Is 24% Manual J inaccuracy common? Is this overestimation desirable?
Aaron_Williams
| Posted in General Questions on
I have both measured and calculated thermal load numbers. There is 24% disparity. Measured load is 15127btu @23f. CoolCalc Manual J calculates 18820btu. I have used other online load calculators with varying results. I always use aggressive inputs but they still overestimate. Is it normal for a Manual J calculation to overestimate? If it is the norm, should I size my heating equipment to the Manual J because this is the proven method of sizing?
Located in the pacific northwest, the home is an 1100 ft 1970 rancher on a vented crawlspace. Construction is original apart from modern windows and 18in fiberglass insulation in the vented attic. Baseboard heat.
I am concerned that when specifying heating/cooling equipment its easy to apply safety margins repeatedly, compounding over specification of capacity. This appears to be fraught process. How inaccurate is the data that forms the foundation? Is there evidence that the Manual J results in over sized equipment? Or does it provide a desired standard margin of capacity?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
How did you measure design day load?
I measured electricity consumption at design temperatures at 4am on several days. 100% of kW consumed contributed to heating the house. I just noticed I did not take into account that two people are inside the house. Can you see other errors in this method of measurement?
At 260 btu/ hr each, the occupants of the house bring the measured thermal load to 15651. This closes the disparity to 20%. The occupants were already part of the Manual J.
Consider that the Manual J may be accurate (for its intended purpose) and your measurement was low.
Infiltration changes with wind. Ie, Manual J may assume closer to worse case wind conditions than the day you measured. There are also thermal mass effects that make it hard to say that your 4am load didn't have some assistance from earlier.
It's not clear to me that Manual J attempts to be as accurate as possible - it may purposely add some margin to avoid the greater impact of under-sizing.
Also note that design day load isn't coldest/hottest day load - so there is some margin needed to cover this. Depends on climate, but often about 24%.
Jon, infiltration and thermal mass are very good points. I agree with you, a few thousand btu either way is inconsequential. I am concerned this margin of error repeated a few times along the way to specifying equipment may be an issue. I have no idea what the right way to choose equipment is. I apologize if I am over-complicating this.
This is my best effort to integrate thermal mass and infiltration into my plan to evaluate the accuracy of my CoolCalc Manual J. I have to assume the CoolCalc worst case infiltration estimate is reasonable. At 3169btu( the difference between the measured and Manual J results) the infiltration would be 37CFM *Edit 65 CFM. As for the thermal mass, I assume its effect is mute as the design temperature is transient, only briefly occurring at night. The thermal load never catches up to the total thermal loss. If these assumptions are reasonable, the accuracy of the CoolCalc Manual J is very good.
So, I have obeyed the mantra, "do a Manual J." Jon you say "You don't size equipment to Manual J design day anyway." So what do I do with this Manual J. I thought it was the foundation for choosing equipment. I'm sure it has been asked and answered. Someone please point me in the right direction. Where do I go from here??
Thank you for the help and education.
> As for the thermal mass, I assume its effect is mute as the design temperature is transient
A brief 4am dip to design temp is exactly the case that results in a less than design day load (and so a measurement error). But long term dips to well below design temperature happen and equipment should be sized to handle it.
Differences in wind alone can cause differences much greater than what you calculated. Here in MI, high winds can add 20K btu/hr.
> what do I do with this Manual J
You do it and then add some margin to cover the issues. How much is open to debate, but avoid under-sizing and huge over-sizing and it will be fine.
Back when I took my classes for HVAC design, the instructor told us not to pull our hair out wondering if we made a mistake and our client would be cold as a result. He said Manual J has a lot of safety factors baked in, like most engineering calculations, just follow the process as well as possible and the results will be satisfactory. We are talking about a fairly imprecise science here, followed by equipment selection that often jumps in size 10,000-20,000 btus at a time. Do you manual J as well as possible, size per the results, don't worry about it. If you aren't confident in your Manual J, hire a professional.
The comfort issues related to oversizing are from MASSIVE oversizing, often double or even triple what the house needs. The safety margins in a manual J will not cause comfort issues.
Jon
> long term dips to well below design temperature happen and equipment should be sized to handle it.
>You do it (Manual J) and then add some margin to cover the issues.
Following this procedure, we calculate the load at one condition then extrapolate the worst case scenario. If the worst case is the number we want, why not calculate it to begin with?
is Yupster's advice contradicting Jon's?
>Manual J has a lot of safety factors baked in
>size per the results, don't worry about it
I will continue try to see if my measured, and calculated thermal losses numbers will agree. If the Manual J has safety factors baked in, I want to know how much. I will need to learn more about what assumptions the Manual J makes when calculating wind infiltration and the effect of thermal mass. I expect I will have to preform the manual J without the aid of a online calculator so I can see what assumptions the calculation makes regarding wind infiltration and thermal mass.
I am flogging this dead horse because the various online calculators don't come close to agreeing, and the Manual J's builtin safety factor is an enigma.
I still believe the various margins of error could result in I gross over sizing. Inaccurate or poorly preformed manual J, + inappropriate safety margin, + few equipment option eg. must buy one size up.
There are may ways to approach sizing. I imagine the science must inform a practical approach. Without any experience, and unable to find much consensus, I feel fiddling with a few load measurements is my only anchor in reality.
Any more comments or direction to threads are appreciated
The design temperature is the point at which your area stays above 99% of the time. Only 1% of the time does it drop below this temperature, or about 88 hours a year. Your house has some thermal mass to help it ride those extreme lows, maybe with an interior temperature fluctuation of a couple degrees. But the coldest days of the year are often bright, sunny days because of complicated weather reasons. So passive solar gain also helps mitigate those days.
Insulate and air seal well, and you really don't have to worry! My house is not on the extreme end of air tightness or insulation, and it can sail through the whole day with NO heat on at -20°C with a temperature drop of ~5°F.
Calculated numbers will never match measured numbers because your calculation are at some theoretical state (perfectly even 72°F inside, perfectly even temp outside, infiltration based on a LOT of assumptions, etc.). The real world has gusting winds, internal gains varying wildly, missed insulation spots, over insulated spots, a bunch of air leaks in one room, none in the other, sun shining in here and not over there, exterior doors opening, etc. That's why engineers spent a lot of time and effort developing this imperfect but good enough method for sizing equipment.
Safety margins usually exist for a reason. Leave those in.
Don't poorly perform your manual J, not a problem
Match your equipment as closely as possible, unless you are going to manufacture your own equipment you are stuck with these choices anyway. Modulating equipment is also available, which would completely alleviate any problems from minimal oversizing caused by minor Manual J errors and safety factors.
Manual J is a good enough method. You can trust it. You can also choose not to. It's your time. :) The rabbit trail is long and deep...
You should trust Manual J provides sufficient safety factors. It makes no accommodation for wind, solar gain, or thermal mass. For heating, it is really a simple calculation.
There is no need to add margin, but that margin is often added in for you just by the equipment sizes that are available. The equipment capacity usually doesn't perfectly match the load calculation. When your load is just barely above the capacity might be the only time to look in more detail.
For using load vs energy use, you don't need to be at the design temp. Instead calculate the load vs difference between indoor and outdoor temperature. You get a btu/hr/deg. Use that number to get your load at the design temp. For heat pumps you can then plot the load btu/hr/deg vs the published equipment capacity as shown in the attached chart.
>"Modulating equipment is also available, which would completely alleviate any problems from minimal oversizing caused by minor Manual J errors and safety factors."
Modulating ranges are not infinite, which is why it's important to be as aggressive as possible in the Manual-J input assumptions, and minimize the oversize factor from the Manual-J.
When working from a fuel-use measurement 1.4x is fine, but when upsizing from a Manual-J something smaller is usually better. If the Manual-J is overshooting by 20% (typical, common) that's a 1.2x oversize factor. Upsizing from there by the full ASHRAE recommended 1.4x oversize factor would be 1.2 * 1.4 = 1.7x oversizing, which is the AFUE test assumption. That's fine for efficiency on non-modulating equipment, but more than ideal for most modulating equipment. Upsizing modulating equipement by 1.2x from a Manual-J would still be under 1.5x total oversizing- enough to cover Polar Vortex disturbance coolth, yet still a decent modulation range even on equipment with only 2.5:1 turn down ratios.
> Upsizing from there by the full ASHRAE recommended 1.4x oversize factor
Do you have a link to such wording? All I see is that anything from 1.0x to 1.4x is allowed for a furnace (less for other equipment) with no actual recommendation for either.
Your points seem very reasonable Yupster. For the Manual J to always work it must have an innate safety factor. Having appropriate insurance for the margin of error is a good thing. I have been trying to reinvent the wheel. Using a perfectly measured load is not a proven process. Trying to validate the proven processes is a waste of time. I will use CoolCalc for my thermal load. It produces near average results and is recognized by the ACCA. Before heading for the Manual S, I need to Investigate further the need far an additional safety factor. I have seen it mentioned frequently. I suppose only the professionals that have seen a Manual J result in under-sizing can verify its necessity. Because I cant find the ACCA prescribing it anywhere, omitting the additional safety factor seems responsible. While still keen to avoid oversizing I will shift my focus to following proven procedure and finding corroborating statements regarding the omission or addition of additional safety factors. Surely real life application of the Manual J process has been studied. Do these studdies shed light on the need for additional caution.?Once again I am probably overcomplicating this. As a rank amateur armed only with google. I lack any common sense that a little experience would provide.
This is the reason for the preceding chaos. The door to my crawl space is small. Ultimately I am trying to convince myself that the largest medium static unit that will fit the door, will heat my house.
Sorry. I did not see the replies from Dana and Matt. Looks like there is consensuses, the Manual J has an adequate margin of safety preventing under-sizing. I dont know of any practical reason to be more cautious when specifying equipment. Now I have to try to convince a contractor.
Dana
>Upsizing modulating equipement by 1.2x from a Manual-J ...
When upsizing by 1.2 do you use rated or maximum capacity of the modulating equipment?
Thankyou
Matt. Thank you for the education of on load vs energy use measurement. A btu/hr/deg chart would be useful. I am a little confused about plotting the split system capacity, are the manufacturers capacity charts showing rated or maximum capacity? Im not even sure what number I want. Do I want to sacrifice efficiency on the coldest day in order to be more efficient on days with less load?
The equipment capacity comes from the capacity tables in the engineering guide:
https://portal.fujitsugeneral.com/files/catalog/files/(D&T)%20ARU12-48RGLX.pdf
You use these tables to select equipment.
The chart I show is for a 18RFLD.
Are you installing new ductwork? Where are you located and what is your design temp?
*Clarifying that the charts show max capacity at combinations of indoor/outdoor temps. The COP they show is interesting, but not terribly informative for efficiency. The rated HSPF number is what you should use to compare efficiency.
Thank you, the capacity charts make sense now. I plan to install ductwork and insulate the perimeter of the crawlspace. Located in Port Alberni BC, CoolCalc recommended a deign temp of 25F. I have been looking at a ARU18RGLX. it produces 20,820btu@23FDB. At 11% over the 18820btu Manual J is this a good match? The ARU24RGLX would produce 25,660btu, a 36% margin. Given I will still have electric heat as backup any thoughts on which unit would be most efficient? The minimum output of 5,400 is the same for both units.
I thank all who have educated me. I could not stand the idea of blindly following an unknown contractor's recommendation of what I should buy from him.
Not sure of this, but doesn't code (IRC) require following Manual S which requires <= 25% over-sizing for a heating dominant heat pump? Ie, isn't 36% over a code violation?
Your design temp looks correct.
http://ashrae-meteo.info/old_version_2015/index.php?lat=49.32&lng=-124.93&place=%27%27&wmo=714750
The ARU18RGLX looks like it should work.
How big is the place? I would consider the 18RFLD standard unit. You get a .4 higher HSPF, lower modulation at 3100 btu/hr, a much easier to duct profile, and a bit more capacity of 22k btu/hr @23F. Don't be scared about the .36 static unless you have particular reason to be scared. If you design everything around a velocity of less than 400 ft/min you should be fine. This is particularly true if you can do a central return with a 20x30x2 filter.
The link attached was for the GLX series.
See RLF Technical Manual.
https://portal.fujitsugeneral.com/files/catalog/files/(DT)ARU9-18RLF-AOU9-18RLFC2015061.pdf
Its good to know an 18RLFD can be used in a whole home ducted system. A large airfilter/return grill is easy to accommodate if placed in a corner. The 3100 btu/hr minimum of the 18RLFD and 11.3 HPPF are appealing. its 22,000btu max @ 23FDB is is 1180btu higher than the 18RGLX and %17 over my Manual J. I assume 18ARLF and 18ARLFD are the same.
A system like the 18RLFD @0.36 static looks like it will require a greater level of precision when deigning ductwork. Could I expect an average contractor to get it right? I would request copies of his calculations showing the predicted static pressure. And also verify the measured static is within spec once the louvers and dampers are adjusted (and a half plugged filter is simulated?). It would be very disappointed if I installed a lower static unit for efficiency, only to end up operating it outside spec, resulting in unknown performance/efficiency losses. I will shoot for less than 400 ft/min. I cant wait to see what margin of error this affords.
I just realized you want to put this in a vented crawl space, which would add a fair bit more load with all the duct in unconditioned space. I have run through how much.
Any chance the main hallway can drop 10”?
You should consider converting the crawl space to a condition space https://www.buildingscience.com/documents/information-sheets/crawlspace-insulation
Ideal is to have an engineer design the system and a contractor install per the design.
John Semmelhack has a very good presentation
https://www.phius.org/NAPHC2018/Think%20Little%20Slide%20Deck%20-%20NAPHC%202018.pdf
John or Energy Vanguard are two groups involved here that do design for a lot of these systems. I don’t know who is doing them up where you are, but I am sure there are a few groups.
Matt - See post #18. He is insulating the perimeter of the crawl. I assume this means that it will be a conditioned crawl.
Yup missed that, this should be good.
Aaron, I assume there are some doors to the 179sf room. Would the soffit space be available for a low wall return?
You will likely need transfer grilles in the bedrooms. You can get about 80cfm around most doors. https://www.energyvanguard.com/blog/easy-retrofit-return-air-bedrooms
Heat that crawlspace up to about 100F (design day, usually less) and (depending on flooring) you may not need any ducts/returns/vents at all. But balance would be cumbersome, so only practical as supplemental heat.
I can't vouch for the wisdom of this idea, but if you put the thermostat for the crawl space in the conditioned space it's self-balancing.
Without tweaking, a centrally located room will have the same radiant heat input as a corner room - but their loads are different. Not like hydronic, where you can vary the amount of tubing and/or use zoning.
Yes there are doors in the 179sf room. A sliding glass door to the south west and an interior door in the north and west. A soffit down the north-south center line would be practical and the addition of of a mechanical closet with return at the north end of the centerline would be unobtrusive.
I had assumed it was a worthwhile asset the crawlspace ducting provided, allowing conditioned air to be distributed to the far sides of the house and at floor level. A known asset of building in the crawlspace is less drywall dust in the house and an improved quality of life, mostly by keeping my wife happy. I cant find info on how much of an asset register placement is.
* I didnt look hard enough. For discussion on register placement see...
https://www.greenbuildingadvisor.com/question/hvac-supply-registers-in-a-mostly-heat-dedicated-system
https://www.greenbuildingadvisor.com/question/location-of-supply-ducts-ceiling-or-floor-depends-on-heating-or-cooling-climate
Is the "CD" in RLFCD meaningfull?