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
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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?
:^)
Jim
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Ivan Vegvary says...

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 savings.
Ivan
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Ivan Vegvary wrote:

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 transferring heat. I'd go with a small, highly-efficent tank heater.
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Ivan Vegvary wrote:

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 tank.
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 closets.

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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 in diameter Water temp 125 degrees Average unconditioned ambient temp 40 degrees (pessimistic) 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 heater tank 2) The price of energy is higher there
The thermal mass of the concrete is high enough that Ivan Vegvary wrote:

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"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.
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So true, when I went to a shop stove that used outside air for combustion I was amazed. ANY negative pressure in the building will draw cold air from every nook and cranny!
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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 and hot on demand.
Martin Martin Eastburn @ 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:

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Ivan Vegvary says...

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.
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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. <sigh>
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.
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winters.
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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.
Jerry
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Ivan Vegvary wrote:

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.

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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.
Dave Hinz
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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 equipment.
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.
jk
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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 an issue.
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.
Pete Stanaitis
Ivan Vegvary wrote:

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I work for a HVAC company that installs Buderus boilers like this, http://www.buderus.net/Default.aspx?tabidw&cid &&ctitle=wall%20hung%20heaters Great 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. Greg
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wrote:

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)/0.9248 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.
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On Sat, 31 Dec 2005 03:00:34 GMT, Ecnerwal

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 LOT.
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
--<< Bruce >>--
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Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
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Actually, the retail uplift cost to get a full forced vent, no pilot hot water heater from Rheem is about \$100 to \$150 over the equivilent standard heater. (I picked Rheen because they are stocked locally) http://www.rheem.com/consumer/catalogRes_detail.asp?idh
Bruce L. Bergman wrote: