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:
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.