Radiant heat floor LP usage

wldlif7

I am trying to determine how many gallons of LP will be required to heat my workshop ( an existing installation) or at least a guesstimate so I have an idea for budgeting. I am located in northern UP of Michigan. I maintain an approximate 60 F temperature in the workshop all winter.
Here’s my installation:

• 720 sq ft w/ 9ft ceilings
• R36 in the ceiling and R19 in the 6” walls w/ 1/2” osb on the inside and still siding on the outside. 3 walls are exterior and 1 wall is interior
• I’m using a 190,000 btu on-demand hot water heater as my heat source
• My radiant heat setup is a closed system, no influx of colder water into the system once past startup
• Average temp delta of approximately 45F I believe

not sure what else to expand on, I feel I may be asking a question that can not be estimated. I know this is not explaining all the woodworking equipment and items in the workshop that will absorb heat from the room and it could affect the usage amount mainly at startup.

Please feel free to request any additional details as I did the installation so I have knowledge of all materials and construction methods

Any help or input at all would be appreciated,

pecmsg

That heater sounds Huge for the total sq ft.

It takes 10 - 15 mins of run time for everything to stabilize

I think your going to cycle it to death.

hot_rod

If you ballpark 25 btu/ sq ft, possibly a high side guesstimate, if you are maintaining 60F. that is 18,000 btu/hr at design day!

An electric water heater tank with a 5KW element would probably heat that shop. What do you pay for electricity UP there?

That tankless water heater in not a very good match for that proposed system.

Jamie Hall

As @Steamhead said, the tankless water heater is not a good match more that use. I'll go further: it's a very poor match indeed. Get a proper boiler which won't cycle itself to death.

That said, whether you use the right boiler or the tankless, based on just the floor area, you will use perhaps 5 gallons of propane per day on cold days.

hot_rod

Use this to compare the cost differences of various fuels. Probably natural gas or electricity are you choices?

Use 85% for the gas, 100% for electricity. Use actual delivered cost for both after tax, distribution, etc charges. The bottom number on your bill.

https://coalpail.com/fuel-comparison-calculator-home-heating

DCContrarian

I assume when you say "Average temp delta of approximately 45F I believe" you mean that average outdoor temp is 15F and indoor is 60F?

A better measure is heating degree-days. If you can get that, and then go to this website and look up your county:

https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County%20Level%20Design%20Temperature%20Reference%20Guide%20-%202015-06-24.pdf

and get your 99% heating temperature that should give us what we need.

Just spitballing, at 720 square feet I imagine it's about 36x20. If one 20' side is against the house that gives 92 linear feet of exterior wall, at 9' high that's 828 square feet. The heat flow through an assembly is temperature delta times area divided by R-value, so at R19 you'd get 828*45/19= 1961 BTU/hr. The ceiling is 720 square feet and R36 so that gives 720*45/36= 900 BTU/hr.

Those two give 2860 BTU/hr. There are going to be three other sources of heat loss: What's the floor going to be? And do you have windows and doors? And how tight is the exterior?

Looking at the county list, it seems most places in the UP have a design temperature of about 0F and about 8000 degree-days of heating. By heating to 60F instead of 70F you can knock that down to about 6500 degree-days. Keeping on with the spitballing, let's say the heat loss I identified is half of the total, or about 5000 total BTU/hr at 15F. At 0F it would be about 6700 BTU/hr, that's your design point. So yeah, 190,000 BTU/hr seems a tad large.

If you were to run at 0F for 24 hours you'd use 24*6700= 160,000 BTU of heat. That would be 60 degree days. The heating season is 6500 degree days, or 110 times that, so figure 160,000*110= 17.7 million BTU for the season. One gallon of propane is 91,000 BTU, figure 80% efficiency when all is said and done which nets 73,000 BTU/gallon. So figure on 242 gallons for the season.

The more additional information you can give the more accurate this estimate will be.

One more thing: at 6700 BTU/hr, with 720 square feet, that's 9.3 BTU/square foot. Figure 2 BTU/sf for every degree above room temperature the floor surface is, that gives a floor surface temperature of 4.6F above room temperature, or 64.6F. So it's not like the floor is going to be toasty warm on your toes. For working, cooler is probably better, you don't want your feet getting hot. But given the low load and the low floor temperature I'd also price out a minisplit and the cost of electricity.

psb75

You provided all the R-values of insulation above the floor. What about UNDER and on the sides of the radiant slab? That's kind of important. Some would say, "critical".

hot_rod

overhead garage door? Insulated? What size?

Lots of infiltration loss around an overhead door. I have a 12x12 insulated door on my shop. On windy days is has some serious leakage. Im adding inside weatherstrip, it looks like a like a long paint brush that seals that big moving slab of sheet metal better

I would do a heatloss for that shop if you are trying to nail down more exact answers

You mentioned a tankless so you must have gas or LP? A Lochinvar 55 boiler would run in the 90% efficiency range at the supply water temperature you are probably looking at

If you have tube in the floor already, I would certainly use that for a clean quiet a comfortable shop heat system

Try a free heat load demo if an easy to use calculator at www.hydronicpros.com

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1809886#Comment_1809886

My expectation is we're going to learn there's a large garage door that has more heat loss than the rest of the structure combined.

GroundUp

It could be as low as 100 gallons a year, but it could be more than 1000. You left out the most important parts of the insulation equation. If you were using the proper heating appliance rather than something 1,000% oversized, you could easily gain 10% efficiency regardless of where it falls on the spectrum, however this sounds like a typical 2 car attached garage so the common wall helps a lot. If you have minimal doors and windows that are well insulated along with minimal air exchange (door opening, exhaust fan, dust collector, etc) as well as full R10 under and around the entire perimeter, shoot for the lower end of the spectrum. If it's got nothing under and around the slab along with a bunch of windows and a constant outside air exchange, shoot for the upper end.

hot_rod

Is it an attached or detached shop? That makes a difference on the heatload. If it shares a wall with the heated house, of course loads are lowered.

Even with an R-7.5 insulated overhead door, I'b be surprised if the loads on a detached shop are in the single digits, teens is doable, I think that is what @GroundUp finds in Minnesota.
Loads are temperature dependent of course, look up degree data online from you closest weather station, or local online news weather data.

GroundUp

https://forum.heatinghelp.com/discussion/comment/1810020#Comment_1810020

He did say in the OP that "3 walls are exterior, 1 wall is interior" which I took to mean attached. Perhaps that was lost in translation though. Personally, I've got a 9x16ft door with a rating of R18 (actual R value, hard to say) in my 32x48 shop which is R40 everywhere else aside from 2 half-glass service doors and 24 sq ft of low-E windows and that calculated out to at about 11 BTU/SF on a design day of -30F. Actual usage over the past 3 winters has averaged approximately 20M BTU, keeping it 60* all the time and 67-68 while I'm in there. That would come to maybe 250-300 gallons a year (I heat it with a wood boiler primarily and electric boiler auxiliary so I don't have an actual fuel consumption figure) if I were running a high efficiency tankless like the OP.

DCContrarian

Note that garage door r-values are not regulated like other building materials, and are subject to what some might call "aggressive marketing" and others might call outright deception. You're not going to get more than R-4 per inch with a door, look at the thickness not the published r-value.

hot_rod

the various board foams XPS, EPS, and the poly types are all around R-5 per inch. I don’t know of any insulation, that is tested or certified that is higher then that?

Maybe two layers of bubble foil😉

Teemok

Being that your tank-less water heater is already installed, I'll add this sometimes unpopular opinion. The tank-less mismatch everyone has pointed out is real. A tank-less heater is not ideal but that doesn't mean it won't work or has to be horribly inefficient or that it must cycle to death. So much of life is less than ideal. I've seen tank-less heaters work very well and last. They are in specific use cases matched well for them. There have been so many horror stories around tank-less heaters being misused as boilers that folks around here don't like them at all.

I'm assuming this is an insulated concrete slab. Your tank-less heater has a minimum firing rate. If the structures heat load is greater than that minimum firing rate, the heater will not death cycle. Proper pumping rates are important. The problem is, sometimes your heat load will be less than the heaters minimum firing rate and cycling will be an issue. There are mitigation efforts you can use to minimize cycling. Timed pulsed calls for heat with wider thermostat differentials can let the slab cool enough to offer periods of non cycling burn time. When the slab is cold the heater typically won't cycle. The problem shows up when the water starts to return to the heater warmer and that's when the cycling starts. The rate of the cycling determines how bad your problem is. Raising the supply water temperature a bit and certain piping and control arrangements can create wider water differential and or anti cycle lock out. If keeping the space within a tight temperature band is a requirement these mitigation approaches aren't very good as they result in bigger space temperature swings off the set point. Adding a buffer tank to the system can help elongate cycles but that's more money and space. The truth is there really aren't a lot of boilers small enough to be ideal for your load. They will all cycle as they all have minimum firing rates. Some do have anti cycle features built in. If LP is must over electric as a heat source the buffer tank and pipe needed is worth looking at. Compare your existing tankless heaters minimum firing rate to the best matched boiler, if there's a big difference going for the better sized boiler might be worth it but a buffer tank would help both.

GroundUp

https://forum.heatinghelp.com/discussion/comment/1810049#Comment_1810049

Big box store doors with EPS inside, sure. Commercial doors with injected high density poly, double that. Even your typical closed cell spray foam used in building envelopes (poly) is R6.5-7 per inch.