I'm trying to diagnose why heating bills for a home with all-electric heat are sky high. The home used 9729 Kwh of electricity in Jan of 2014, and the average temperature that month was 26 def F. I'm going to assume about 90% of that was for heating. (Which is conservative based on spring numbers.) The home is very large - over 4600 sqft over 2 floors. My best estimate is that the exposed wall area + 2nd floor ceilings are about 6169 sqft in area. The home uses almost entirely electric baseboards for heating, which means they used about 40157 BTUs/hour to maintain a temperature differential of around 44 degrees. (Assuming thermostats set at 70 deg F.) So R = ((6169)(44))/40157 = 6.75. Does this math seem correct? Any faulty assumptions? Does this R value seem way off? The home was built in 1981, with some renovations (that may have included insulation work in part of the home) in 2010. About half the windows are single pane, the other half double. One wrinkle is that there is no basement and it's built on a concrete slab - so do I need to add that to the square footage of "exposed walls"?

You need to know the exact meter reading dates to get close to the exact number of hours. It's also reasonable to subtract off the amount of power used per hour during non-heating months.

It's also good to look up the base 70F HDD on a nearby weather station for that exact meter-reading period on degreedays.net to come up with something more accurate than what the utility company put down, since they may be using data from a location at a different elevation or far-removed from where you were.

The average U-factor would the BTU input divided by the number of hours, divided by the total square footage of the exterior surfaces of the house: U= BTU per square foot per degree difference, or BTU/(degree-F x ft^2) The effective R-value is 1/U. So your arithmetic is right, but the accuracy of the inputs can be tuned up a bit.

Heat losses through the slab vary only minutely with outdoor temp- it's effectively a constant, and there are no simple models for how to calculate that constant, but it's a relatively small error unless you live in permafrost country.


The U-factor for 2x4/R13 16" o.c. construction is typically about 0.08 to 0.10 BTU/F-ft^2 (R10-ish whole-wall). The U-factor for wood sash single panes is typically about 1.0 BTU/F-ft^2 (R 1-ish) for 80s vintage clear glass double panes it's about U-0.6 (R1.7 ish), so the windows likely to be dominating the loss numbers and dragging down your averages by quite a bit. If the windows are in otherwise good shape, installing tight low-E exterior storm windows can make a huge improvement at half the installed cost of a code-min replacement window. (Harvey makes the tightest storms in the biz and has a low-E glazing option, but the Larson low-E storms sold through box stores don't suck if you upgrade to something other than the "bronze" series.) Go ahead and put them over the old-school double panes too. In rare instances that can break the seal on a 1980s clear glass double pane unit due to high temperatures when the sun is shining on the window, but it usually doesn't. A low-E storm over a wood sash single pane delivers ~U0.32-U0.35 performance, comparable to code-min replacement windows. Over 1/2" clear double panes it's more like U0.23-U0.30.

The biggest wild card is the amount of air leakage/infiltration you are getting, and it's often as much as 1/4-1/3 the total heat load on houses that haven't been investigaged and sealed. Blower-door directed air sealing (after fixing the obvious big leaks) can be HUGE! Air sealing is best done prior to any insulation upgrades, since it may require moving some insulation around to get to the leaks, particularly in the attic. How much fluff do you have up there? With a blower door running using infra-red cameras it's easy to spot the leaks, but it's also easy to spot missing or deteriorated insulation.

Any house that's burning through 8-9000 kwh in a month for space heating would benefit greatly from heat pump technology. The better air-source heat pumps would average a COP of 3 or better in a location where the average January temp is 26F (which is probably US climate zone 5), which means it has the potential of cutting the heating bill by 2/3. In a slab-on-grade house finding somewhere to put ducts where they are inside the insulation and air-pressure boundary of the house (as in "NOT up in the attic, above the fluff") can be complicated. But a few ductless mini-split heat pumps covering the more open zones can be quite effective at delivering most of the benefit, even if you end up still using the baseboards in the doored-off or more remote rooms far from the area with the ductless heads. Running ducts in the attic puts big holes in the pressure boundary, and any duct leakage ends up driving air infiltration with the air handler while it's running. If you already have an AC system in place with ducts in the attic, those penetrations can be a significant part of the heat load in winter.

Great info. Thank you. The reported usage was from actual meter reads for the month in question, and the temp was from a local weather station that I obtained myself. I'll look into trying to determine exact degree days and use that figure instead. To complicate things a bit, the home *does* have older mini-splits (circa 2010), but I have no way of knowing how much they were used. (This is a home I'm considering buying.) I'm going to visit the home again on Saturday, and will catalog both the mini-spit models as well as the number/type of windows. I was planning on switching out all 5 existing heat pumps for the Fujitsu RLS3h heat pumps, and likely add several more... but the payback period is hard to figure out. Could be 6 years, could be 15. All depends on what the current owners used as their primary heating source. Lastly, there really isn't space for ducts.

Yep, you didn't account for any air infiltration. It can quickly dominate the heat loss equation, especially in a really large house.

Probably going to get a brick thrown at me for this but go here and plug some numbers into this calculator:
http://www.borstengineeringconstruction.com/Heat_Loss_Analysis_Calculator.html

It's free for personal use and you don't have to register or anything. Read the instructions(link at the top of the calculator) they will explain a lot. Dana1 was kind enough to provide some window and wall R values for you, so along with areas and volumes you have what you need to go and play with it. The thing I found cool about this calculator(besides being free) was that it incorporated air infiltration losses. It might help you get your brain around where all your heat might be going...

If they were using the mini-splits at all, it's futile to try to assess the overall heat load based on power use. Don't even try. You also don't have the thermostat setpoint information, nor do you have the exact dates on which the meter was read (or do you?) Meters aren't normally read at midnight on the first of the month or anything like it.

Take it all back to first-principles, run a Manual-J or I=B=R load calculation on EVERY room individually ,and add them up. The air-infiltration wild-card is something that you can quantify (and fix) with blower door and infra-red imaging directed air sealing.

Most Manual-J load calculators have ridiculously high air infiltration numbers, that seem to presume all air enters and leaves the house at the full indoor or outdoor temperature. That may happen if all of the air infiltration is coming through large round pipes, but there's a VERY significant heat-exhanger effect that happens with crack leakage.

The minimum modulated output of any of the RLS3 is 3100 BTU/hr @ 47F which is WAY above the heat load at 47F of individual rooms. If a room/zone doesn't have a design temp heat load of at least 6000-7000 BTU/hr the thing will be cycling on/off with big room temperature swings during weather cold enough that it's a comfort problem. Mini-duct cassettes can be snuck into the tops of closets, into partition walls, etc, and can split the output between a few rooms. Done right, they modulate at high efficiency and high comfort almost all the time.

In 2010 the most popular Fujitsu units were the RLS2 series. They are pretty efficient even by today's standards, and they too will deliever a seasonal average COP north of 3 in this climate. They have a fully specified output down to -5F, and keep going even at -25F (according to my friends in Quebec.) There is NO "payback" in swapping out an RLS2 that has another 10-15 years of life in it for an RLS3. But there may be reasons to re-commission the existing equipment in a different location, depending on how your post-insulation & windows heat load numbers work out. The cost of moving one is pretty tiny compared to buying a new one. The efficiency gained by swapping an RLS2 with an equivalent RLS3 is less than 20%.

The RLFCD mini-duct cassettes are 5% less efficient than an RLS2, 20% less efficient than an RLS3, but it more than makes up for that by distributing the heat to rooms that might otherwise need baseboard heat on a regular basis. And by not having an oversized on/off cycling ductless head with big temperature swings, the place is just plain more comfortable.

If you ARE going to stick an oversized ductless head into an individual room, the Mitsubishi FH09NA would be a FAR better choice than the Fujitsu 9RLS3, since it can modulate down to 1700 BTU/hr @ 47F. Even in a room with a 3000 BTU/hr design heat load it'll still modulate at least half the time, whereas the 9RLS3 would start cycling on/off as soon as it got up to ~25F outside.

Bottom line- more ductless heads are definitely NOT better. And yes, there is almost room for some ducts, even without ripping the place apart, or making it look like a scene out of the movie Brazil.

Thank you for the plug Ronmar. Dana has provided great info and guidance. Infiltration is often a primary heat loss driver, especially in older existing buildings. The only thing I would add is that sometimes the actual effective R value of an older existing building is not consistent with the standard R-value calculation because some building envelope insulation materials can degrade significantly over time. As such, you might want to take some actual measurements and apply some math as a sanity check. Here is an excerpt from our Heat Loss Analysis Calculator instructions that explains how this may be accomplished:

“It should also be noted that if you have an existing building and you would like to determine the total R-values of the building envelope, this can often be easily accomplished without knowing anything about the building material that were actually used in the original construction. All you need is an IR temperature gun to measure the interior and exterior surface temperature of the building envelope, know the indoor temperature, and apply some math. Any existing building envelope R-value can be calculated using this equation:

R = 0.64 (Tis - Tes) / (Ti - Tis)

where R is the building envelope R-value, Tis is the interior surface temperature of the building envelope, Tes is the exterior surface temperature of the building envelope, and Ti is the indoor temperature. This equation assumes that the indoor air film R-value is 0.64 and that both the indoor and outdoor temperatures are reasonably constant for a couple hours prior to measuring the interior and exterior surface temperatures. This approach is very accurate for 2x and SIP construction, but may be less accurate for ICF construction because the thermal mass effect time lag associated with ICF is typically much longer than the time that the outdoor temperature stays reasonably constant prior to measuring the interior and exterior surface temperatures. Furthermore, the effective R-value performance of ICF is often vastly different than the conventional R-value and is highly dependent on the daily outdoor temperature variation as explained in the instructions for our ICF Performance Calculator.”