Propane tankless water heater users?

 Hello all,
    I am wondering if anyone out there is running a tankless water heater
on 100lb portable propane tank(s)?
    I am in the process of installing a Rheem RGT53PV(p) and it calls for a
propane supply of 141,000 BTU with 10"wc min and 14"wc max. I made a
quick call to our propane supplier and the guy was going to check with
their tech but felt pretty sure that that wasnt possible from a single,
two, or three, 100lb tanks manifolded together. I found this a little
hard to believe as I regularly run 250,000 btu mushroom heaters on
jobsites in the dead of winter off 100lb tanks.
    The Energy star rating on this heater calls out an annual cost of
operation at about $240. That is 4-5 100lb tanks a year around here and
we likely be operating it at about 1/2 capacity as we oversized a bit.
    The cabin is fairly remote so there is no way we can get a propane
truck up there. Stil waiting to hear the tech's verdict but just thought
I would throw it out here and se what others experience was.

Thanks for any input,
Mark

  I run a 10 gal water heater from propane but not a tankless. I
considered it except for the outlay cost.

Some thoughts on this.

 aprox 20,000 BTU per pounds of propane.

100# x 20K = 2,000,000 BTU from a tank
 2,000K BTU / 141K BTU = 14 hours of usage at full output from each
tank.

I am not familiar with the pressures but can a regulator to do this
pressure handle the 141K BTU volume? Do the newer 100# tank have the
flow shutdown like the 20# and 30# BBQ units?

 
All kinds. Heck, I've even run my 60 gallon tank hot water heater,
stove, fridges, and propane space heaters on one.


It can be a matter of the regulator, as they are rated for throughput
as well. But yes, it can be done easily. And do note that the actual
piping diameter of your propane system becomes an issue as well at
that size consumption. No little 3/8" copper lines are gonna feed that
heater if they're going any distance ;-).

I've got one client that has TWENTY 100 pounders manifolded together
(multiple manifolds). It can be done. Same deal as yourself, access
only for portable tanks, but high consumption requires extravagant
storage.


Or even forty pounders, I've seen.


It can be done, just get an installler or "tech" who knows his stuff,
and picks the right gear to do the job.

DJ

 

The energy star rating is government fiction and doesn't matter here.

The issue is the rate-of-withdrawal the tank(s) can withstand.  A
single 100 lb tank can withstand 141Kbtu for awhile.  The problem is,
propane is a superb refrigerant, so the remaining propane and tank
will cool.  This results in reduced pressure in the tank.  And as
frost forms on the tank, it becomes insulated from ambient so even
less heat can be conducted in to make up for the heat carried away on
the expelled gas.  The worst case is when the tank is almost empty.
Only a little propane is in contact with the tank, that area is
heavily frosted and the propane rapidly approaches the zero pressure
temperature.

Since the high withdrawal is intermittent (only when you're using hot
water), the curves can be eased a bit.  I imagine that for a
continuous 141Kbtu load, two or three or more 100 lb tanks might be
required.  OTOH, if you only run the heater for 10 or 15 minutes at a
time then one tank should handle it.

For a certain tank size and anticipated ambient conditions, there is a
limit on the withdrawal rate.  This should be available to your dealer
as a set of charts or curves.

Here's what I'd do (and coincidentally what I did at my cabin for an
85kbtu furnace) I'd install two 100 or even better 500 lb tanks (the
largest that can be moved on a dolly) and connect them together with
an RV-type changeover regulator.

This type of regulator draws from one tank.  When the pressure in that
tank drops to a designated minimum, it flips to the other, full tank
and raises a red flag to indicate that it's time to fill the empty
tank.

In this situation, the regulator could change over because the first
tank is empty or because the first tank got too cold and the pressure
dropped.  In that case, you simply determine how much propane is left
in the first tank (by the frost line on the outside) and if there is
enough to continue using it, flip the manually regulator back over and
continue using from the first tank.

The advantage of this setup vs drawing from multiple tanks all the
time is that when the first tank is empty, you still have a full tank
to use while you fill the first.

To extend this a little, you could have two pairs of two tanks, each
pair connected with a simple header and the header feeding the
changeover regulator.  That way two tanks would empty at once and two
would remain in reserve.  That would let you get two tanks filled on
each trip to the propane station.  Might be important if the distance
is great.

The tanks aren't too expensive (about $80 at Tractor Supply and a buck
or two more at Northern Tool last time I looked) so you can have
several.

John, licensed propane dealer.
---
John De Armond
See my website for my current email address
http://www.neon-john.com
Cleveland, Occupied TN
Don't let your schooling interfere with your education-Mark Twain

 Why tankless? At most all you save are the stand by losses of a
regular tank, which with todays foam insulation are pretty low, and in
the winter the lost heat helps keep the building warm.

MUCH easier to support a 40K BTU or even 75K BTU tank.

Most tankless also require electric and are way harder to maintain and
require regular service. Something to consider for a remote cabin