From: John De Armond
Subject: Re: Propane tankless water heater users?
Date: Fri, 09 Mar 2007 11:52:45 -0500
On Thu, 08 Mar 2007 23:45:39 GMT, M&S <email@example.com> wrote:
> 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.
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
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
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
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
John, licensed propane dealer.
From: John De Armond
Subject: Re: Propane tankless water heater users?
Date: Sat, 10 Mar 2007 13:00:15 -0500
On Sat, 10 Mar 2007 14:27:09 GMT, M&S <firstname.lastname@example.org> wrote:
>Thanks for the info. All makes sense and I really like the idea of the
>changeover regulator and 4 tanks manifolded in pairs.
You're welcome. Glad to help.
>Did a quick google
>They have a HD model for 115.00. I am thinking through the summer I
>could run two 100lb tanks but will add two more manifolded in pairs for
That looks like a good unit. For your application, what you'll want
to do is to split the changeover unit and low pressure regulator.
Install a 2 or 5 PSI regulator (color coded red, whatever pressure is
in common use in your area.) in place of the low pressure one. Then
install the low pressure unit at the heater.
The reason to do this is to save on piping cost. At 11" of water
pressure, it will take probably 1" line (off the cuff estimation
without looking it up) to supply the heater. At 2 or 5 psi, 1/2" soft
drawn copper (quite inexpensive) will do.
This is known as "point of use" regulation. Aside from the material
savings, having a regulator at each appliance eliminates appliances
interacting with each other. With a low pressure setup, that heater
kicking in could very well cause a propane stove eye to go out from
the momentary drop in pressure.
If you need more point of use regulators and can find someone to buy
on their wholesale account, the cost should be in the $11-15 range.
Twice that if you walk in off the street to a wholesaler's "city
When you go with two pairs of tanks, you'll notice something
interesting (and totally harmless). The pair of tanks plumbed
together will always maintain the same level of propane. The reason
is that as the propane is used, the one with the least liquid propane
gets cold faster because of less thermal mass. That lowers the vapor
pressure in that side. Propane gas flows from the other side and
condenses back to liquid in the smaller side. This imparts both
liquid and heat to the smaller side until an equilibrium is reached.
Eventually, both sides will have the same amount of liquid and they'll
I've seen this really confuse customers who simply plumbed two tanks
together. One tank would be almost empty so the customer would hook
in a new one, leaving both valves open, thinking that the almost empty
one would go ahead and empty and then he could unhook it and refill
it. When he comes back, both tanks are partially full and at the same
level. Head scratching time until one considers the gas laws.
>The cabin is very small and space is a premium so the tankless is a good
>fit and usage is minimum. Standing pilot and heating and then sitting on
>40 gallons of water for 23+ hours a day for 30-45 minutes of usage just
>doesnt make sense. With all the new Gvt. BS a 40 gallon propane water
>heater is pushing 400 bucks around here so the 300+ extra for the
>tankless is well worth it given it will likely run less than half the
>annual operating cost at our ultra low demand.
I've personally been using the Paloma brand of tankless heater for >20
years. Best one on the market, IMO. Fully proportional flame control
(water stays at the same temperature regardless of flow) in a purely
mechanical design (no electricity required.) Even the largest (5 GPM
100 deg rise) one hangs nicely on the wall in some out-of-the-way
nook. The burner is almost silent so no problem having it near
Saving on the fuel bill is half the benefit. The other is unlimited
hot water. when you've had a hard day doing whatever and feel like a
nice soaking 1 hour shower, no problem! I'd not take 10 tanked
heaters for any of my Palomas.