Better than predicted energy efficency?
Ok I’m into the third full heating season with the house being sealed up and the numbers I’m getting simply don’t add up..
Too good!
5500 sq.ft. house 107 windows and in the worst weather I’m only spending $200 a month heating it.
The only way I can figure it is is there R value in rocks?
My walls are 16 inches thick but 4 to 8 inches of that is stone which I didn’t put any R value down in the calculations..
By my calculations I should be using at least $100 more per month. Neighbors with similar sized homes tell me they burn around $500 to $800 per month. (conventional framing, fiberglas insulation) Since their homes are all brand new I assume they too have modern high efficency furnaces.
All of the homes have a lot of windows facing east and west with few on the north side like my place.. None seem to have as many windows.
PS going up from 1/8th inch glass to 1/4 commercial glass doesn’t get me a higher U value does it?
Replies
Yes you do get higher U-value w/ thicker glass ... but not much. Have you had a life style change (e.g. are you now an empty nester?)? Maybe you are closing off a lot of rooms that aren't used? Sealing a house can be a substantial change in energy. Air leakage can be half of your bill if the house is leaky.
East/west windows in the cooling season are absolute killers. The heat gain on a west window compared to your north windows is VERY significant. Sounds like you also may have much better insulation (8inch plus walls?). Are your windows low-e glass?
You are a typical lay person homeowner. You are making comparisons w/ your neighbors w/ only a smatering of details to do so (and maybe lots of assumptions). You are talking only heating? Or are you talking gas consumption (i.e. heating AND domestic hot water)? If the later, your neighbors hot water habits may be a big part of their use. Maybe they have more people using water. Maybe your shower, clothes washer, and dishwasher are more water efficient.
Some food for thought to get you started.
Edited 10/29/2009 8:17 am ET by Clewless1
The home is oriented east and west due to the lake and bay views which while they added a lot of cost and energy loss would have been a sin to lose.. I have 22 windows facing west and 29 windows facing east. I know I have heat loss in those areas through the glass. All the Andersen windows are low E, argon filled. The home made doubled glazed windows made to cover stained glass are not. Those windows however have the double 1/4 inch commerical glass on them. They are argon filled but not Low E
The walls are 16-18 inches thick if I count the exterior stone and timber. The insulation is SIP's so I didn't use the .20 factor for studs since there are no studs between the interior and exterior walls..
The neighbors I'm comparing myself with are similar to us both in family size and presumed life style.. If anything my consumption should be higher than theirs since I'm at home 24/7/365 and most of them are working families..
"East/west windows in the cooling season are absolute killers."
Not with Low E II Argon they're not! We have many!
Frenchy - your stone walls contribute quite a bit toward 'thermal lag' making daily swings (and therefore thermostat cycles) less abrupt.
Jeff
Low-e and argon do not automatically mean that the SHGC is low as well. Low-e can have a SHGC of say 0.6 Argon, I'm guessing really doesn't change that value much. It requires tinting or a specially ordered low-e coating (which most people don't do) to pull the SHGC down. Double low-e (I'm assuming that is what low-e II is) is likely to reduce the SHGC some. On east/west windows, I go for 0.30 max and try to get under 0.20 depending on the application.
Next week the guys from the gas company will arrive and change out your meter. Thankfully (for you) state law only lets them "claw back" 12 months of underbilling.
DanH
That's exactly why I have concerns! Same furnace as the old house (I reused it) and that house 1/2 the size with only 33 windows cost me $500 a month to heat during the coldest months.. less than half the cost, in spite of price increases?
It's also possible that the load calc program you used had a lot of "fudge factor" built into it to compensate for shoddy airsealing and insulation. Even by following ACCA Manual J very aggressively, you will still get a heat load that's about 15% high.
You appear to be a guy who would do better than expected, not worse.
Also, load calcs for heating don't factor in any other sources of heat gain, like appliances, lights, people, etc., and they are typically designed to calculate heat load at the winter design temperature, which is calculated over decades to be the 99% temperature for your area--in other words, 99% of the time, it will be warmer than the design temperature--usually much warmer. (BTW, did you use the 99% winter design temp in your calcs, or did you go a few degrees lower, "just to be safe"? If you used a lower temp, then your calcs would exaggerate the load and the projected cost.)
Are you really looking for an answer to your question, or did you just use this as a way to make the rest of us jealous? ;-)
I'm not familar with the 99% number.. I plugged in the actaul degree days number to the formula I used.. My projection was $350 a month during the coldest months.
Oh by the way because of the thermal mass inside the house I don't use a set back thermostat besides when I'm sitting here at the desk or watching TV I hate to be chilly..
Just for information I'm in a T shirt and barefoot as I type this and trust me I'm a weather wuse! I've been know to wear a winter parka on a 50 degree day!
Oh and by the way I really do have a concern because the old house 1/2 the size and with only 33 windows ued to cost me $500 a month during the coldest months. Since I reused the same furnace and more than doubled the size plus all those windows I failed to see how I could be that efficent!
As DanH pointed out the gas company can back charge me if the meter proves to be wrong.. I couldn't afford that!
Edited 10/29/2009 11:26 am ET by frenchy
Hmmm.
HDD should be be an accurate number if you plugged it in according to the month-by-month number, which apparently you did, because you say it predicted the cost for coldest months.
If we discount the possibility of the meter being wrong, or perhaps the gas co. just estimating the usage, then I can only think of two likely factors that could reduce your actual load from what was calculated: Infiltration rate and unaccounted-for heat sources.
As for infiltration, someone already questioned your number of 0.7 ACH--this is very high for the type construction and attention to detail that you obviously did. And, it's a fact that the larger the house, the lower the infiltration rate will be given the same type of construction and air sealing--in other words, a house with twice the surface area (thermal envelope) will not have twice the infiltration as similarly-built house that's got half the surface area. Every time the area is squared, the volume is cubed. Also, did the program you used assume a heat loss for ventilation? Until you install your planned HRV, you don't have a ventilation load, since intermittent exhaust fans don't count, generally speaking.
Unaccounted-for heat sources could be several things: TV, computers, refrigerators, lights, occupants, various appliances. They will contribute heat to the envelope at the rate of 3.4 BTU's per watt. And the furnace blower itself contributes about 1500 to 2500 BTU's whenever it runs.
One more thought: has your new house dropped some of the gas usage by having an electric water heater or kitchen range?
Unaccounted-for heat sources could be several things:
But this is accomodated in a standard HDD calc since HDD are based on a temp that is 5 Fdeg below the 'assumed' thermostat setpoint. In residential applications, the intermittent use of relatively small items and people is well accomodated by this concept. I wouldn't expect but a very small variation in the estimated energy use by the application of internal loading beyond the value the HDD already accounts for.
I'm not familiar with the load calc program(s) you might be referring to, and it appears that frenchy used one similar to what you mention, that is, he and you both refer to HDD.
The ones I know are those required to meet ACCA Manual J (which is the ANSI standard now required in the IRC). With these, the number you end up with is the total BTU's/hr lost when the outside is at the winter design temp.
For calculating the heat loss, Manual J calculates the load based on the of winter design temperature, in the dark, no lights, no appliances, no people or any other heat source.
For calculating heat gain, it uses the summer design temp and adds in other heat sources.
It appears that maybe the HDD programs are intended to project energy use, whereas the Manual J programs are intended to size equipment, and to calculate room-by-room heating/cooling needs.
Is that the way you see it?
That's exactly right. There is a difference between design (i.e. static) load calcs in Btuh and estimated energy consumption calcs that result in Btu's consumed over a period of time (e.g. month or year). One is a static peak load for determining the maximum equipment size. The other to estimate energy use in real world/weather conditions.
Many erroneously interchange the two methods/concepts. Many also think that system designers are also skilled at estimating energy use. While they both use similar concepts and elements (e.g. U-value), they aren't really the same and they both require a little different skill set.
Sorry ... got a bit on my soap box.
Thanks, I think that statement best explained my situation..
It was well thought out and logical.. and frankly It's one less thing I have toworry about.
Thank you..
"Sorry ... got a bit on my soap box."
No problem, you followed the 2 rules that make it OK: 1) You have to know what you're talking about, and 2) You know when to get off.
those are all good points.. adjusting for everything I can get closer but I'm pretty sure The numbers would be still more generous than they are.
Just thought of another area wherein the original load calc could have gone wrong:
Did it assume ductwork in unconditioned space, when in reality you might have ducts all contained within the conditioned space? This can account for about a 20% difference in the load.
So recalc using the actual HDD for the month you got the bill. Our utility used to tell us the HDD for the month on our bill. But you can get the actual. You used the long term average for the month, probably. The actual can vary from the average by an easy +/- 25% and more.
I think that with the SIPs panels and the good windows, you are above average in terms of construction. Depending on roof and other details, the SIPs should give you an ACH down around 0.20, maybe lower I would surmise.
BTW, the SIPs I've seen do have some framing factor ... 2x6 @ 48oc plus the top/bottom plate. Very small, but when you think about it, not a huge change from 24oc.
Sounds like you are mixing design heat load concepts with energy consumption analysis a little. Design temp won't appreciably change his consumption of energy. He could design and size for -40, but he will largely use what he needs for the current weather ... which is what he is referring to. Design heat loads usually don't account for internal gains/loads. That concept is somewhat arguable, but residential internal loading is pretty small compared to e.g. commercial.
The use of the HDD heat losss methodology automatically accounts for internal loading since HDD are normally on a base temp of 65 degF rather than the assumed setpoint of 70 deg.
what did you guess for an infiltration rate?
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Because I haven't spent enough time doing all the little things that add up to so much I calculated it at .7 air changes per hour.
I do know that fundamentally the house is well sealed up because I focused on energy matters carefully during construction. The big obvious air leaks have been dealt with but I haven't done a blower door yet..
that's a pretty high estimate. if you are actually at all tight you're probably no more than half that, and you might even be as low as 0.2 equivalent or so if you are using an HRV and are otherwise very tight.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
I intended to purchase a HRV unit but loss of job stopped those plans.. and while the house was built with great care to seal everything during construction I accept the fact that the fine tuning hasn't been done yet.. I'm delaying that until I can afford a HRV. I'm familiar with the too tight problem. That's why my air exchange numbers are high though.
But you could be correct, A lower air change rate would help get the actual numbers closer to the theoretical.
Back out what the ACH would have to be to make actual heat loss match up with calculations. What does that give you, and is it within expected range for a carefully sealed up house (not a spec house to typical construction standards)?
0.7 ACH air leakage is quite large! Standard energy code houses in the NW were targeting 0.35 as the 'standard' or 'average' if you sealed per code, but didn't really go overboard. I saw some guys seal and test down to 0.10, but those were the exceptions to the rule. Based on blower door tests I've done and results of others, the 0.35 ACH value was a pretty good value to use for average construction using minimal effort for air sealing (e.g. do the windows and the plumbing/electrical penetrations of the envelope).
Neighbors homes may be similarly sized but the devil is in the details and your home is anything but conventionally built.
And you can not assume they have high efficiency furnaces or that their heating systems were desighned,installed, or adjusted correctly.
As I remember it, you built/renovated your place. The other houses were built by the cheapest people around. This alone will make a big difference in energy use. I'm sure you took the time to install the windows correctly, and not just "dropped in". That's just one item of the many you told us about. I'd bet there is next to nothing in your house "good enough" (not talking high end things).
Life style will make a difference but not beyond 10% in most cases for the same house.
Take my house: we use about the same amount of electricity as the previous owners but we paid more because we were not the first owners. They got a discount rate for being all electric. Power company install gas lines in the street so they could stop the discount! By the way we are white euro types and they were Indian and cooked all the time! Add to that heatpump systems in a "cheapest built" home make you want to run to the store just to warm up.
Yes I understood about air infiltration and took great pains as I built to eliminate that.
I keep the house barefoot and t shirt warm in fact about once an hour my dog needs to go outside and cool off or he starts panting like he's having a heat stroke..
Frenchy,
It must be because of all the shellac that you have used :-)
In reality, You probably did a great job plugging and sealing, then a good insulation job.
In upper Michigan my house about the same size as yours has been heated with an 85K BTU furnace using propane. The second furnace has no need to start.
I used a lot of "Great Stuff" and dense pack cellulose in the walls and R-50 in the attic.
Sealing is so very important. I do not think there is a contractor out there that will be as careful in sealing up as a careful home owner can. Needless to say, but every tiny hole or crack leaks a lot of air and it all adds up in a hurry. Contractor would lose the bid if he or she figured in a "perfect sealing job".
Enjoy the benefits of your careful sealing and insulation job.
Virginbuild