Electric Baseboard Heat Calculation

[mkgrady]

I have a customer that wants electric heat in their basement. I never really learned how to calculate this...just always guessed and the rooms were always finished rooms with insulated walls. This situation is for a basement that is not finished or even insulated, in fact the heated space on the first floor above will be insulated from the basement. The basement ceiling is going to have fiberglass insulation placed between the joists.

It sounds like a wacky situation but this home is about to be converted from a summer cottage to a year round home. The owners goal is to keep the pipes in the basement from freezing. In previous winters the place was just shut down and the pipes were drained to prevent frozen water pipes. His other goal is to keep the basement warm enough to be able to run a washer and dryer. I can't imagine doing laundry in a room that is only 40 degrees.

The house has cement block foundation walls that rise about three feet out of the ground. No insulation on the block walls. The space height to the bottom of the ceiling joists is five feet. The space is about 30' by 33' and the outside temp goes as low as zero Fahrenheit. He is right on the coast of Massachusetts so below zero pretty much never happens because the ocean moderates the temps.

Any ideas on how to calculate the wattage this will need?

 

[JFletcher]

here's a thread on it:

http://forums.mikeholt.com/showthread.php?t=108071

1W per cubic foot is 8W per sq ft in most places (8' ceilings).

The wildcard is how much outside air gets into the basement. This should be close to zero if he wants to heat it.

You've got one room that is almost 1,000ft²? I dont have a good calculation for you, tho I'd start with 1 8' heater (2000-2500W ea, 4 total) on each wall. 8' is generally good up to 350sq ft, tho I'd go half that for uninsulated space.

He could probably maintain 40*F with one heater tho if it fails he's got frozen pipes... unless he heat traces them.

 

[Dennis Alwon]

That's ridiculous and an outrageous waste of energy. You will need about 10kw of heat min, IMO

 

[JFletcher]

That's ridiculous and an outrageous waste of energy. You will need about 10kw of heat min, IMO

I agree, tho if the pipes are (presumably) at ceiling level, and the ceiling is insulated, it shouldn't take much to keep it above freezing. Personally, I'd heat trace the pipes and put in a portable for doing laundry, but that's me. Our old house, the dryer was in the carport, and if you wanted dry clothes, you go outside year round... tho it never gets to 0* here.

 

[mkgrady]

That's ridiculous and an outrageous waste of energy. You will need about 10kw of heat min, IMO

I agree but I'm not sure what to suggest to the customer.

 

[Dennis Alwon]

I agree but I'm not sure what to suggest to the customer.

Tell her that energy codes will not allow you to wire in the BB heaters unless the room is insulated to the energy code. This is true around here

 

[mkgrady]

Tell her that energy codes will not allow you to wire in the BB heaters unless the room is insulated to the energy code. This is true around here

I wish I knew a lot more about heating and insulating. I'm wondering how I would respond to the following hypothetical questions from a customer (either in KWH or .20 per KWH):

If I do not insulate the basement walls what will it cost to heat this basement to 40 degrees during the winter?
If I do insulate the basement walls whatever will it cost?

 

[wwhitney]

How to do the calculation, with a bunch of assumptions along the way:

If it's 0F outside and 40F inside, that's a delta T of 40 degrees. Assume the area losing heat is 4 feet by 125 feet = 500 square feet. That includes 1 foot of buried wall, but none of the rest of the buried wall or the slab. Ignore the heat gain from the presumably conditioned space above.

Assume the concrete block walls, uninsulated, are R-2, and ignore any contribution from "still air" layers. That means the heat loss is 40 F * 500 ft^2 / 2 = 10,000 BTUs/hr. That's about 2900 watts per Google.

As to the cost, you would need to know how often the outdoor temperature is below 40 degrees, and by how much. The instantaneous rate of heat loss will be proportional to the temperature difference.

Cheers, Wayne

 

[mkgrady]

How to do the calculation, with a bunch of assumptions along the way:

If it's 0F outside and 40F inside, that's a delta T of 40 degrees. Assume the area losing heat is 4 feet by 125 feet = 500 square feet. That includes 1 foot of buried wall, but none of the rest of the buried wall or the slab. Ignore the heat gain from the presumably conditioned space above.

Assume the concrete block walls, uninsulated, are R-2, and ignore any contribution from "still air" layers. That means the heat loss is 40 F * 500 ft^2 / 2 = 10,000 BTUs/hr. That's about 2900 watts per Google.

As to the cost, you would need to know how often the outdoor temperature is below 40 degrees, and by how much. The instantaneous rate of heat loss will be proportional to the temperature difference.

Cheers, Wayne

Thank you Wayne. Good to know there is a way to figure it out but it is really over my head. Is it safe to assume insulating the block walls will make a big difference in operating cost?

 

[wwhitney]

Thank you Wayne. Good to know there is a way to figure it out but it is really over my head.

The idea is to size your heat source to just keep up with the heat loss during the coldest design day of the year, so that under those conditions the heater will run continuously. (Assuming you've calculated the heat loss correctly.) The basic heat loss formula is

Heat Loss (in BTUs/hr) = Area (in square feet) * Temperature Difference (in Fahrenheit) / R value (in American units)

Google will do unit conversions, so I just ask it to convert BTUs/hr to watts.

The tricky part is modeling the boundary area properly. In my sample calculations I just considered the concrete block walls (including 1 foot below grade), and I ignored the heat loss through the slab below and the heat gain through the floor from the presumably conditioned space above. That made the calculation easy and meant I only had to use one R-value, for the concrete block walls.

Is it safe to assume insulating the block walls will make a big difference in operating cost?

You can definitely say that the heat loss though the walls (and thus operating cost attributable to that) is inversely proportional to the R-value. It seems concrete block walls have an R value of about 2. If one added 1 inch of EPS foam board insulation to the inside of them, with an R value of 4, to get a total R-value of 6, then the heat loss through the walls would 1/3 as much. [But if the block walls were R 3 to start, then the new R value would be 7, and the heat loss would now be 3/7 as much.]

Cheers, Wayne

 

[kwired]

the fact you have insulation in the ceiling helps, otherwise most heat loss will be to the floor above, and probably still will be the majority of the heat loss unless you have windows - especially poor windows. Otherwise general rule of thumb for general living spaces is 10 watts per square foot, but if you only want to keep it above freezing you can get by with a lot less then that. I have put 5 kW heaters in garages that are maybe near 10k square feet and they do just fine keeping it warm enough to thaw snow and ice off a car - and those have overhead doors that don't have the greatest R value and they do get opened to let vehicles in and out as well.

 

[retirede]

Most unfinished basements I have been in would never get below freezing with no heat in them at all. In this case, 3 feet of wall above grade is more than normal, so it may not be true here.

 

[mkgrady]

the fact you have insulation in the ceiling helps, otherwise most heat loss will be to the floor above, and probably still will be the majority of the heat loss unless you have windows - especially poor windows. Otherwise general rule of thumb for general living spaces is 10 watts per square foot, but if you only want to keep it above freezing you can get by with a lot less then that. I have put 5 kW heaters in garages that are maybe near 10k square feet and they do just fine keeping it warm enough to thaw snow and ice off a car - and those have overhead doors that don't have the greatest R value and they do get opened to let vehicles in and out as well.

My uneducated gut was telling me that because the basement ceiling will be insulated it will make it harder to heat not easier. My thinking was that the heated first floor would cause heat to migrate into the basement. I realize heat rises but I assume it would mix with the colder air below. Not true?

 

[wwhitney]

My uneducated gut was telling me that because the basement ceiling will be insulated it will make it harder to heat not easier.

That is correct if the living space is conditioned, which is my understanding. I believe kwired's comment was assuming the living space would be unconditioned.

My thinking was that the heated first floor would cause heat to migrate into the basement. I realize heat rises but I assume it would mix with the colder air below. Not true?

Hot air rises, but heat can also be transferred by conduction or radiation. The basement will gain some heat from the conditioned living space above it. Depending on how poorly the basement ceiling is insulated, and the location of the pipes involved, that could be enough to keep the pipes from freezing. If all of the pipes are in the basement ceiling, then relocating the basement ceiling insulation to be under the pipes should be sufficient to keep them from freezing.

Cheers, Wayne

 

[mkgrady]

Most unfinished basements I have been in would never get below freezing with no heat in them at all. In this case, 3 feet of wall above grade is more than normal, so it may not be true here.

That has been my experience too. Being partially below grade seems to keep the temps above freezing. I'm torn between thinking trying to heat the space to 40 is going to be very costly to being no big deal. Pretty confusing. I should probably tell the customer that installing the heat is only cost effective if he insulates the walls.

 

[junkhound]

Most states have an energy code with a free spreadsheet to do the heat loss calculations.

Here is the one for WA.
http://www.energy.wsu.edu/Documents/Heating_System_Sizing_Worksheet.xlsx

BTW, electric heat in MA???!! Sell them a mini-split, even just heating to 40F will pay for itself in a year or 2.

Also, if an electric dryer, vent it thru a filter into the basement, no need to waste that heat. Have vented mine into basement for 45 years, no problems. But do hang clothes outside on a line all summer and when outside temp over 60F. Parents ventied into basement from their first electric dryer (cica 1960) and still doing so. Grandma hung out all year around in IL, sometimes took a week for the frozen clothes to sublimate or hang inside if < 0F hmy:

Sealing and adding insulation to any un-insulated space is nearly always the best and most cost effective first approach!

 

[drcampbell]

My uneducated gut was telling me that because the basement ceiling will be insulated it will make it harder to heat not easier. My thinking was that the heated first floor would cause heat to migrate into the basement. I realize heat rises but I assume it would mix with the colder air below. Not true?

Not quite. Heat doesn't rise. Heated fluids tend to rise and carry heat with them. Heat travels from a warmer place to a cooler place, regardless of which direction that might be.

The basement ceiling insulation won't have much effect on the basement heat load. Insulated or not, there won't be much downward heat flow. The temperature difference isn't large enough for significant radiant heat transfer. Convective heat transfer won't be very effective because the air near the warm basement ceiling will be stagnant. And conductive heat transfer won't be very effective because air isn't a great conductor of heat.

If the house is heated with something more economical than electricity, install a duct & fan or radiator & pump instead of resistance heaters. Residential heating plants almost always have excess capacity available.

 

[mkgrady]

Most states have an energy code with a free spreadsheet to do the heat loss calculations.

Here is the one for WA.
http://www.energy.wsu.edu/Documents/Heating_System_Sizing_Worksheet.xlsx

BTW, electric heat in MA???!! Sell them a mini-split, even just heating to 40F will pay for itself in a year or 2.

Also, if an electric dryer, vent it thru a filter into the basement, no need to waste that heat. Have vented mine into basement for 45 years, no problems. But do hang clothes outside on a line all summer and when outside temp over 60F. Parents ventied into basement from their first electric dryer (cica 1960) and still doing so. Grandma hung out all year around in IL, sometimes took a week for the frozen clothes to sublimate or hang inside if < 0F hmy:

Sealing and adding insulation to any un-insulated space is nearly always the best and most cost effective first approach!

Mini split sounds interesting. I wire them often but have never installed one. Not sure how the line set install works. Seemspulling a vacuum is required. Is this something electricians typically get into? Not sure the thermostat, which is usually a remote would have a setting as low as 40 degrees.

 

[GoldDigger]

Mini split sounds interesting. I wire them often but have never installed one. Not sure how the line set install works. Seemspulling a vacuum is required. Is this something electricians typically get into? Not sure the thermostat, which is usually a remote would have a setting as low as 40 degrees.

A common technique for minisplit line sets is to ship the compressor unit fully charged with freon, the evaporator coil (inside unit) pumped down to a good vacuum, and the line set either pumped down to a vacuum or carrying an amount of Freon appropriate for the length of the set.
Each component has a seal at its connectors which is automatically broken as the connector is mated and tightened.
The result is an install without the need for either a vacuum pump or a bulk container of refrigerant.

 

[drcampbell]

Mini split sounds interesting. ...

That would be the worst selection possible. This basement will need heat only on the coldest days of the year, during which a mini split (or any other heat pump) will switch over to resistance heat.