Best space heater for an energy efficient home
okapi222
| Posted in Mechanicals on
In general, what are the most efficient electric space heaters for spot heating small rooms (eg 90 sf) in a tight and well insulated home, a hydronic baseboard or a convection wall heater?
Any opinions?
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Replies
Daniel,
By "convection wall heater," do you mean a hydronic unit or an electric-resistance unit? Or something else entirely?
The convection wall heater I was looking at was electric resistance. Something like this
http://cadetheat.com/show_product.php?prodid=1051
versus a hydronic unit like this
http://cadetheat.com/show_product.php?prodid=1010
Every heating solution needs a heat load calculation to be able to come up with the rational solution. Tight and well insulated 90 foot room could mean almost anything.
For auxiliary heating, the room's purpose and the approximate occupancy duty cycle makes a difference too.
What is the fuel/heat source for your baseboard vs. wall convector?
Daniel,
Electric resistance heaters operate at 100% efficiency.
Most hydronic systems burn fossil fuels and have an efficiency in the range of 76% to 92%. So electric resistance wins.
But it doesn't really make sense to choose a space heater based on efficiency.
Hmmm.. seems we cross-posted.
That resistance fan unit puts out 1500Watts or about 5000 BTU/hr, which is RIDICULOUSLY oversized for a 90 square foot room in a tight well insulated house, unless you keep the windows wide open, or unless you let it stagnate to freezing temps and need to bring it up to a comfortable room temp quickly for intermittent use.
Like I said, heat load calculations come first, and how you use the room matters.
Just to clarify, both heaters electric resistance. The baseboard uses a hydronic system for thermal mass. So I understand that this room requires very minimal heating. The question is how to minimize electricity usage for that heating.
Daniel,
All electric-resistance heaters have exactly the same efficiency: 100%. Buy the heater that's cheapest (as long as its heat output matches your design heat loss).
They're not really all the same, unless you look at it in the crudest macroscopic sense, and control it just as crudely.
What you're buying with that power is creature comfort. You don't really care what the air temperature in the room is, you just want to be comfortable, and that has more to do with the average radiant temperature of the room than it does the air temperature. The lowest energy use resistance electric solution is usually going to be a low-thermal mass radiant cove heater, under both occupancy sensor and thermostat control. Radiant cove heaters are more comfortable at lower room temperatures than either fin-type baseboards or fan-units, at about the same price-point as baseboards. Comfort levels are comparable to the panel-radiators and hydronic baseboards, but respond more quickly and will use less power if controlled properly.
Controlling them with both an occupancy sensor and thermostat works well, since it heats up the objects in the room directly (rather than the air), which includes the humans. Running full-on during the warm-up ramp it's quite comfortable, like standing in your T-shirt in the sun outdoors at noon on a dead-calm 25F winter day. When the room is up to temp, the thermostat turns the heater off. When nobody is using the room, the occupancy sensor turns it off. And since it hasn't heated up a slug of thermal mass next to an exterior wall, it's not raising the temperature of the wall or the room to increase the heat loss to the exterior.
That's about as good as it gets in resistance heating. It's operating cost won't be half that of your other solutions, but it'll be well into double-digit percentage savings in most applications. (And you still haven't mentioned how this room is being used, or the anticipated heat load.)
Thanks for these answers. Will look into the radiant cove heater. The room serves dual purpose as a guest bedroom and an office.
Just be sure to size it reasonably for the actual load- you don't want it to feel like you're in the broiler when the room is coming up to temp, but you DO want it come up to temp even on the colder days.
If used as a bedroom you may have to set it up to be able to bypass the occupancy sensor control, since occupancy sensors need the occupants to move to "see" them.
Mounting it at the crown molding above a window helps average out the radiation temperature from that direction.
Here is an electric heat pump that runs at an efficiency of 250%.
http://www.livingdirect.com/EdgeStar-Extreme-Cool-14000-BTU-Dual-Hose-Portable-Air-Conditioner-Black/AP14001HS,default,pd.html?mtcpromotion=PLA%3EHome_Air_Quality%3EPortable_Air_Conditioners%3EResidential%3EAP14001HS&src=SHOPPING&kpid=AP14001HS&CAWELAID=120128520000041628&gclid=CL-2yNuq97oCFbA7Mgod8noAfw
It's not appropriate for a room under 150 sq. ft. and it is clunky to install, however.
Given that the heat load of the room at design temp could easily be less than 1000 BTU/hr, it's hard to find any heat-pump solutions that make any sense at all.
In an office with a couple hundred watts of computer & lights and a human emitting 300-400BTU/hr the resistance heater would likely only need to run for the warm-up ramp most days. As a bedroom, a sleeping human is only good for ~250BTU/hr though, probably not fully covering even the heat load of the windows unless its a single modestly sized triple-pane. The key to making resistance heating work effectively is to keep it off whenever it's not actually needed, which is why only thermostatic control isn't the best approach.
In this application you can probably get away with a 400-600W 115V cove heater (600W is over 2000 BTU/hr- unless this room is mostly-windows the heat load isn't likely to exceed that) and use a cheap wall-switch occupancy sensor rated for handling 600W of incandescent lights, wired in series with a line-voltage thermostat. To disable the occupancy sensor a standard wall switch wired in parallel with the sensor switch, so that when the switch is "on", the occupancy sensor switch can't interrupt power to the line-voltage thermostat.
We are heating most of our house with unducted mini splits. The solution I was looking for was for for the odd rooms where a mini split would be overkill. We also do not want a ducted solution. Thanks Dana and Martin for your answers. We are definitely looking into radiant cove now. The motion detector sounds like a great idea, especially for the bathroom because the use is so intermittent. Thanks again. Daniel