what onDemand Propane Tankless water heater to choose
Last Post 16 Jul 2015 12:22 PM by jonr. 9 Replies.
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dkyleUser is Offline
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08 Jul 2015 10:02 AM
I have a Wood Shop, concrete floor with about 200 feet of 3/4" PEX tubing in it.  The footprint is 16'x27'.  I want to be able to simply run a motor to circulate the fluid (anti-freeze) and have an "OnDemand Propane Tankless Water Heater" heat the fluid as it circulates.  It should be a manual control.. I turn the pump on, I turn on the heater.
What brand, or type of heater is available that fits my need?
Dana1User is Offline
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08 Jul 2015 12:09 PM
That's not necessarily going to be the best or cheapest solution. But SFAIK Takagi is the only vender that doesn't void the warranty when (ab)used in that type of application, and even they might take issue with anti-freeze.

In almost any radiant slab w/tankless solution would require two pumps and a primary/secondary configuration to work reasonably well, since you'll want a much higher delta-T (and lower flow) across the tankless than you want across the radiation. It's also worth buying a thermostat & zone relay to power up the pumps.
BadgerBoilerMNUser is Offline
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08 Jul 2015 01:44 PM
Electric boiler.

Why 3/4" PEX.

Does it have an oxygen barrier?

What kind of anti-freeze.

What percentage?

Why anti-freeze?
MA<br>www.badgerboilerservice.com
dkyleUser is Offline
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10 Jul 2015 10:27 AM
The shop is remote & not used frequently. I only have 2 30 amp circuits into it.
I'm not sure why 3/4" PEX.. but it is what it is. I don't believe it is oxygen barrier tubing. Antifreeze is 50/50 stuff for autos.. 50% water. The system is expected to remain idle for days in sub-freezing climate.. I don't want it to break.

I was hoping for a unit that would fire up (& off) manually, rather than use a pressure differential (that, honestly, I don't understand).
..and yes.. it would be nice NOT to have rubber gaskets that i would have to replace every couple of years. Perhaps they can be replaced by a nylon, or some material that won't get eaten by the antifreeze.

I don't understand the statement "..require 2 pumps & a primary/secondary configuration..". Maybe you can elaborate more; perhaps a schematic?
jonrUser is Offline
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10 Jul 2015 11:05 AM
The system will heat up so slowly that you will probably find a thermostat helpful. Then you can turn it on well before use and not worry about overheating. It might be that the heater sees fewer hours/year than a domestic hot water application.

For intermittent use with limited electricity, consider overhead propane infrared heaters. These will heat up very quickly. Low E floor/wall/ceiling paint (or foil wallpaper) would be beneficial (more efficient and more even).
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10 Jul 2015 11:20 AM
First, there isn't a building in the world meeting any current energy code, that cannot be heated with 1/2" nominal PEX tubing.

Then there is the oxygen barrier that is the current standard and should not be ignored unless you have advanced hydronic design skills.

Third, automobile anti-freeze should never be used for hydronic heating in any application as it is toxic and has silicates that will foul common hydronic heating components such as pumps. The use of toxic automobile anti-freeze will void the warranty of all hydronic component manufacturers including PEX tubing, pumps and most especially gas-fired equipment.

So as I see it, whether you have one or ten pumps it is all a waste of time.

There are many on-line resources for radiant floor heating, most of which don't even sell the products. If you can't afford professional help I suggest you avail yourself of this free information before wasting any more time and money.

We design and install radiant and infrared low-intensity tube heaters. Though the tube heater may "heat up" more quickly than the floor, the output is the critical question. A two-stage residential might be in order, but it is no more DIY than a Menard's radiant floor system. Too little information and too much of a rush.
MA<br>www.badgerboilerservice.com
Dana1User is Offline
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10 Jul 2015 02:44 PM
Posted By dkyle on 10 Jul 2015 10:27 AM
The shop is remote & not used frequently. I only have 2 30 amp circuits into it.
I'm not sure why 3/4" PEX.. but it is what it is. I don't believe it is oxygen barrier tubing. Antifreeze is 50/50 stuff for autos.. 50% water. The system is expected to remain idle for days in sub-freezing climate.. I don't want it to break.

I was hoping for a unit that would fire up (& off) manually, rather than use a pressure differential (that, honestly, I don't understand).
..and yes.. it would be nice NOT to have rubber gaskets that i would have to replace every couple of years. Perhaps they can be replaced by a nylon, or some material that won't get eaten by the antifreeze.

I don't understand the statement "..require 2 pumps & a primary/secondary configuration..". Maybe you can elaborate more; perhaps a schematic?
I'm not sure what means " ...a unit that would fire up (& off) manually, rather than use a pressure differential..." in your vernacular(?).

To not flash-boil on the heat exchangers most tankless HW heater need a minimum pressure of ~15psi.

Pumping toward the tankless will increase the pressure at the tankless a few psi while the pump is running, but it'll be nowhere near sufficient for suppressing the boil.

A tankless burner will fire off at something like 0.5-1 gallons per minute of flow.

The modulation level and total heat you get out of it is a function of flow and the temperature difference across the tankless. ( 2gpm is about 1000lbs/hr. The temperature difference x lbs/hr= BTU/hr.)

The max gpm through a tankless might be 6-8 gpm, but you will absolutely run the thing into an early grave if you pump it that hard.  Something like 1.5-3 gpm can last awhile, but takes a bigger temperature difference to get the same amount of heat out of it.

Running a tankless at more than 1/3-1/2 of it's maximum firing rate at a high duty cycle will also drive the thing into an early grave.

The appropriate temperature difference across the radiation will typically be no more than 20F, and often in the 5-10F range- too low to be optimal for a tankless. You will usually (but not always) have to separate the flow through the tankless from the flow through the radiation to get the appropriate delta-Ts and total heat output in the necessary ranges.

To be able to control the flows & delta-Ts separately, it's common to use a primary/secondary configuration, either with a hydraulic separator component, or closely spaced tees, either of which requires two pumps:



https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcSAS696e-gFwGNBND-W2xserWPkqS3SwSmpx-7RcV4NCSHPcR0c

To know where to set the output temp of the tankless you need to know the heat load, the water temperature requirements of getting that much heat out of the slab, then use that to determine the necessary flow rate through the tankless and where to set the output temp.  The output temp of the tankless at flow rates that won't kill it while it it is young will almost certainly need to be much higher than your radiation water temperature requirements, which it the other reason you'll be going to be using a primary/secondary (or sometimes a thermostatic mixing valve, if the flows can be made to work.)

If you're not going to do the math on it ahead of time, the odds of achieving satisfactory performance and/or reasonable equipment lifespan are miniscule.  It's not just a plumbing project.

If you don't have oxygen barrier PEX you'll have to use bronze impeller pumps and other less-oxidizing system components- no iron.

It may not be legal to use automotive anti-freeze in the heating system, even if there WEREN'T material compatibility problems.

Hydronic systems that are not under automatic control in deep sub-freezing climates are a disaster waiting to happen under any circumstances.

Are the two 30A circuits both 120V the same phase, or is 2-phase/240?

What is the design heat load of the building?

A single phase 120V 3kw electric boiler pulls 25A, and delivers 10,000 BTU/hr, which could easily be more than the heat load of a 425-450' insulated shop building.  (If the heat load is over 10,000 BTU/hr it's probably cost-effective to insulate to where it is lower than that.)  Electric boilers come smaller than that too. The installed cost is cheaper than a tankless propane hot water heater or a propane boiler.

It's probably about as cheap to go with electric ceiling radiant panels or oil-filled panel radiators on the walls of equivalent power (which don't have freeze-up issues)of the requisite output, than a propane tankless and radiant slab.  As others have pointed out, even with a big burner propane tankless it'll take hours to bring the slab up to temp.  Electric radiation sized for the design heat load would be far more responsive.  In many markets the operating cost of propane burners isn't significantly cheaper (and often more expensive than) resistance electricity at recent years' prices.

Like any other heating problem getting to the right solution starts with the calculated heat load at the local 99% outside design temp.  Without that you can't even size the heat source or radiation appropriately.  With the already build radiant-slab radiation cart well ahead of the goat you may still be able to get there with that radiation (even though it's not really all that appropriate in an intermittent use situation), but you're still a long way away from picking a heat source.
jonrUser is Offline
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11 Jul 2015 10:52 AM
If you aren't going to heat it all the time and aren't going to rely on infrared to make it feel warmer than it is, then you want much more heating capacity than the typical design temp load indicates. For example, it might take 200,000 btu just to warm up the slab and 20+ hours of advance planning might not be convenient.

How to calculate (or measure) the amount of infrared to be comfortable quickly - good question.
dkyleUser is Offline
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16 Jul 2015 11:45 AM
Thanks for the input. I appreciate the time Dana1 put in to the response.
I admit I approached this as a simple "plumbing project"; I'll need to research this further, & may abandon the "tankless heater" altogether. I am not relying on this as primary heat for the shop.. just wanted to take the chill off the concrete floor.
jonrUser is Offline
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16 Jul 2015 12:22 PM
> just wanted to take the chill off the concrete floor

Nothing better than overhead infrared to do that quickly.

Here is a calculator that can help:
http://comfort.cbe.berkeley.edu/
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