Tankless water heater question for beginner

Users who are viewing this thread

marwitz89

New Member
Messages
3
Reaction score
0
Points
1
Location
Wisconsin
Hello, I'm building a shed with a 16 X 24 foot room that will be heated. I have installed pex in the concrete for in floor heating. I'm considering purchasing a Takagi instant hot water heater that would only be used to heat this area (closed loop, no potable water) but had a few questions:

1. The area I would like to place the Takagi heater is not inside the heated room and it gets very cold in WI. Is it ok to run anti-freeze in this water heater and what kind?

2. If I run anti-freeze, is it still acceptable to leave this water heater in a very cold room or will I have issues?

3. I'm running 3/4 IPS tubing underground from my propane tank to the heater. It's about 250 feet total distance. Is 3/4 inch line large enough to supply an instant hot water heater? The model I am getting is: Takagi T-H3M-DV-P

If it's not ok to run anti-freeze or leave this heater in a very cold room, does anyone have any suggestions? I was thinking of enclosing the heater and piping inside an insulated small room and placing a small electric heater in the room that would keep the room at 40 degrees.

Thanks for any help,
Joe
 
Last edited:

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
Hello, I'm building a shed with a 16 X 24 foot room that will be heated. I have installed pex in the concrete for in floor heating. I'm considering purchasing a Takagi instant hot water heater that would only be used to heat this area (closed loop, no potable water) but had a few questions:

1. The area I would like to place the Takagi heater is not inside the heated room and it gets very cold in WI. Is it ok to run anti-freeze in this water heater and what kind?

2. If I run anti-freeze, is it still acceptable to leave this water heater in a very cold room or will I have issues?

3. I'm running 3/4 IPS tubing underground from my propane tank to the heater. It's about 250 feet total distance. Is 3/4 inch line large enough to supply an instant hot water heater? The model I am getting is: Takagi T-H3M-DV-P

If it's not ok to run anti-freeze or leave this heater in a very cold room, does anyone have any suggestions? I was thinking of enclosing the heater and piping inside an insulated small room and placing a small electric heater in the room that would keep the room at 40 degrees.

Thanks for any help,
Joe

Getting to the optimal HVAC solution starts with load calculation. Either an I=B=R type or an online Manual-J with a so-so freebie tool would be a good start.

Even at minimum fire a tankless water heater would be RIDICULOUSLY oversized for the heat load of an insulated ~400' shed, even in WI. The minimum input to the T-H3M-DV-P is 19,500 BTU/hr. It's never going to modulate, and it'll most likely short cycle itself into an early grave unless you add some controls to prevent that. The design heat load of a 2x4/R13 shed with R19 in the roof/ceiling would be under 10,000 BTU/hr @-15F unless there is a huge amount of window area (or the windows are left open.) If insulated to something like IRC 2018 code minimum it could be half that if there isn't a lot of window.

The highest heat rate you could possibly to get out of the floor at surface temperatures that won't feel like it's frying your boots is about 15,000-20,000 BTU/hr, or roughly the minimum fire output of the Takagi.

I'm pretty sure anti-freeze would void the warranty on a Takagi tankless, but it would probably still function well enough to cover your load, but it's really the opposite of a "right" solution. A small propane fired tank type water heater wouldn't short cycle, and would have enough burner for the load, but that's not necessarily a good solution either.

Without knowing what the intended use of this shed is, a 3/4 ton ductless cold climate heat pump like Fujitsu's AOU/ASU 9RLS3H would probably fill the bill (they make bigger versions if it doesn't), and probably at a cheaper operating cost than a short cycling propane tankless. That model has a "minimum heat" function that keeps it at 50F for freeze control, if desired. But that might not be the right solution either, even if it's more appropriately sized for the load.

So, run some load numbers and/or get back with the building particulars.

Did you install any insulation under the slab?

The R-values for walls & roof are...?

The U-factors and sizes of the windows & doors are...?
 

marwitz89

New Member
Messages
3
Reaction score
0
Points
1
Location
Wisconsin
There will be a small window and an exterior door with a half window in the room. The walls are 2X6 and there is 2 inches of foam under the pex and the pex is at the bottom of 5 inches of concrete. I considered a LP water heater but they are not nearly as efficient as a tankless and are more expensive. Thanks for the help.
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
No insulation other than what's under the slab?
 

marwitz89

New Member
Messages
3
Reaction score
0
Points
1
Location
Wisconsin
Sorry, all of the walls will be insulated with Kraft faced R-21 and the ceiling will have blown insulation in it to an R-value of about 30. I definitely will not be going with an instant hot water heater anymore. I talked to a Takagi rep and they stated they don't warranty them anymore for radiant heating. I'm leaning toward a 40 gallon LP power vent water heater as I don't want to vent thru the roof. Thanks for your help and suggestions, I appreciate it.


The use of this room in the shed is a man cave. It may not get used much during the week so I would keep the temp around 55 and it would get used on the weekends so I would heat it up to 70.
 
Last edited:

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
A 2x4/R21 wall with typical siding & sheathing options has a U-factor of about 0.06 BTU/hr per square foot per degree F difference. An R30 attic is good for about U0.35. I'll assume you have about 10 square feet of U0.032 (single low-E double pane, argon fill) window and 20 square feet of U0.50 uninsulated hardwood exterior door.

With 9' ceilings and 16' x 24' foot print there is 80' of perimeter x 9' of height for 270 square feet of gross wall area. Taking out the 30 square feet of window & door leave 240' of wall. Assuming a -15F outdoors, 70F indoors (roughly Ashland's 99% design condition) that's an 85F temperature difference, for wall losses of:

U0.06 x 240' x 85F= 1224 BTU/hr

With 16 x 24= 384 square feet of U0.035 ceiling you're looking at ceiling losses of:

U0.035 x 384' x 85F= 1142 BTU/hr of ceiling loss

The 20' of U0.5 door is worth another...

U0.5 x 20' x 85F= 850 BTU/hr

The 10' of U0.32 window is worth...

U0.32 x 10 x 85F= 272 BTU/hr

Add it up and you're at 3488 BTU/hr of conducted losses.

Assuming a fairly leaky 50cfm or 300 cubic feet per hour of infiltration loss, the specific heat of air by volume is about 0.018 BTU per square foot per degree-F, so the infiltration & ventilation could be as high as:

300 cf/hr x 0.018 x 85F= 459 BTU/hr

That brings the total to a whopping 3947 BTU/hr assuming you're smart enough keep the window and door closed when it's -15F outside. :)

A 3412 BTU/hr per kilowatt, that's less than what a 1500W space heater delivers (!). If you insist on radiant floor there will be some significant floor losses, call it another 1000 BTU/hr but you're still within 2000 watts. There are purpose made electric boilers that would fill that bill for less than a grand, eg: The next to smallest Electro Industries EMB series has a 2.5kw version (more than twice your design load) that runs on 120VAC.

With systems & loads that small the pumping power can be a large fraction of the total power using a cheap 1/25 HP circulator, so it'll be worth installing a ECM drive smart pump and dial in to a flow that's reasonable, using less than 10 watts instead of 75-100W. The lowest-power version in the industry that I can think of is the AquaMotion AM55-FVL "Einstein", available at the big orange box store for about $200 (shipped, w/ installation flanges.)

If you're going to be air conditioning the place in summer, a half-ton Mitsubishi FH06NA ductless mini-split might be a better option overall. Per the AHRI submittal sheet it's good for 6430 BTU/hr @ -13F, yet can modulate down as low as 1600 BTU/hr @ +47F. It's possible to adjust the setpoint down to 50F using the "SMART SET" function on the remote. If that's not enough of an oversize factor to give you confidence, the FH09 3/4 tonner has the same minimum modulation, and delivers 7630 BTU/hr @ -13F (almost 2x your design heat load.) During the sub zero temperatures it's only using half as much electricity as an electric boiler, but when it's 20F+ it would be using less than a third. There are others (Fujitsu AOU/ARU9RLS3H, or LG LAN/LAN090HYV1 ), designed to deliver high efficiency & capacity in cold climate applications that would cover those loads.

Of course your neighbors and friends might think you're crazy to try and heat it with a ductless heat pump, but you'd have the last laugh. They're super quiet, and very efficient compared to window-shaker AC or a ridiculously oversized propane burner or electric boiler. In my neighborhood a name brand 3/4 ton cold climate mini-split runs $3-3.5K fully installed with warranty in competitive bidding. If you want to explore that option more there are installation details to get right.
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
I must have been woefully under-caffeinated this AM:rolleyes::

"With 9' ceilings and 16' x 24' foot print there is 80' of perimeter x 9' of height for 270 square feet of gross wall area."

Uh.... make that 720 square feet of gross wall area, no 270. (DOH!) 270, 720, 207, what's the difference? :)

Reduced by the 30' of window and door it's 690' of net wall and wall losses of

U0.06 x 690 x 85F= 3519 BTU/hr (and not 1224 BTU/hr)

So I underestimated the total heat load by 2295 BTU/hr.

The total heat load is then 6242 BTU/hr or about 1.8kw, (not 3947 BTU/hr)

That's still well within the output of a 2.5kw EMB series electric boiler, but pretty slim margin on the Mitsubishi FH06. You'd definitely be looking at a Mitsubishi FH09 or an even beefier Fujitsu 9RLS3H if going with a mini-split solution.

With more precise information about framing design and door & window specs it could be fine-tuned, but that's still the range you're looking at- less than 3kw of electric boiler or a 3/4 ton cold climate mini-split.
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks