Installed radiant floor (concrete) in a walled off 21'x60' section of the
shop. Works wonderful. (Northwest, Oregon). Have used it for two winters.
My propane supplier said that I should have gone with a "tankless" heater
instead of the ordinary (heavy duty) tank type water heater. Claims the
savings would be tremendous by not keeping 50 gallons hot, on tap 24/7.
I'm trying to do the math and need some advice. Logic tells me that I would
have to sit in the shop for 24 hours and add up all the RUN times, while the
water is circulating through the floor, in order to calculate the OFF time
within 24 hours. Then I would have to figure out the cost of keeping water
hot during all of the OFF times. This would be a factor of heater
insulation etc. and might be available from manufacturers etc.
BTW, in addition to floor heat I will be adding one bathroom (maybe three
handwashings a day?) with shower (maybe two three showers a month, if I am
too dirty to be allowed into the house).
Any simpler advice from any of you out there? These on demand units are not
cheap. My supplier tells me that he runs a four bathroom house (4 family
members) with a single "on demand" unit.
Thanks, Ivan Vegvary
Unless I'm missing something here, it doesn't cost anything to keep
50 gallons of water hot. Any losses from the tank to the surrounding
area will show up as heat in the area where the tank is. And seeing
as that's the idea of the radiant heat in the first place (to
actually heat the building) I can't see where the 'tremendous'
savings would come from.
Does this guy want to sell you a tankless heater, by any chance?
Excellent point Jim!! While my tank sits in the 'unheated' area of the
shop, you are right, heat loss normally would add to the ambient unless its
summer and you don't want the heat. I did not think of your good point.
No, he doesn't sell these units. I looked into them because you see them
all over Europe most often just a small unit sitting on the bathroom wall.
You turn on the hot water in the lav and it scares the bejeezus out of you
when the tankless fires up, flames and all. Supposed to be tremendous
I assume that the "run time" you want to measure is that of an electric
recirc pump. Simply connect an old analog electric clock to the motor.
When the motor runs, the clock runs...
I doubt that there is much cost in keeping the water in the heater tank hot.
And, if the tank is inside the shop, any heat loss from it contributes to
the heating of the shop. If not, make sure the tank is well-insulated. But
I think you will observe that the heater runs very little unless you are
taking hot water from it.
My first reaction is "what's in it for him".
Does he know you're using it for heat and not just hot water? Estimates
I've seen range from 10% to 30% savings when used only for hot water and
not for heat, and the higher estimates are making a lot of assumptions
about infrequent usage and flow controlled fixtures and the like. When you
use it for heat the thermal loss from water circulating through the heating
coils vastly outweighs any loss through the insulated tank so I would
expect the savings to be even less.
If you were using it only for the occasional shower or handwash then an
on-demand unit would make a lot more sense.
One can run a four bathroom house on a total loss system passing long pipes
over an open fire too--this doesn't address efficiency at all.
For actual hot-water supply, it makes sense. You don't want to maintain
50 gallons of heated water for the 30 minutes in 24 hours that you
actually need it.
But for radiant heat supply, you are requiring constant heat. The
main thing you need efficient heat transfer from heating elements to the
water. I bet a good tank heater is more efficient than a tankless in
I'd go with a small, highly-efficent tank heater.
I agree, although there are some stack losses during standby in a naturally
aspirated water heater that might be significant. Setting up the control to
only maintain temperature in the tank when heat is called for might save a
little. Since warming up a radiant floor takes a while anyway the lag would
be no big deal, until he starts using domestic hot water. At that point a
switch could be thrown for DHW priority.
Bear in mind that in Europe many domestic hot water systems are retrofits to
buildings that were old when the US was new, and in that situation it's a
lot easier to hang something small on the wall than to find a place for a
Even in the US that's sometimes the case for different reasons--for example
my mother had one in her condo because it was originally a rental apartment
with hot water provided by a central boiler for the whole complex and when
they went condo the association decided to do away with that service
without considering that there was no room in the individual units for a
conventional water heater unless one wanted to give up one of the two
If you plan to use the space at least several times a week, and you
have a concrete floor and radiant heat, you are typically forced to
keep the floor at least warm through the heating season. Otherwise it
takes too long to heat up the thermal mass of the concrete.
For this usage pattern, there would be little benefit from a tankless
heater. And as Jim Rozen points out, if the water heater had been in
the enclosed space, any heat loss just serves to heat the building
anyway. There would be almost zero benefit to a tankless heater, even
if it were free.
For a water heater in an unconditioned space, it is not difficult to
compute the standby losses of the tank full of water. It's a basic
problem in heat flow that any decent HVAC contractor should be able to
work out for you. And you should be able to follow this yourself.
Assume: 50 gallon tank, call it a cyliner 60 inches tall and 16 inches
Water temp 125 degrees
Average unconditioned ambient temp 40 degrees
Water heater tank insulated to R11 (this varies from one
model to the next)
80 percent efficient water heater
Heating season is 5 months
Cost of propane is $2.00 per gallon
Surface area of tank is about 3400 square inches or 23.6 square feet
Heat loss = (125 - 40) * 23.6 / 11 = 182 BTU per hour or
131000 BTU per month
Propane yields about 71000 BTU per gallon
Propane used per month for standby = 131000 / (71000 *
0.8) = 2.3 gallons
Cost per month = $4.60
Cost per year = $23.00
So your cost per month for the tank of water "just sitting there" is
about $5.00 per month. If the tankless unit reduced it to 0.00, is this
a "tremendous" savings?
Assume a suitable tankless unit costs $600 and has a service life of 15
years. Your "savings" per year from a tankless unit is NEGATIVE $17.00,
and this does not consider the net present value of money. If you move
within 15 years, and do not take the tankless unit with you, the
"savings" picture is far worse (a much bigger loss). To put it another
way, your payback period for the capital investment is "NEVER".
As the cost of energy rises, the equation becomes more favorable for
the tankless unit. If the cost of propane doubled, you would just start
to see some savings on a straight line basis. Factor in NPV of money,
and the cost pf propane would probably have to quadruple for this to
make economic sense.
I suspect the Europeans use these units because:
1) The houses are smaller, and there is less room for a big water
2) The price of energy is higher there
The thermal mass of the c> > > >
Doesn't make sense. You're using that energy to heat the shop. The
water heater, presumably, is in the shop. So the heat that leaks out of
the water heater, well, heats the shop. Which is what it's for.
You're overanalyzing it. I recognize this because I tend to do the same
thing. Set the thermostat on the water heater as low as it'll go -
warmer than your air, but not at normal bath termperature. Sure, it'll
be on sometimes when you're not in the shop, but that little bit of
waste heat is minimal, and not really wasted because you need to have
some warmth in there.
I'd say keep the water heater, turn it down to minimum, and switch off
the circulation pump when you don't need the floor to be heated. It's
what I do in my shop and it works well for me.
"Jacket losses" are only part of the standby losses calculated for
boilers/water heaters. What you are neglecting, if the propane heater is
normally aspirated and draws inside air for combustion, is the heat and air
loss caused by the movement of air through the stack. In that case you are
losing heat through the center of the tank and also losing heated air
through the stack, which draws in outside air through cracks in the
building. That loss can be mitigated by using an ID fan fed by outside air.
If the water heater is serving radiant heat only it can be also be
controlled by outside reset and kept at the lowest non-condensing temp
possible based on outside air. The controller can be overridden for DHW.
If the water heater is in the same room that you are heating, what's the
difference? Any heat lost from the tank is just heating the room
anyway. I suppose that "tremendous" is a relative term, but I doubt
that it would make a very big difference unless the tank isn't insulated
at all. In any event, you sure could add insulation to the tank if it's
Come to think of it, Even if you keep the water hot in the summer, a
little extra heat in the Seattle area might just help minimize rusting
of your tools.
Ivan Vegvary wrote:
I work for a HVAC company that installs Buderus boilers like this,
boilers, efficiencies run in the high 90% for floor heat. For domestic
hot water heat you will need to add a indirect hot water tank too. Now the
down side! These units are very expensive, the indirect water heater will
run you a $1000 alone, the good side of it all is the efficiency, plus the
quality of the equipment. The indirect water heater may be the last one you
will ever buy.
On the other hand I would not tear out a working system just to upgrade it.
Keep in mind you may trim 15 maybe 20% off your propane bill, so you may
save a $100-$200 a year on propane, but spend $5000-$6000 doing so! It may
take 25 years to break even on the deal! You will not see "tremendous"
savings!! Say you spend $1000 a year on propane, 20% saings will be $200. Is
it worth it to you?? (I doubt you will save 20% on your fuel bill!)
You really need to do the math for your specific situation to decide which
is best. The best answer for you maybe two standard water heaters, one for
the floor heat, one for domestic hot water.
I am going to disagree slightly with this, and everyone else who is
saying the same thing.
Yes the heat goes into the same overall space, and results in the same
average air temp, but how it is getting there makes a difference.
The radiant floor is heating from the floor up, heating him, and his
The stray loss heat, is mostly convective, and while it heats the air
immediately around the tank, is convected upwards and out of the part
of the space you really want to heat.
In my shop space this would make a big difference.
Not likely true, especially if you have a good tank system. Good as in
high-efficiency - but in any case, at this point, your payback on
replacing it will stink, as you already bought it and it's not worn out,
so it would have to be terribly inefficient to warrant replacing,
dollars and sense-wise. Some of the better (and costly) tank type
heaters have better efficiency numbers than the tankless units.
If you are concerned about this, (and I think your supplier is probably
selling the tankless units to be so gung-ho on them), you'll want to add
insulation to your hot water pipes, and (if permitted, as permitted) to
the outside of the hot water tank. You can't do much about flue losses
on a gas heater.
Figure it this way, and you might be less concerned - where is any heat
loss from the tank going? Into the heated space. So the "lost" heat is
heating the space you want to heat - so it's not "lost" until it makes
it through the wall/floor/ceiling, which it would do anyway if you made
if by some other means and then used it to heat with as needed.
So, your cost comparison should be the present heater's cost, and
efficiency, .vs. the cost of a more efficient heater (tank or no tank),
and compare the propane you use with this heater to the propane you
would use with a more efficient heater. No need to look at on or off
times at all.
For example, let's say you use 400 gallons of propane per year and your
present heater has a rated efficiency of 80% (lousy for gas water
heaters these days). You replace it with one that's 92% efficient
(pretty good, and costly). (400 * 0.8)/0.92=348 so you save 52 gallons
of propane per year (in this example). Guesstimate life of the expensive
heater, guesstimate cost of propane, you'll soon know if it will
actually save you any money.
I really don't understand why you are using a hot water heater to run
radiant space heating. What you really need is a small gas boiler with a
propane burner assembly. I think they make some with less than 100,000 btu
output, about the size of a breadbox.
Kinda comical to see the bunch of mechanics here claim that a hot water
heater has no standby losses, since it only loses heat to the room. Losses
up the flue are one of the many reasons oilheat is inherently more efficient
than gas, since the retention head oil burner greatly reduces standby losses
by convection of air cooling the contents of a boiler/furnace as compared to
an atmospheric gas burner.
I don't have hot water in the shop - but figured if I wanted it,
I'd put in an electric -in - line unit - demand switch and instant hot.
On the short term, I have the same thing in my water cooler - room water, cold
hot on demand.
@ home at Lions' Lair with our computer lionslair at consolidated dot net
NRA LOH & Endowment Member
NRA Second Amendment Task Force Charter Founder
Ivan Vegvary wrote:
Well, you can cut the flue losses when you use the larger
'commercial' gas water heaters - they have a powered flue damper at
the top of the water heater that is interlocked with the gas valve,
with safeties and time delays both ways. When the damper is closed,
it cuts heat loss up the flue to a bare minimum. Combine that with a
well-insulated tank and water lines, and you cut the heat losses a
I don't think it can be retrofit at a reasonable cost. Most
commercial water heaters are running electronic ignition controls with
spark or hot-surface ignitors, some are running combustion blowers and
forced-air "inshot" burners for low NoX. And almost all 'residential'
water heaters are mechanical click-disc thermostats and normal venturi
burners with standing pilot and millivolt safeties. In most cases
there isn't a power receptacle in the area.
And if you went through all the rigmarole of changing it over to
have a flue damper, the Impossible Mission For... Umm... I Mean the
American Gas Association and Underwriters Laboratories will deny all
knowledge of your actions. ;-P
On 30 Dec 2005 09:06:26 -0800, with neither quill nor qualm, jim rozen
No doubt. One of the things that irks me (more than the 4,000+ s/f
home sizes nowadays) is their penchant for putting in the conveniences
like recirculating hot water to make instant tap water available.
They often don't insulate the hot water pipes, either. What a waste!
Some idiot shown on This Old House did it "for convenience" a decade
ago and it has become a national energy-wasting nightmare.
Tankless heaters are nice, but the last time I looked at specs, even
the residential models were fairly expensive and couldn't handle the
flow for a shower or bathtub. The commercial models were in the
thousands of dollars.
Do the voices in my head bother you?