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.

 

[BillK]

Seems to me that this is one of those situations where it might be worth paying a professional to come out and look over the situation and give you some advice ?

 

[Rc_Guy]

Seems to me that this is one of those situations where it might be worth paying a professional to come out and look over the situation and give you some advice ?

Or just put a boiler in that’s made to do that type of thing

 

[u3b3rg33k]

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.

I don't think size matters as much as what the application specifies.
I happen to have a rinnai i120cn, and i've got it set up for simultaneous DHW & CH (not the default), and the only downside is while you're drawing hot water, you get 140F CH water at whatever rate it decides it can give you. since my fan coil doesn't care, that's not a problem for me.

if I had underfloor heating, 140F might be way too hot, so I'd either need to turn that off or put in something like a taco X block. but my setup is pretty simple so I just live with occasionally very hot heating air (normally it does outdoor reset and drops the heating temp way down).

The main reasons I could see wanting separate appliances would be more DHW demand than a combi/single unit can provide, or being less space limited than I am. it's quite nice having a single combustion appliance for both uses. as it stands even if I turn on ALL my hot taps at once, i still have plenty of hot water and heat.

 

[Jackfre]

Do not use a tankless water heater for your space heating. The warranty is void in this application. a tankless water heater is designed to have widely varying flow rates and a high detla T. Navien make good units. You want a combi boiler which they and many others make. One of the nice things is these, at least in the better brands have outdoor reset control built into the unit. You will be happy with a combi. You will be unhappy with a water heater.

 

[jblnut]

I have a 3,888eq/ft shop and heat it with an 80k btu heat exchanger off an outdoor stove and it runs around 1/3 of the time when it’s **** cold out. Depending on your insulation the BTU’s of that unit are fine but it isn’t designed to do floor heat ……

 

[Dh3256]

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

That's the point of a tankless heater - you are not paying the standby cost of a tank and have endless hot water.

What issue are you trying to address by adding a tank?

 

[Sumboodie]

That's the point of a tankless heater - you are not paying the standby cost of a tank and have endless hot water.

What issue are you trying to address by adding a tank?

The minute long water flow before water is hot, everytime water is turned on.

Can't run hot water and 30 seconds later, run hot water and expect it's still hot. It'll just be the hot that’s in the pipes, then cold for a bit, then warm and then hot.
Like when washing dishes or washing clothes... it causes the washer's internal heater to come on, which is $$ for power.

Some units have an insulated buffer tank to solve that. Putting a small water heater in line is also very common. The one I have is a couple gallons. Standby cost is minimal... cold probably leave it unplugged even.

 

[u3b3rg33k]

The minute long water flow before water is hot, everytime water is turned on.

Can't run hot water and 30 seconds later, run hot water and expect it's still hot. It'll just be the hot that’s in the pipes, then cold for a bit, then warm and then hot.
Like when washing dishes or washing clothes... it causes the washer's internal heater to come on, which is $$ for power.

Some units have an insulated buffer tank to solve that. Putting a small water heater in line is also very common. The one I have is a couple gallons. Standby cost is minimal... cold probably leave it unplugged even.

I don't have this issue OR an internal buffer tank. my only issue is old galvanized hot water lines that are waiting for replacement.

 

[Dh3256]

The minute long water flow before water is hot, everytime water is turned on.

Can't run hot water and 30 seconds later, run hot water and expect it's still hot. It'll just be the hot that’s in the pipes, then cold for a bit, then warm and then hot.
Like when washing dishes or washing clothes... it causes the washer's internal heater to come on, which is $$ for power.

Some units have an insulated buffer tank to solve that. Putting a small water heater in line is also very common. The one I have is a couple gallons. Standby cost is minimal... cold probably leave it unplugged even.

Interesting, I haven't really had that issue with the tankless heaters I have installed. If it is several minutes between uses, yes, but not 30 seconds, and the temperature drop is minimal since the heater "hot starts" within a few seconds.

For those concerned about this issue and the initial delay to flush the cold water out of the pipes, a recirculation loop is a common solution and likely more energy efficient as well as more comfortable.

 

[Sumboodie]

I don't have this issue OR an internal buffer tank. my only issue is old galvanized hot water lines that are waiting for replacement.

It's very common.

 

[Sumboodie]

Interesting, I haven't really had that issue with the tankless heaters I have installed. If it is several minutes between uses, yes, but not 30 seconds, and the temperature drop is minimal since the heater "hot starts" within a few seconds.

For those concerned about this issue and the initial delay to flush the cold water out of the pipes, a recirculation loop is a common solution and likely more energy efficient as well as more comfortable.

Not sure how a holding tank wouldn't be more efficient than flowing water all day.

 

[Dh3256]

Not sure how a holding tank wouldn't be more efficient than flowing water all day.

Typically, recirculation loops have a switch on a timer so they don't run continuously.

Another reason a holding tank is not an efficient or common solution is that it increases the initial time to hot water. With a tankless it's best to minimize water storage to decrease the time to get initial hot water. If you add even a small tank, you are increasing the volume of cold water in the pipes that must be displaced before you get hot water. For example, if you add even a 6 gallon tank and have typical 1 GPM flow shower head, 6 gallons of water plus whatever is in the pipes have to be displaced before you will get hot water - so six minutes or more.

These are all reasons why holding tanks are not commonly installed on tankless water systems and likely why no one else has encountered that kind of system.

 

[Dh3256]

It's very common.

If it is common, why has no one else every encountered it?

I guess it's possible it's unique to your area or something like that, maybe local conditions allow it to work in your area?

 

[Sumboodie]

Typically, recirculation loops have a switch on a timer so they don't run continuously.

Another reason a holding tank is not an efficient or common solution is that it increases the initial time to hot water. With a tankless it's best to minimize water storage to decrease the time to get initial hot water. If you add even a small tank, you are increasing the volume of cold water in the pipes that must be displaced before you get hot water. For example, if you add even a 6 gallon tank and have typical 1 GPM flow shower head, 6 gallons of water plus whatever is in the pipes have to be displaced before you will get hot water - so six minutes or more.

These are all reasons why holding tanks are not commonly installed on tankless water systems and likely why no one else has encountered that kind of system.

It decreases time. It's a holding tank of hot water, why would it need to warm up?

It's definitely a common thing. Several brands have them built in. Navien for one.

 

[Dh3256]

It decreases time. It's a holding tank of hot water, why would it need to warm up?

It's definitely a common thing. Several brands have them built in. Navien for one.

The water will cool down over time. That's why it will take longer to get hot water.

What models of tankless water heaters have holding tanks? I can't find any Navien model that has that.

 

[Sumboodie]

The water will cool down over time. That's why it will take longer to get hot water.

What models of tankless water heaters have holding tanks? I can't find any Navien model that has that.

Sure, if you don't use hot water for a few days.

All the A series

 

[Shiftless]

What models of tankless water heaters have holding tanks?

If a tankless water heater has a holding tank, is it no longer “tankless”?

 

[finn]

If a tankless water heater has a holding tank, is it no longer “tankless”?

Looks like the Navien A series does have an internal buffer tank, and a recirculating pump.

In my opinion, they’re no longer tankless.

From what I remember, tankless water heaters don’t qualify for energy credits, as they don’t reduce energy consumption, I’d those credits still exist.

All they really do is provide unlimited hot water, and reduce the space footprint.

I have a tankless in my space constrained outdoor wood burning Sauna, and a hybrid electric heat pump in the house basement.

I prefer the hybrid heat pump, if you have room.

 

[u3b3rg33k]

idk how long it really takes my washer can slam the hot solenoid on/off/on/off/on/off all it likes and there's no cold water sammich.

 

[Dh3256]

Looks like the Navien A series does have an internal buffer tank, and a recirculating pump.

In my opinion, they’re no longer tankless.

From what I remember, tankless water heaters don’t qualify for energy credits, as they don’t reduce energy consumption, I’d those credits still exist.

All they really do is provide unlimited hot water, and reduce the space footprint.

I have a tankless in my space constrained outdoor wood burning Sauna, and a hybrid electric heat pump in the house basement.

I prefer the hybrid heat pump, if you have room.

Yes, the Navien A series have just moved the recirc loop into the unit, with the corresponding reduction in efficiency.

Tankless do qualify for energy credits, since they reduce energy consumption by 25% to 50% and are more efficient than any tank heater can be.