I have a 440 sqft garage/shop/apartment that has two zones for radiant floor heating. from what i can tell, navien is the way to go as far as combination gas hot water heater/radiant floor heat. they have three models a 150k, 180k and a 199k btu units. is there any reason to not buy the biggest? it's a $300 difference online between smallest and largest, and i doubt that the installer would honor that difference on their bid.

The minimum-fire output of any one of those hulking beasts going to be 2-3x oversized for the heat load of the whole 440', let alone a subdivided zone thereof.

A saner approach to a pipsqueak heating load with domestic hot water service would be a condensing tank type water heater with a 76k burner and a heat exchanger isolated hydronic system with a single pressure modulated ECM drive pump and two zone valves.

Have you even done a heat load calculation on the place? If you had, and compared it to the MINIMUM firing rate of a combi heater you'd find that it would NEVER modulate, only cycle on/off at min-fire. You don't get the modulating efficiency out of something that's doing at most a 50% duty cycle on the coldest 50 hours of the year. It's a lousy solution.

There is currently no good solution for these small garage loads, but Dana has it right. This is especially true if you need DHW.

If you don't or this is not a full time gig, then an infrared low-intensity makes more sense, even to a wet heat guy like me.

Don't get me wrong, I would like a Navien CH-240 for Christmas!

If I had a sporadically heated shop/garage and couldn't avoid using a radiant slab, I'd go with a larger than normal heat source (to get a reasonable warm-up time).

I believe that the NCB-180 is rated at 14-80K btu/hr for heating (still much bigger than needed).

As you know, the response time is directly related to mass and limited by the heat transfer coefficient of PEX and concrete. More is not necessarily more...

Agreed, a given slab has some upper limit on how fast it can move heat from water to cold concrete without damage. But I'd guess that for most, there is a huge gap between this BTU limit and the BTU from a boiler just big enough to produce steady state room temperatures on design day.

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Great info. All I can say is yikes. I have tried doing a heat load calc but I keep getting lost at all the info needed.

Is there a commonly recommend condensing tank type water heater with a 76k burner or at least a good brand to look into?

I'm trying to learn and educate myself as best i can. I've called every navien dealer listed within 30 miles of my location (a city of 130k people) and never get answers or return calls. The only plumbers that do return my calls do not know about heat load calcs etc and just want to sell me whatever their supply house has in stock.

My natural gas and water lines are run to the "attic" space, which is the dead triangular space between the roof and the short walls in the loft. This is the only realistic spot to have a hot water heater. If there isn't an ondemand option that i can use on my floors, i will abandon having heated hot water floors and just install a ductless mini split heating/cooling unit. I'm a glass blower which requires a strong exhaust system... having the heated floor would be ideal as it wouldn't suck all the heat out of the room but if its too small of a space to do realistically then fuck it.

I dont have room for a full size Tank water heater. the tankless one will fit nicely in some deadspace that has the natural gas run to it.

i just tried my best to do a heat load calc. it comes up with 9051btu for heating. I have no idea if i did it right.

Heres the info i know.

Shop is outside dimension 22'4"x24'4", 2x6 walls, will have r21 or 19 insulation

North and south walls are 22'x9' tall with 2 3x4' dual pane windows. South wall has metal man door (3' wide)

East and west walls are a total of 540sqft (22x9 but the roof is steep (14:12 i think) for the second floor loft so i added in 50%. there is a 5'6"x4'6" window in both east and west walls). East wall has wooden french doors (6'x7') west wall has an additional tiny 2x3 bathroom window or so.

Roof is 20*24 and will have R30 insulation probably

I'm confused on the "Linear feet of flooring" section on the calculator - not sure what that means. My floor is a poured slab.

I believe i have about 7128 cubic feet of space to heat.

here is a link to picture of the load calc i did:

http://i.imgur.com/1TyQuSS.jpg

Dana:
Sounds like the general recommendation you are making is 1) upgrade the DWH (if too small) and 2) create a heat exchange system to create a closed system.

Im not going to even mention the word 'open' system (aside from just mentioning it!), but is there any downside to just getting a smaller DWH whose sole purpose is for the garage heating system rather than setting up a exchanger?

That way if your DWH dies in the winter, you aren't stuck without heat..... although your little pony DWH may die and leave you cold anyhow.

(just more curious as to the downside of the small DWH for a small heat load....my DWH isn't that big, and I may just rather buy a small exclusive tank rather than upgrade my domestic one)

That was my original plan. Just get a small propane powered water heater for the floor heat and a separate unit for the domestic heat. Then I was told to do navien. What's the downfall of using the navien? Will it just keep cycling on and off or something? What if I was to.only use one of my zones? Would that matter? I ran the zones parrell to each other so each zone covers the entire floor, my thinking was to run hotwater counter clockwise in one zone and clockwise in the other.

It's basically going to come.down to either getting the smallest navien and having in floor heat or just abandoning infloor heat all together, or getting a cheap natural gas hot water heater and spending the leftover on a mini ductless electrical heater/air con

So just to clarify the question, three options have been mentioned for small hydronic loads and NG/propane:

1) add a heat exchanger to a DHW heater (Taco X-Pump block, $1300?)
2) use a dedicated tank water heater ($1000 + $200 for pump?)
3) use an oversized boiler (which with the high thermal mass of a slab, won't short-cycle and when operating, will be at the most efficient load point (min-fire)) ($2000 + $200 pump?)

Plus other items needed (thermostat, zone valves, etc).

The linear ft is the perimeter measurement around the entire base of the wall(93ft?). A slab is calculated a little differently than a framed floor. If you are insulated under the slab, the downward heat loss is typically outward thru the earth around the perimeter, so they want to know how long that perimeter is... You could have also just put in sq/ft(calculate like framed floor) which will probably be close enough to help you make a decision on heat plant.

Personally I would keep the heat and hot water separate.

Is this something that needs an inspectors permit signoff? If so, using a non heating rated appliance(standard domestic water heater) to drive the radiant heating system may not fly with the inspector. Or it might not pass a future sale home inspection... So either a DHW heater that is rated for both DWH and domestic heating service with a heat exchanger to keep the water and heating systems separate, or a standalone heating rated appliance.

At that small a heat load have you considered an electric boiler to drive the radiant heat system? My new home which we just started building has a very low heat load like your project. I need less than 4KW/HR(13,600 BTU/HR) worst case, and the only dedicated heating appliances that have that low an output are electric. I also do not have gas so it was going to be electric or propane for me anyway. The advantage of the small output electric boilers is that they are very small.

Ronmar, which electric heater did you use? That's not a very bad idea. Was it expensive?

Ideally everything gets inspected. technically, its a garage soni don't think I need a heat source. My inspections got cancelled for today. If need be, I can abandon the infloor heat and sneak it in later.

Ronmar, which electric heater did you use? That's not a very bad idea. Was it expensive?

Ideally everything gets inspected. technically, its a garage soni don't think I need a heat source. My inspections got cancelled for today. If need be, I can abandon the infloor heat and sneak it in later.

I have not decided yet, but I think it is down to the thermo2000 or the Nextgen. In this radiant heating section scroll down to the bottom of the page, I started a thread about small electric boiler opinions.

Both the two I just mentioned have digital controls, so you set the target temps you desire and it does it's best to meet those settings. The others use an Aquastat so you make a setting change and have to monitor where the temp actually steadies out at. Once setup of course it should maintain that, so not a big issue, just a little more involved with setup and fine tuning the operation. They also both offer several outdoor reset curves to chose from, so they are a little more customizeable to different applications IMO. They are of course a little more expensive. I am leaning toward the Nextgen as it has the added benefit that there is virtually no other system components needed beyond a room/floor thermostat. It is heater/boiler, Controller, Pump, expansion tank and relief valve all in one tidy case. Just hook it up to your distribution manifolds and electric and fill and purge the system... It is the most expensive, but when I factor in my time and cost to design, procure all the parts for, and fab the pump and distribution portion of the system, it is cost competitive(but way easier to install) with the Thermo-2000 which is my other preference.

Posted By ahreno on 10 Dec 2015 12:51 AM
That was my original plan. Just get a small propane powered water heater for the floor heat and a separate unit for the domestic heat. Then I was told to do navien. What's the downfall of using the navien? Will it just keep cycling on and off or something? What if I was to.only use one of my zones? Would that matter? I ran the zones parrell to each other so each zone covers the entire floor, my thinking was to run hotwater counter clockwise in one zone and clockwise in the other.

It's basically going to come.down to either getting the smallest navien and having in floor heat or just abandoning infloor heat all together, or getting a cheap natural gas hot water heater and spending the leftover on a mini ductless electrical heater/air con

The downside to the Navien is that it's minimum-fire output is well above your calculated heat load, and you're risking short-cycling the thing into an early grave. What's the point of a modulating boiler if it literally never modulates, and is ridiculously oversized for the load even at min-fire.

What's your 99% and 1% outside design temperatures? (The Mitsubishi FH09NA mini-split is good for about 11,000 BTU/hr @ +5F outdoor temps, the Fujitsu 9RLS3 is good for substantially more.)

Not sure I know what you mean by 99% and 1%. The coldest I've ever ever seen it get is about 5°F (record low I believe), winters are generally closer to 20s on a cold day. Usually the lows at night are closer to mid to high 30s.

The heated floor is a nice feature to have for 1) comfort and 2) glass blowing. If imnjust heating the air, I could have a huge propane burner going full blast and my exhaust fan will just suck the air out as fast as its heated. There's literally at least one if not two or more 1000-1300cfm fans going when the torches are on. That being said, I put the pex in with the thinking it might not ever get used. I don't want to buy a $2200 Hester if it's going to die in two years. If I can do it electrically without killing my wallet, I'm totally fine with that too

The 99% design temp is the temperature your location stays above 99% of the hours in any given year. Or put another way, it only gets colder than the 99% temp for 1 percent of the time every year. The 1% design temp is the summer temp that you will only exceed 1% of the time.

These temps in conjunction with your overall insulation values and areas of your structure are used to determine your heat load/cooling load and size your equipment correctly...

In his case he has to calculate the heat load of the 1000-2000cfm of ventilation against the heat gain of the glass blowing furnace too, but it's still a nothing load compared to the minimum you can get out of the Navien.

Assuming it's 20F outside and you want it to be 65F inside so you're not too sweaty when blowing the glass, and assume that the side losses on the glass furnace is at least 5000 BTU/hr. The specific heat of air by volume is about 0.018 BTU per cubic foot per degree F. You have a 45F delta-T between the indoors and outdoor temp. At 1000 cfm (60,000 cubic feet per hour) the heat load of that ventilation at that unsually cool daytime temp in your area is then 0.018 x 60,000 cfh x 45F= 48,600 BTU/hr, less the 5000 BTU/hr of glass furnace comes to 43,600 BTU/hr.

If you're trying to get that much heat out of 440' of radiant floor you shoes would feel downright hot, the opposite of comfort. If you tried to get that much heat out of 200' of radiant floor your shoes would melt (actually, the PEX probably would melt.)

So forget about heating the place at the peak ventilation rate with a radiant floor. Keep the floor at a comfortable low-70sF with small water heater, which should be doable with a 5 gallon electric tank.

Maintain the AIR temp with a 1-ton package terminal heat pump (the type you see in hotel/motel rooms), which in your climate will be nearly as efficient as a mini-split, but much cheaper up front. Be looiking at something like the 1-ton Amana Digismart PTH123G35AXXX . It's overkill for your average heat load, but would recover quickly, and it'll air condition the place (also oversized to cover the load of the glass equipment) so you can stand to blow glass in there in the summer. At internet pricing it's under $800, and with the wall sleeve and wiring up the breakers it'll still be under a grand in raw parts cost.

These things switch over to resistance heating when it's below ~25 F outside but it sounds like it would be operating in heat pump mode almost all the time during daylight hours when you're likely to be using the place. They make a 1.25 tonner, the (PTH153G35AXXX) but it's probably not going to appreciably affect your comfort level. Specs for the heat pump series is on page 4:

http://www.h-mac.com/product-catalogs/amana/Amana-PTAC-Specifications.pdf