Tankless water heater capacity questions

[ironwelder85]

Good day guys! So, I bought this little tankless water heater to heat an insulated 30x40x14 shop.

Now, after I keep looking at it, I’m skeptical if this can adequately heat the shop to 50-55° tops. It doesn’t seem like the inlet/outlet for the fluid isn’t large enough to move enough through the in floor tubes. But I’m not a hvac specialist by any means.

The floor setup has 6 loops. Any help would be awesome!

Located in northern Minnesota.

 

[Rc_Guy]

If it moves too fast, will the water be too cold returning to recover going through the water heater?

 

[Shiftless]

Northern Minnesota certainly gets cold in the winter but 100,000 BTUs sounds like plenty to keep a well insulated 1200 sq.ft. shop warm. How will you be controlling what water goes through each of your 6 loops? Do you plan to run hot water through all 6 loops all the time or ???. What is your pump situation?

 

[ironwelder85]

If it moves too fast, will the water be too cold to recover going through the water heater?

I have no idea. The fittings I bought for a shutoff/fill are 3/4” npt and the unit is 1/2” npt. I think the gallons capacity is there but I want to triple check with people on here who know a whole lot more than I do before I start drilling holes through the wall.

 

[ironwelder85]

Northern Minnesota certainly gets cold in the winter but 100,000 BTUs sounds like plenty to keep a well insulated 1200 sq.ft. shop warm. How will you be controlling what water goes through each of your 6 loops? Do you plan to run hot water through all 6 loops all the time or ???. What is your pump situation?

So far I do plan to run through all 6 at the same time. I have one circulating pump that’s rated at 23gal/min

 

[Shiftless]

I have one circulating pump that’s rated at 23gal/min

I wonder what the flow rate will be when it’s pumping water through what 1000 feet of maybe half inch tubing with all the fittings and bends.

 

[Shiftless]

I’m certainly not a hydronic heating designer or HVAC guy. Just a retired high school Physics teacher sitting around in the morning drinking coffee and reading GJ.

 

[Shiftless]

From the web…

Standard residential tankless units can struggle or void their warranties if subjected to the continuous, low-temperature cycling of a closed radiant loop unless installed with specific supporting hardware.

 

[Shiftless]

More…

The optimum flow rate for an individual closed-loop in-floor hydronic heating circuit is typically 0.5 to 1.0 gallons per minute (GPM)—or about 1.5 to 3.0 liters per minute (L/min)—aiming for a small temperature drop (delta T) of 5°F to 10°F (3°C to 5°C) between the supply and return lines. [1, 2]

 

[Rc_Guy]

This is what I have, about 1600 square-foot garage zone and about 1600 square-foot house zone.

I live about an hour north of Twin Cities in Minnesota.

 

[fitter30]

Pipe size, loop lenghts, pump brand and model numbers. Compleat btand and model of boiler. Pics of the piping and boiler.

 

[u3b3rg33k]

Good day guys! So, I bought this little tankless water heater to heat an insulated 30x40x14 shop.

Now, after I keep looking at it, I’m skeptical if this can adequately heat the shop to 50-55° tops. It doesn’t seem like the inlet/outlet for the fluid isn’t large enough to move enough through the in floor tubes. But I’m not a hvac specialist by any means.

The floor setup has 6 loops. Any help would be awesome!

Located in northern Minnesota.

you probably want the combi/heat only version not the domestic water version.
generally speaking you let the loop do it's thing with some hydraulic separation (e.g. a closely spaced T). the heat versions of these appliances generally have a pump included in the unit. you may need to provide one (or two) of those.

 

[Shiftless]

Obviously…
In floor hydronic heat systems are not as simple as some people think.

 

[Sumboodie]

Why would you try to use a water heater as a boiler?

 

[u3b3rg33k]

Why would you try to use a water heater as a boiler?

an unforseen quirk is that many of the water heaters are intended to take in "cold" water while the boilers are meant to take return-temp water.

a water heater may internally make 140F water and then bypass cold to make the setpoint. a fundamentally different method of operation than only heating to setpoint in a system that doesn't need as much speed or accuracy in temperature.
e.g. a sudden increase in demand may be possible to handle via cutting down bypass flow, even before the burner has a chance to ramp up, and the opposite is true - a sudden decrease in demand can be 'met' safely by increasing cold water bypass without risk of scalding. a CH only boiler doesn't need to do those things.

it's also easier to get that high efficiency target if you can throw 40F water at your heat exchanger even while making 140F+ water. that's gonna be more difficult if your return water is in the 130s.

 

[paulsomlo]

Why would you try to use a water heater as a boiler?

$ - the one shown in the 1st post retails for $299.99 on Amazon.

 

[dcg9381]

Standard residential tankless units can struggle or void their warranties if subjected to the continuous, low-temperature cycling of a closed radiant loop unless installed with specific supporting hardware.

I think this one is out of China, so "realistic warranty" doesn't apply. Might or might not beat spending 5-10X as much for something with a warranty.

Lets assume 100k BTU is realistic (probably isn't).
Match that with flow rate and internal water temp, I think you can probably come up with a "is this sane or not". BTUs, I'd assume can be calculated from performance of the heater... Not calling "********" but do the calcs. If it cant rise temp 50 degrees at 3 GPM, you can probably calc actual capacity.

I have one of these water heaters in my shop. 3-5 GPM (look closely, it depends on internal water temp). It has been an outstanding purchase for about $200 back 5-6 years ago. But I don't use it for floor heat.

 

[Sumboodie]

an unforseen quirk is that many of the water heaters are intended to take in "cold" water while the boilers are meant to take return-temp water.

a water heater may internally make 140F water and then bypass cold to make the setpoint. a fundamentally different method of operation than only heating to setpoint in a system that doesn't need as much speed or accuracy in temperature.
e.g. a sudden increase in demand may be possible to handle via cutting down bypass flow, even before the burner has a chance to ramp up, and the opposite is true - a sudden decrease in demand can be 'met' safely by increasing cold water bypass without risk of scalding. a CH only boiler doesn't need to do those things.

it's also easier to get that high efficiency target if you can throw 40F water at your heat exchanger even while making 140F+ water. that's gonna be more difficult if your return water is in the 130s.

I'm not an expert, but they don't make 2 different units just because.

Some of the new construction I've seen install both vs a single combi.

Combi ***** in that there's no water storage. Easy to REmedie with a small tank water heater THOUGH

 

[u3b3rg33k]

I'm not an expert, but they don't make 2 different units just because.

Some of the new construction I've seen install both vs a single combi.

Combi ***** in that there's no water storage. Easy to REmedie with a small tank water heater THOUGH

there are operational compromises with a combi for sure. by default many of them operate in 'hot water priority' so NO CH during DHW.

 

[Sumboodie]

there are operational compromises with a combi for sure. by default many of them operate in 'hot water priority' so NO CH during DHW.

Yes, but unless you're running hot water constantly it's not an issue.

Probably not great for a big house with lots of activities.

Me.. hot water is used a couple times a day, maybe. Mostly for washing greasy hands or the shower.
Neither take but a few mins.