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# sizing heater for 200 sqrft worksop

Member Posts: 5
I recently built a 200 sqrft workshop with r- 21 in the walls and r-32 in the roof. I live in Alaska but the temps never get below -10F and i'd like to keep the shop around 50-60F max. From my calcs its only 2500 btuh of heat loss. I have 200 ft of 1/2 inch pex in the 6 inch r-10 insulated slab already, and I'm wondering what would be a good option for heating that tube. Tankless? water heater? how many gallons/gpm? Electric or gas? I don't think they make a boiler that small but what do I know. Any help would be much appreciated. Thanks!

• Member Posts: 3,206
You math suggests you're only looking for 12.5 BTUs per sq. ft. Are you sure that's right? Seems low to me.
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• Member Posts: 7,472
Are you talking propane or natural gas? do you know your electric and gas rates? A small electric water heater will cover the load. I agree your heat loss seams low but even if the real load is double, a 4,000 watt water heater will put out 4,000 * 3412 = 13,648.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 4,201
Are you planning on maintaining the temperature all the time? then in floor is the way to go. If not then in floor is good enough to keep the floor "Warm" if your laying on it but poor trying to come back from setbacks. You'll need a forced air blower with higher water temps for that!
• Member Posts: 1,048
Normally I wouldn't suggest, but yes like @zman suggested a standard water heater would do you fine. The load is so small it's hard to justify a boiler or on demand water heater.
D
• Member Posts: 19,958
A 2-1/2 or 6 gallon electric water heater works well for small areas like that. On a shelf, under a counter. change the element to the power you need. they usually come with a 1300W 120V=
4400 BTU/hr
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 5
Thanks for the responses! I did my original calcs wall by wall and came up with that heat loss but i just did a questionairre at the us boiler site and came up with about double. I have natural gas run to the building. I am planning on maintaining the floors at about 50-60 at all times and maybe adding a heater if i need higher temps for finishing something. I think a 6gal mini water heater makes a lot of sense.
• Member Posts: 7,472
Natural gas is cheaper to run than electricity. A small gas water heater would work as well.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 19,958
An atmospheric gas fired appliance in that small square footage, with combustion air requirement, hmmm?

You would want at least a sealed combustion type of heater or boiler.

This little droid intrigues me for small space jobs.

Look at the actual operating cost savings in that small space between gas and electricity and the cost difference for the components.
A few hundred bucks buys a small electric tank, scratch and dents are dirt cheap when you can find them. I have installed many small electric tanks for bathroom only radiant jobs, in the 100- 250 sq ft range they make sense..
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 5
I just ordered the Bosch 4 gallon mini tank electric water heater from Amazon. I have a taco 007-f5 sitting around do u think that would work for an application like this?
• Member Posts: 726
I was thinking what @hot_rod wrote. In most areas natural gas is much less expensive to operate, however if you go with a conventional run of the mill gas water heater, you also have to deal with providing fresh air. Not sure the fuel savings is worth it with that small of a load.
• Member Posts: 5
@hot_rod do you think just putting a little electric mini tank on a timer so it only could turn on every 30 minutes or so would be a good idea to limit power consumption? I just cancelled my Amazon order to do more research cause I'm worried about getting a \$400 electric bill or something.
• Member Posts: 1,656
A small electric WH tank has almost no standby heat loss and will likely only run on heat call anyway, no need for a timer of any sort. At 1300W, as Bob mentioned, even if it ran 24/7 for the entire month you wouldn't break \$100 if your power costs \$.10/kwh. You're probably looking at less than half that in the coldest month if the design heat loss is 5,000 BTU and pocket change in the shoulder months like this. That 007 will do you just fine
• Member Posts: 19,958

@hot_rod do you think just putting a little electric mini tank on a timer so it only could turn on every 30 minutes or so would be a good idea to limit power consumption? I just cancelled my Amazon order to do more research cause I'm worried about getting a \$400 electric bill or something.

If you want to crunch all the numbers you could come up with a fairly accurate cost of operation.
You know the load, the cost of the energy, just calculate in how often it runs and how many days you are at or below design.
Most areas spend about 80% of a heating season above, warmer than, design conditions, that data, days of occurrence, is also available at ASHRAE and other weather data sites if you want to crunch the n umbers that tightly.

Wire to water with an electric tank is near 100%. With a gas fired appliance the efficiency needs to be accounted for.

The beauty of hydronics is you could change or add a different heat source at any time if a less costly energy source becomes available.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 7,472
I would be concerned about continuous operation with a little heater like that. It is really meant as a booster. A 20 gallon tank would be better.
The other issue I have seen with systems like this is that without outdoor reset, they overheat the slab and waste energy that way. You could put a little I-series mixing valve in to prevent overheating. You could use a 3 way and a single circ with your setup. http://apps.taco-hvac.com/uploads/FileLibrary/102-145.pdf
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 5
I get what you're saying. Our electricity costs are a about .19 cents per kwh but even then it would only be 2 weeks out of the year where it could potentially run like that. I'd think the whole of the year shoudn't cost much more than 600-800 and that's with doubleing my btuh calc. I'll probably turn it off for 6 months out of the year anyway. I went ahead and re ordered the Bosch but this time I got the 7 gallon. Thanks guys I'll check back in a couple months with an update!
• Member Posts: 2,633
Any other building close by? If so how are they heated? If you have a house less than 100' away and it has a boiler.....
Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
• Member Posts: 1
I spent 30 years in Anchorage and as I recall electric rates where pretty good. Not that I'm big on water heaters for hydronic heating but, your load is less than 1KW (3412Btu/h). I'd consider a 6 gallon electric water heater. They usually have a 1.5KW element and are 120V. Plumb like a boiler with 12psi expansion tank, fill valve and backflow device. A small bronze pump, and don't forget an air eliminator. Set the temp at 110F for starters. Let your stat control the pump. Honeywell TL8100 can be wired 110 or 24V.
• Member Posts: 2,633
This will be a closed system, no need for a bronze circulator. As an electrician, I'd hardwire the heater, avoid a cord and plug for continuous use, outlets (receptacles) and factory cords always seem to melt on extended medium amperage draws....ask me how I know! Even when well below their ratings.
Serving Northern Maine HVAC & Controls. I burn wood, it smells good!