptane, etc., and the industry-accepted figure for energy content of "standard" LPG is 86,100 BTU/gal.  If what you are buying is being sold as propane and not LPG, then you may be getting the higher-energy gas.  But be advised, the terminology is unregulated -- many "propane" dealers are selling mixed LPG.  (BTW, interestingly, whereas LP in the US is mostly propane with amounts of other hydrocarbons mixed in, in Mexico LPG is mostly butane, again with some other hydrocarbons mixed in.)

Quote
You also wrote the national average price for propane is $3.00. According to every source I found, the national average price per gallon for LPG is $1.85, with the regional low/high prices listed as being $1.63 in the Midwest and $2.10 in the Northeast, although I'm sure it would be easy enough to get gouged for $3.00 a gallon out on the road somewhere.


You are correct for bulk LP delivered residentially, or picked up at a bulk dealer in a bulk tank.  And I suppose I should have considered that, since most folks building a conversion with significant LP usage will install an ASME bulk tank and fill up at bulk suppliers.  (BTW, the latest figures I had for nationwide average bulk LPG price was $2.30/gallon, but that was from March.)

I was referring to walk-up pricing to have DOT cylinders filled, where it is not uncommon to pay $15-$20 to have a nominal 5-gallon cylinder filled.  Many bus converters opt to use this type of LP storage.  (As a side note, we have a pair of even smaller, 2.5-gallon nominal, 11-pound cylinders, which we use only to run the stove and the BBQ.  5-gallon cylinders are too tall for us.  In addition to the hassle of not being able to even get LP at many outlets that now do "tank exchange" only, we often have to argue that we should be charged a single 5-gallon tank-fill price, rather than a standard price for two standard-size tanks.  If they don't cut us a break, we move on.  Fortunately, the two tanks last us around four months, so we don't go through this often.)

Quote
Your numbers put the efficiency for gas fired heaters at 70%. Not even the government sites list them quite that low. The heater manufacturers boast around 89%, while the real number for a modern unit is likely closer to 80%, and what I used for my calculations.

Your figure for the electric units seemed a bit too low as well, listing 95% efficiency, when my research finds them to actually be closer to 100%.


Remember, we are talking domestic water heaters here, not space heaters.  I believe the numbers you are citing, 80%-89% for gas and 100% for electric, are referring to space heating.  When talking about water heating, less of that heat is transferred to (or retained by) the water.  I used the latest research figures for conventional storage-tank water heaters.  BTW, an electric space heater is 100% efficient by definition.  All the energy consumed is converted to heat, and all the heat goes into the environment. Since what you are trying to do is heat the environment -- voila, 100% efficiency.  If you think about it, a water heater can never be 100% efficient, because some of the heat that goes into the water will be transferred from the water to the environment before the water is used (even for tankless units).  Since heating the environment was not the goal, efficiency is lost.

Quote
Because I live in a place where the cost of electricity is higher, and the cost of LP is just below the average, I hope you can see my confusion with your first post.


Actually, I did say that the numbers for some folks would work out the other way, depending on local costs of both LP and electricity.  There are some places where electric power is upwards of $0.15 per KWh.

So I don't dispute any of your math or analysis, as it pertains to you.  I was speaking in a generic sort of way about average conditions, which, for someone like me who is always on the move, is pretty close to reality.

We pay for every ounce of diesel or LP that we burn, but we have yet to pay one red cent for electricity to heat water.  Yet I would estimate that more than a quarter of the hot water we use was heated electrically.  And since, while traveling, you can be relatively certain that you will have, at least some of the time, electric service "included" in your rent, IMO it makes sense to include an electric element in your water heating system.  (I feel the same way about space heating -- in addition to six hydronic heaters that are supplied either by waste engine heat or by the diesel boiler, we also have two electric toe-kick heaters, to take advantage of high-current site hookups when they are available.)  FWIW.

-Sean
http://OurOdyssey.BlogSpot.com

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« Reply #24 on: August 09, 2007, 10:16:03 PM »

If you wanted the best of both worlds, an electric tank with heat exchanger feeding into a instant heater would let you use the free heat when available.  It would automatically switch to the instant heater if the supply of heated water from the electric heater was exhausted.  And you would have the electric for redundancy.  This is basically the system used with a Webasto, just slightly rearranged.
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« Reply #25 on: August 09, 2007, 10:19:56 PM »

Tomcat, I had more like 3413 BTU per kwh or 2545 BTU per horsepower hour. I haven't seen the number that you used.

For what it's worth.

Tom Caffrey
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« Reply #26 on: August 13, 2007, 10:51:49 AM »

I guess it's just part of the "do it your way" but, We use a tankless water heater in our mc9 and have been doing so for two years with zero problems.
could not figure out another way, we could fill the whirlpool tub. have arthritus so love the tub, and the water heater.
steve and cindi mc9
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« Reply #27 on: August 13, 2007, 11:32:29 AM »

Steve or Cindy,

Curious, what model are you using?

Propane or electric.

Thanks

Cliff
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« Reply #28 on: August 13, 2007, 12:13:10 PM »


[/quote]

Sean,

 

Unlike some here on the board, I'm a full timer. I'm parked on an 80 acre spot on the Eastern/Central Colorado high plains at 6900' MSL, where I'm under cover, and buy power from the local electric co-op, run into a 100 amp pedestal.
I usually only travel 2-4 times a year, and don't ever plan to have hookups when I do go somewhere. However, if they're some there when I arrive, that's OK too.
I also subscribe to the "Do it your own way", and that is why I made the choices I did, based on the information I collected during my two year research period prior to purchasing a shell.





Jay
87 SaftLiner


[/quote]

Thanks for the Like Darrin   Grin

@ Jay, Id Love to visit your 80 acres.. Sounds like Heaven !!!  Unless , you like to run around Naked  Shocked


 Grin Grin Grin Cheesy
Paul...
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