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Author Topic: How are propane boilers supposedly so efficient?  (Read 4200 times)
belfert
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« on: January 03, 2010, 08:12:18 PM »

Clifford says his propane boiler takes far less fuel than a diesel boiler to produce the same amount of heat.  Considering physics, how is this possible?  Diesel has more BTUs per gallon than propane.  Are diesel boilers that inefficient?

It may be a moot point as I can't find anyone selling a propane boiler for an RV.  The Primus system seems to be mostly sold in Europe.
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Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN
Nick Badame Refrig/ACC
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« Reply #1 on: January 03, 2010, 08:22:32 PM »

Hi Brian,

You are correct, diesel has more btu's then LP but,, Diesel fuel takes more energy to ignite and

diesel boilers are less efficient then LP because you are trying to transfer heat to a liquid substance

rather then air to air. So, it takes more BTU's to heat a pound of water then a cubic ft of air.

But, water will hold longer tempatures then air.. This will take more calculating to better answer

your question! I will work on it!

Nick-
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« Reply #2 on: January 03, 2010, 09:02:15 PM »

Brian,

I know you said "propane boiler" and Nick seems to be talking about a forced air furnace.  But I think they very well may be more efficient and efficiency trumps BTU content if you are spilling those BTU's out in the outside air.

Unless I am mistaken, and just figure the odds of that ever happening, the D is a true boiler in that the flame is directed to the inside of a cast iron pot...boiler while the propane unit I saw was using a formed corrugated sheet metal like chamber to get the heat transferred to the coolant.  The ex temp was way cooler than the 600 degree plus for that Webasto.  Working with a cooler flame maybe there is a better chance to get more efficiency.  There is a water heater that is also a boiler that has a corkscrew 2 inch pipe running up through the tank and that exhaust is ambient and they vent through a PVC pipe.  98 or 99% efficient and there was a guy on the board that used to install them with great success  and profit.

Nick!  Time for you to open the envelope and read the answer. Grin

John
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« Reply #3 on: January 03, 2010, 09:25:33 PM »

Primus propane boilers were sold here for RV's, if they no longer are.  I am looking at a coach that has a Primus propane hydronic system.  I do not know much about them, but the seller says they had a lot of maintenance issues.  The seller believed the diesel boilers like Webasto and Aquahot were better.  I saw a thread on a Wanderlodge board about changing over, but I think that was due to a lack of parts rather than dependability.  One thing that could be thought to play a part in the heating costs is the fact that you are paying road tax and the diesel but not on propane.
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« Reply #4 on: January 03, 2010, 10:14:15 PM »

.... clip .... It may be a moot point as I can't find anyone selling a propane boiler for an RV. 


http://precisiontemp.com/pt_rvmd_twintemp2.html

OK, now all you gotta do is find a money tree.  But, if you decide that this unit is cost effective for you, the Precision Temp folks are great to work with.

Jay
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« Reply #5 on: January 03, 2010, 11:05:20 PM »

That $120 unit was specked at 80,000 BTU.  My conventional HW heater costs almost $400 and that ticks me off cause I know that wages are not the factor that is driving the costs up faster than the cost of living index.  Political?    Mebee.  But it is at least very very personal and relevant.

I think gas is more reliable and  more efficient that oil.....not true.

John

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« Reply #6 on: January 04, 2010, 12:14:50 AM »

According to Precisiontemps claims, their product is a lot more efficient than the diesel boilers they reference-- 82% verses 70%.

http://precisiontemp.com/images/twin_comparison.gif

However, Webasto's seem to be available rebuilt at significant savings

http://cgi.ebay.ca/Webasto-DBW-2010-24v-Coolant-Heater-Reconditioned_W0QQitemZ160388583978QQcmdZViewItemQQptZMotors_RV_Trailer_Camper_Parts_Accessories?hash=item2557e7922a

Webasto says that unit uses 60 watts@24 volts.  I assume that's the pump talking.  I've heard one complaint about them is the diesel smell.  Is it really prevalent?
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« Reply #7 on: January 04, 2010, 01:30:50 AM »

Lin,
I have a 5C with a Webasto in the engine compartment curbside, and I get no diesel smells inside. Since it is hydronic, not sure how I would.
I would think that since propane is a cleaner fuel than diesel, the conversion rate of the available btu's would be better, but only slightly, than diesel fuel.

Enjoy,

Gary
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« Reply #8 on: January 04, 2010, 03:13:42 AM »

Hi John,

I was falling asleep last night while replying... Shocked Lol

Nick-
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« Reply #9 on: January 04, 2010, 04:44:19 AM »

Propane  has more BTU's per pound per gal than diesel,several people are making the propane unit now including Aqua-Hot.  

Lin there are 2 major suppliers of parts for the Primus System in the USA I'll post them for you sometime today the are just about trouble free I had to replace a circulating pump on mine this year and that has been the only problem I have ever had and the pump was a off the self pump nothing special 140 bucks.  


good luck
« Last Edit: January 04, 2010, 05:47:40 AM by luvrbus » Logged

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« Reply #10 on: January 04, 2010, 05:51:25 AM »

Propane dose give more heat per lb. than diesel but it is only half the weight per gal of diesel.  Here is a good link to check out the cost.
http://www.energykinetics.com/savingsHeatingFuelComparisons.shtml
Jack
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« Reply #11 on: January 04, 2010, 06:07:52 AM »

I'm comparing diesel hydronic system versus propane hydronic system, not forced air.  I have no intention personally of going to propane hydronic since I already have a Proheat and I only use the heat maybe a week a year.  If I was full time in a cold climate it might make sense.

I'm just trying to figure out why propane uses so much less fuel than diesel.  It seems to me that a BTU is a BTU.  Clifford made a comment yesterday about not buying a fuel sucking diesel boiler.  He has mentioned in the past that his Primus uses a lot less fuel than a diesel boiler.

Weight of the fuel is really a non issue.  Everybody measures BTUs per gallon as that is how propane and diesel are generally sold.
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Sean
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« Reply #12 on: January 04, 2010, 06:08:36 AM »

Clifford says his propane boiler takes far less fuel than a diesel boiler to produce the same amount of heat.  Considering physics, how is this possible? ...


That depends on what you mean by "less."

I notice Clifford mentions above that propane has more BTU per pound than diesel, which is true.  However, it has less BTU per gallon, as you have already noted.  The two substances have very different densities, so a gallon of propane weighs just a little more than half what a gallon of diesel does.

So, yes, it takes "less fuel" measured by weight.  It takes more fuel measured by volume.

I don't know about you, but virtually every place I have ever purchased either diesel or propane, it has been sold by the gallon, not the pound.  So I find it more convenient to compare fuels by volume.

Right at this moment, the average nationwide prices of the two fuels are virtually equivalent on a per-BTU basis, but that's comparing road diesel to off-road LPG, and average retail price for diesel vs. bulk delivery price for LPG.  In practice, we have found that LPG runs us around $3 per gallon on the road, which makes diesel, even with the road tax, the clear winner at the moment.  Since we have cylinders rather than a tank, we often get gouged for LPG even more, since many outlets have a $20 minimum for cylinders (we usually talk them into filling both our diminutive 11-pound bottles for that price), which puts the price closer to $5 per gallon.

If you compare off-road diesel pricing to LPG, the numbers get even better.  Red diesel is probably as easy to find as true bulk-priced LPG.  We can't use it (no separate tank), but we at least get all the federal road tax (26 cents a gallon) back at the end of each year for our heater usage.  That makes diesel's advantage even more clear cut.

If you are trying to save money on heating fuel over the life of the coach, diesel is a better choice (at this writing).  If you are trying to save weight on heating fuel, LPG is a better choice above a certain amount (below that amount, the extra weight of the tanks negates the savings -- I'd have to do more research to know where the crossing point is).

As far as "efficiency" is concerned, this term relates strictly to how much of each BTU in the base fuel is translated to usable BTU of heating in whatever you are trying to heat (inside of the coach, water, soup, whatever).  Generally, it is easier to extract more of each BTU of LPG than diesel at the burner.  However, more of each BTU of LPG is lost "up the flue" than for diesel due to the nature of the fuel and safety considerations.  For this reason, it is usually more efficient to use propane for something like a cooktop (and diesel cooktops are notoriously finicky anyway), whereas it is usually more efficient to use diesel for something like a boiler (hydronic or otherwise).  Direct forced-air furnaces fall in the middle and are something of a wash; you need to compare them model by model.

None of which means anything without taking into consideration the totality of the system.  Efficiency is lost in every step of the process, not just at the burner.  So it's not possible to make sweeping statements like "diesel is more efficient than propane" or "gasoline is less efficient than natural gas."  So much depends on the design of the rest of the system.  For example, resistive electric heat is the most "efficient" of all (every kWh consumed is turned directly into heat), but it is still likely the most expensive option and of limited practicality in most circumstances, so the 100% efficiency is meaningless.

FWIW.

-Sean
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« Reply #13 on: January 04, 2010, 06:19:20 AM »

Propane dose give more heat per lb. than diesel but it is only half the weight per gal of diesel.  Here is a good link to check out the cost.
http://www.energykinetics.com/savingsHeatingFuelComparisons.shtml


Take that page with a grain of salt.  Those guys sell oil heaters -- they have an axe to grind.

I usually base my comparisons on fuel cost data from the DoE and efficiency data gleaned from specifications pages or aggregated by independent research labs.  That still shows an advantage for oil over LPG, but it is not as clear-cut as the page above would seem to indicate.

-Sean
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« Reply #14 on: January 04, 2010, 06:25:25 AM »

Regardless of how many BTUs are in a pound or gallon of diesel or propane, Clifford has indicated he is using a lot less total BTUs of fuel to heat his bus with a propane boiler versus a diesel boiler.

Are diesel boilers terribly inefficient compared to a propane boiler?  I know propane seems to burn with a nice blume flame compared to diesel.  There has to be some reason a propane boiler can heat with fewer BTUs of fuel.
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