I've had 5 HVAC contractors through my house, and I emphasized to each one that I wanted a Manual J calculation performed before any new systems were installed. Only 1 contractor took extensive notes and measurements of rooms and windows -- he was here for almost 2 hours. He said he's my man because he teaches other contractors how to do the Manual J. Since then I've called him several times and he has not gotten back to me. All the other ones said "yes, of course I'm doing a Manual J" and show me their one-pager on a clip board. From what I can tell the one-pager allows them to note how many rooms are in the house and how many windows are in each room. The contractor then does some multiplication on the sheet and arrives at a load. Now, I thought Manual J's involved computer programs and specific information like construction type, window type, etc. Should I be demanding more? Is more required by building code (I'm in PA)? What should I ask to see? I don't actually want to OFFEND someone who I need to have a good working relationship with, so my natural inclination is to just say "OK!", while having the sneaking suspicion that they didn't do the fancy number crunching that I actually want. Also, is it *imperative* to do the Manual D along with the Manual J? Thanks in advance for any clarity.

Before lap tops were part of my daily routine, a manual J was done on a single sheet of paper. Anywhere governed by the International Residential Code requires a ManJ be performed- enforcement varies.
The more measurements taken the more accurate.
That said a pro usually knows what your home will require and there are all sorts of short cut programs.
Furnaces jump 20mbh or so incrementally so it's hard to oversize by too much. Different sizes may only be $50-$150 more and cost the same to operate. With geo sizing we design much tighter as the next size up might be $2,500 more.
Manual D is the duct design standard. If you are doing a retrofit and existing duct is adequate no reason for it.

Like so many other crafts in life the pros will do the math even though they know the outcome.

As far as choosing the contractor, I certainly wouldn't select one who didn't return calls. When shopping I usually find not one option meets all of my requirements so I go with the one who meets most of my criteria.

Posted By avalanche85 on 16 Oct 2012 11:43 AM
I've had 5 HVAC contractors through my house, and I emphasized to each one that I wanted a Manual J calculation performed before any new systems were installed. Only 1 contractor took extensive notes and measurements of rooms and windows -- he was here for almost 2 hours. He said he's my man because he teaches other contractors how to do the Manual J. Since then I've called him several times and he has not gotten back to me. All the other ones said "yes, of course I'm doing a Manual J" and show me their one-pager on a clip board. From what I can tell the one-pager allows them to note how many rooms are in the house and how many windows are in each room. The contractor then does some multiplication on the sheet and arrives at a load. Now, I thought Manual J's involved computer programs and specific information like construction type, window type, etc. Should I be demanding more? Is more required by building code (I'm in PA)? What should I ask to see? I don't actually want to OFFEND someone who I need to have a good working relationship with, so my natural inclination is to just say "OK!", while having the sneaking suspicion that they didn't do the fancy number crunching that I actually want. Also, is it *imperative* to do the Manual D along with the Manual J? Thanks in advance for any clarity.

The published U-factors of the windows & doors and the calculated U-factor of the walls by construction type have to be correct for Manual-J to have any validity.  A 1- pager based on a phone call won't have much validity.  A proper Manual-J doesn't take all day, but it takes a site visit and use of a tape measure rather than a WAG, and it takes more than one page.

A typical report might look like this:


http://www.wrightsoft.com/Portals/0...xample.pdf

or


http://www.gabuilderenergyservices....Report.pdf

or

https://www.acca.org/Files/?id=359

Most should list the construction type and U-factors used for the walls/attic and windows, and the room dimensions, etc, which are all useful for figuring out if there are any fundamental errors.  A mis-keying of wall construction type/code into the software packages etc can lead to misleading results.


The 99% and 1% design temperatures/conditions should be checked against those in the most recent Manual-J list, and use realistic indoor temps not 65F for cooling 80F heating, etc.

Manual-D is important to get the correct amount of flow & heat/cool for ducted-air systems, but even the best-balanced Manual-D design is easily thwarted by not air-sealing the ducts.  When you stray too far from Manual-D you end up with a greater variation in temp from the coolest to the warmest rooms, etc.

Until fairly recently heat load calculations were not required by code in very many places, but are becoming more common.  In California oversizing by more than some small fraction (it's either 10% or 15%) on heating & cooling equimpment is expressly dis-allowed under Title 24, but any HVAC hack worth their salt can pick the size number first and tweak the inputs to the software to suit (garbage in==garbage out), and even honest brokers tend toward the conservative side on factors rather than risk undersizing (which only very rarely happens in the real world.)  Air leakage rates is one place it's easy to tweak over quite a range, since even a blower door test won't tell you what the real infiltration/ventilation rates are.

How do contractors estimate the air exchange rates on a quick Manual J? That's a fairly significant source of heat loss and it can vary substantially depending on the history of the house. And do they actually poke around to verify that there actually is insulation and other sorts of things present?

Thanks for posting this thread and the replies. I didn't know what a Manual J is. Now, at least I know it exists.

Thanks for the links Dana.
Since my county zoning follows IRC I suspect my future new construction will have to be documented and inspected with the numbers on the links posted.

I did not see square footage on the pdf's. IIRC IRC 2012 uses ASRAE air exchange standards for single family homes. So I expect my new structure will be tested in ways new to even the county building inspectors.

Fresh air throughout the habitat is very important to me.

What do you suggest for me to make it easy for the inspectors to approve?

Air excahange rates are a multiplier (you might go load times 1.13 or whatever is indicated).

I never suggested quick was right- just not to let it be a deal breaker if the only one who did it right doesn't return calls.

Lest we forget the operating cost difference of a 95% furnace one size too big, may be virtually the same while the operating cost of a furnace 2 sizes too big may be less. With gas furnaces bigger doesn't kill your numbers.
With anything using a compressor this is not true.

Manual D is important in design to show someone else the installer knows what he is doing. Again in retrofit work as long as there aren't glaring shortcomings in the duct system the cost of scrutiny will likely exceed benefit.
Air sealing is another question.....

I am beginning to formulate the notion that the HVAC service industry might bear a certain amount of responsibility for energy wastage here in America.

Would I starve selling oversized heating units or would I feast doing energy surveys and retrofits?

Thanks for all the comments and advice! Dana1 - thanks for the very helpful examples. Getting a *valid* Manaul J from an HVAC contractor seems like a grueling uphill battle. At this point I see 2 options for my situation -- 1) I could go with a contractor who proposed replacing a unit, some duct work, adding returns, etc (described in another thread) for $20K. I've seen his work at someone else's house, and he is fastidiously neat and insulates his duct work. His downside is he only does the one-pager Manual J. 2) I could hire a local energy consultant for about $500 to do a full blown energy audit and make general recommendations, and then pay an additional $300 for a proper Manual J. I know the 2nd option is the "right thing to do", but $800 (for no actual physical improvements) is a lot of money! What do you all think? Should I just bite the bullet?

Posted By ICFHybrid on 17 Oct 2012 09:52 AM
I am beginning to formulate the notion that the HVAC service industry might bear a certain amount of responsibility for energy wastage here in America.

Would I starve selling oversized heating units or would I feast doing energy surveys and retrofits?

D'ya think?

The typical oversizing factor on heating systems I see on existing homes in my area is 3x plus. (And it's not because of all that building efficiency retrofitting that transpired between the initial install and the time I peeked at the nameplate ratings.)  And mastic-sealed ducts in residential installations are all but nonexistent here.

The efficiency and upfront cost penalties of gross oversizing are slight for hot air furnaces or old-school air conditioners, but the homeowner pays in lower-comfort.  For high mass boilers the efficiency hit at 3x+ oversizing is significant and for ground source heat pumps the up-front cost hit is gia-normous (unconscionable, really).  I'd bet more geo contractors do a reasonable job of heat load calculation than those popping in gas furnaces, but it's clearly not universal in the industry.

Posted By avalanche85 on 17 Oct 2012 10:42 AM
Thanks for all the comments and advice! Dana1 - thanks for the very helpful examples. Getting a *valid* Manaul J from an HVAC contractor seems like a grueling uphill battle. At this point I see 2 options for my situation -- 1) I could go with a contractor who proposed replacing a unit, some duct work, adding returns, etc (described in another thread) for $20K. I've seen his work at someone else's house, and he is fastidiously neat and insulates his duct work. His downside is he only does the one-pager Manual J. 2) I could hire a local energy consultant for about $500 to do a full blown energy audit and make general recommendations, and then pay an additional $300 for a proper Manual J. I know the 2nd option is the "right thing to do", but $800 (for no actual physical improvements) is a lot of money! What do you all think? Should I just bite the bullet?

For $800 I'd at least want a blower door test and IR imaging pointing out the major air leaks, thermal bypasses, and insulation gaps.

Using a spreadsheet and some crudely estimated U-factors you can get decently close to a true heat load using I=B=R methods.   But to guesstimate the wall & attic U-factors we'd need to know the construction of the house.

For an example of how that works, let's say you're designing for a 99% outside design temp of +10F, and indoor temp of 70F, which is a delta-T of (70-10=) 60F.

A wood-sided 2x4 stick built with typical framing density comes in at about R9.5-R10 with fiber insulation in the cavites with the thermal bridging of the framing factored in, about R1.5-R2 if they're empty.  Let's assume you have cellulose or at least R11 batts in those cavities, call it R10. The U-factor is 1/R, or (1/10=) 0.1 BTU per hour per square foot per degree-F.  For a 60 degree delta that's (0.1 x 60=)  6BTU/hr per square foot. Measure up all of the exterior wall area (subtracting out the windows & doors), multiply that by 6 and you have a BTU/hr number for the walls.

Say you have ~6" of rock wool or cellulose in the attic between joists, that's about R20 center-cavity, but about R15 after thermal bridging of the joists, so the U-factor would be about (1/R15=) ~0.07.  Times the 60F delta that's (0.07 x 60=) 4.2 BTU per square foot.  Measure the attic floor area, multiply by 4.2, you have the total loss for the attic.

If you have single-panes with tight fitting clear storm windows or pre-1985 clear glass double-panes use U0.5 (x 60F = 30BTU/square foot). If you have low-E storms over single panes or purdy-good post-1990 replacement windows call it U0.34 (x 60F= 20 BTU/square foot).  Solid 2" exterior doors, call it U 0.5, if panelized with thin spots call it U 0.8.

For unfinished poured concrete foundations, count only the portion from about a foot below grade on up, and call it U1.0.

Add it all up and you get a reasonable picture of the heat load, but add a fudge factor of 10-15% for air leakage and unseen gaps in insulation, etc.  You can usually tighten up the place a lot at low cost if you happen to underestimate the air leakage factor, and making it as tight as possible is the right thing to do. If it's so tight that your mid-winter humidity is north of 35% RH @ 70F (monitor it with a $10 AccuRite) even when it's well below 20F outside, it's time to get serious about active ventilation, and that would be a GOOD thing.  Energy recovery ventilation limits the heating & cooling penalty to a small fraction of the hit from high "natural" or exhuast-only ventilation approaches would be, and you can set the ventilation rates pretty high, most of the time. (In winter that would have a drying effect though, so backing off or controlling it via dehumidistat to 30-35% makes sense when it's cold out. That doens't work in summer though, since the outdoor air is often more humid than interior air, so you can just duty-cycle it in summer.)

This is just an example- the true construction types and insulation depths & types matter- if you have recent windows and know their U-factors (or can look them up) use those.  With a description of what you have for siding/framing/insulation  it's possible to come up with reasonable guesstimates of the true U-factors.

If you have a full season of heating history on the place and the bills & meter-reading/tank-filling dates you can work backward from those and get similar numbers too.

A home only requires the amount of heating BTU's it requires. Oversize gas furnaces deliver BTU's more quickly (and can cost less to run than "rightsize").
They then stop burning gas as thermostat is no longer calling for heat- thus not wasting extra gas.
It is fallicy to say that oversized furnaces always waste energy particularly in the ECM blower age. It is political not rational to always suggest the smallest furnace for the job.
People change- get older- take blood thinners and suddenly like it at 75+ degrees in their home. I would rarely put my Amana 45MBH 95+% furnace in a home with a41MBH load even though the capacity is supposed to be there. A 70,000 btu furnace is indicated to all but the engineer who thinks only within the manual J box.

Set back strategies do not work well with tight sized furnaces.

With multistage and truly modulating gas furnaces you are going to have to get grossly oversized (manytimes requirement) to burn more energy than a smaller unit.

This is not true of air-conditioners or heat pumps (air or water source) as compressors drawing more amps during lighter loads often use more gross amps in spite of shorter cycles in extreme loads.

The original Manual J heat loads were again on a single sheet of paper. What da ya guys think we did before computer spread sheets?

I'm not suggesting we ignore the envelope but we are putting a lot of baggage on a relatively inexpensive furnace replacement. I certainly don't expect the installer to spend more than an hour loading a home for a $3,000 job.

Doesn't mean one shouldn't do an energy audit and purchase a high efficiency furnace. I'm suggesting they can be done independently of one another as there is no harm caused by the slightly larger than needed furnace.

Rare is the situation where oversizing a low-mass hot air furnace has a lower operating cost than a right-sized furnace- you'd have to read all of the footnote qualifiers to determine that in the particulars. But it's true that 1.5-2x oversizing doesn't take an efficiency hit, most of the time. But there's no advantage to oversizing either.

The difference in design condition heating load between a home kept 78F instead of 68F is less than 15% for US zone 5 & cooler (where you're more likely to have heat loads over 40KBTU/hr), and not a good rationale for up-sizing. Design condition or cooler temps account for only 1% of the heating season hours(hours when most folks are in bed), and rare is the situation where Manual-J hits exactly the BTU output rating for a furnace. Rarer still is the house where the built-in margin on Manual-J is substantially less than 15%. In real-world situations the aging person with lower tolerance for cool can just turn up the thermostat and never take a hit in comfort, even if the output of the furnace is dead-nuts-on the Manual-J calculated heat load at 68F. Bumping the size from 45MBH to 70MBH for a 41MBH Manual-J on the grounds that at 78F the load becomes 47MBH is just silly, IMHO. A 2MBH shortfall that allows room temps drop to 75F between 4AM & dawn on the 3 coldest nights of the year rather than 78F is not a condition worth paying even $10 extra for.

With continously variable modulation or multi-stage systems you don't really WANT to use setback strategies, since the highest efficiency & comfort occurs at the lower speed & firing range, and while oversizing one of those doesn't take a huge comfort hit, it's likely to take a slight efficiency performance hit if used with a setback strategy simply because it can due to oversizing factors, since it runs full-blast at lower efficiency on the recovery ramp.

The typical systems in my region are ~3x oversized for no good reason, and even 5-6x oversizing isn't off the radar- they're out there! While 3x+ oversizing doesn't take a performance hit for a hot air furnace, it's usually a substantial hit with high-mass boilers. And even with ECM drive modulating gas furnaces 3x oversizing takes a hit in comfort.

Yes, a Manual-J can make it onto a single sheet of paper, but it takes a pretty long telephone conversation to deliver all the necessary information over the phone for most homes. Sure, taking more than an hour would be a burden, but taking more than 5 minutes isn't. The number of HVAC installers using " 35BTU a foot times X-square feet of space" rules of thumb for a 30 second heat load calc are still unfortunately rampant.

Question out of curiosity, I don't mean to hijack the thread. I've read other places that when people have gone to geothermal, or when they tighten up the house and do all the right things that they go from a 100,000 btu furnace to a 32,000 btu furnace. From a 5 ton to a 2 ton air conditioning. I am curious if perhaps the geothermal guys know better what they are doing? I am guessing based on several contractors I had come in to bid on a furnace install. My situation was a single story ranch, on a slab, 1200 sq ft (going by external dimensions). From the ground to the peak of the roof was only 12 ft. and no cathedral ceilings. Furnace was installed in 1981. Was a 100,000 btu input, 80,000 btu output natural gas. The blower motor was giving out. I had entertained the thought of installing a new furnace and out of the 5 contractors that bid the job, none of them wanted to do a manual j. They all said they didn't need to, that they all had the experience to know, based on square footage, what the house needed. 3 of the 5 wanted to install a 97% efficient gas furnace at 100,000 btu input. That didn't make sense to me. I highly doubted the existing furnace was still 80% efficient some 30 yrs after it had been manufactured, I wouldn't have been surprised at 65% efficient. So, going to a new furnace at 97% efficient would have been overkill in my estimation. I ended up just replacing the blower motor on account that I didn't have enough money saved up to replace the central AC (R-12) in addition to the furnace. Later on, when I remodeled the master bedroom (converted from a single stall garage way back when), I found out that most of my heat loss was through the off the shelf fireplace and flu (metal pipe) rather than the sliding glass door that replaced the garage door. I'm pretty sure, had I not had to move for my job, that I could have easily done all the necessary work to only need a 32,000 btu furnace.

Perhaps Avalanche 85, you could look into a geothermal to do some of this stuff. Be open and honest with them, and see what they would do the calculation for. Otherwise, if you want to push your schedule back a bit, I think the ACCA sells the calculation books for the Manual J and the Manual D for one residence applications available to homeowners. I don't remember what they went for, I'm going to guess around $100. It might be worth it to just detail it out yourself, design yourself and then hire the contractor to just install the stuff. Perhaps the others can see if that is a good idea or not, but I would think if you are having trouble finding a trusted source for design, this might be the way to go. Just because you may not trust the contractor to design, doesn't mean they don't know how to install.

Hot air furnaces don't degrade from 78-80% AFUE to 65% AFUE, even if 3x oversized and 30 years old.  At 6-10x oversizing it might dip below 70% though.

Yours might have started out at 78%  and dropped to 75%.  Were it as low as 65% you'd have to be burning SOOOooo inefficiently that you'd have soot accumulation clogging the flue and your neighbor's carbon monoxide detector would be going off. :-)  If the flame is mostly blue, not orange, even a rusty heat exchanger would still be giving you better than 65% steady-state efficiency. Unless it runs seriously short burn cycles measured in a few 10s of seconds rather than minutes, it's average efficiency will be pretty close to it's steady state efficiency.

But there's no excuse for replacing it with a similarly oversized condensing unit, even if it's multi-staged, since there are plenty of smaller units out there.  It takes 15 minutes to run a whole-house calc based on fuel usage against heating degree-days (maybe a half-hour if you're really slow at math or don't know how to use the internet to get the heating degree-day data. :-) )

Geothermal contractors who oversize by 3x don't get the contract, since the upfront cost of the oversizing is daunting, and their competitors who have confidence in their calculation methods can beat them on price while demonstrating their own competence about what the true loads are.

But oversizing a gas furnace 3x is typically a few hundred USD, with no downside to the contractor, and the cost of paying a contractor for a Manual-J may exceed the cost of oversizing it "just to be sure".  (Just case it ever gets down to -120F or something like that. :-) )

Dana, silly question perhaps, but while it's easy to pick an indoor temperature, what temperature do you use for an outdoor temp? Geographically I'm in zone 5, but with altitude that's a bit iffy itself I'm looking at 9000 degrees days according to charts. And there is normally a 40 degree difference between daytime and nighttime temps. I've seen it reach -35F, but normally -20F night is common in the dead of winter. With so much variability, what do you use for ambient outside for design?

LarryT-

For areas near cities, you can usually do a search for "design temperature (name of city)" and find some data. I think Dana often suggests using WeatherSpark, although I didn't find the design temps there in a quick look. It sounds like you might be out in the boonies, so in that case you can guess a design temp from the data you presented, -35 F in the extreme, and -20 F regularly. If you took -30 F as your design temp, then the differential temperature (from 70 F) would be 100 F. If the "proper" design temperature were -35 F instead, then the differential temperature and the computed heating plant output would only be off by 5%. Heat losses are assumed to be linear with the temperature differential.

Weatherspark does not give you a design temp, but the weather history is good enough you can usually eyeball it with the cursor if you compare the average January temps to design-temp delta for a few nearby locations with known design temps.

If there's a nearby city with a ACCA Manual-J 99% design temp listed but is at a different altitude, you can also hit pretty close using a 3F/1000' adiabatic cooling rule of thumb, eg:

Say you live in Estes Park CO (~7500'), which is pretty close to Fort Collins (~5000') that has a listed 99% design temp of +1F.  The 2500' difference in elevation results in about a (3F x 2500/1000=) 7.5F cooler outdoor temp. Subtracting 7.5F from the listed +1F gives you a design temp of about -6F or -7F.  Even though it sometimes gets down to -15F sometimes in Estes Park, it doesn't stay there for hours/days on end- it's simply part of the remaining 1% of binned hours in the heating season.  Even if you designed the heating system EXACTLY for -7F doesn't mean you'd experience 63F indoor temps instead of 70F, since the thermal mass of the house limits how fast the interior temps will drop when the heating plant is running a bit short of the actual load, and the duration of the colder-than 99% events are usually pretty short in duration- a handful of hours at most, except during the cold snap of the century.  And if the heating plant is even 10% oversized you're already covered.

Lee, thanks. So it appears that I ignore day/night variation and go closer to the extreme lows. That makes sense when you think about it, as you really need to size for the larger loads. It WAS a silly question. Dana, Regarding choosing that low temp, I am in th boonies (Angelfire) and using information from say Taos or Santa Fe doesn't fit well because ofthe unique location of AF. It sits in a valley surrounded by mountains and traps cold air masses as they pass through. At 9000 feet, it is 2000 feet above Santa Fe and while that should be 6F difference, the difference is generally much more during these air mass encroachments. For example, Feb 2010, an 18F difference between the two sites. Not at all unusual from the 4 years I've been watching it. I'll probably look at -25 and -30 and as I'll normally have large zones that are normally unoccupied and thus the inside design temp for those will be lower, I think I'll be be able to size a boiler that will work. Thanks for the help!

Angel Fire isn't really close enough to Santa Fe to be a useful comparison- it's close to 50 miles!) Whereas Estes Park CO is less than 20 miles Fort Collins. Unfortunately the short-list doesn't even cover Taos, which would still have a relevant design temp, even though there are daily differences on any particular day that exceed the 3F/1000' rule of thumb by quite a lot. If you take the deltas that occurred from a Weatherspark dataset on the coldest days and factor that delta into a published ASHRAE or ACCA design temp for Taos you'd be pretty safe without going overboard.

You can also estimate a 99.5%design temp by taking the absolute annual winter lows logged at a nearby weatherstation over the past 10 years and averaging them, which will usually put you within a degree or two of a 25 year binned hourly number.

Thanks Dana. When you mean close, you really mean close! I'll start looking for historical data then for Angel Fire as you suggest.