I disagree with your commentary on fuel use analysis there dana. *In some cases* it's better, but it's all about the quality and purity of the knowledge you have access to for either calculation or measurement. Even if you know the house is left at 68 24/7: is it running at 95%, 90% , 85%, or 80% efficiency, including distribution losses? Duct systems with leakage can be even worse than that. Should you use base 65, 60, or 55 for the HDD or even lower? Is DHW included and what impact does it have, is there uninsulated recirc on an indirect? atmospheric water heater using the same fuel? tankless coil? etc, etc etc.

It's very easy to be off by 25% or more with fuel use analysis. The quality of the available information is critical for either MJ or Fuel use analysis, and both often involve significant judgement calls for various unknown factors. In some.. dare I say many... cases, evaluation is simply not possible at a degree of precision greater than an experienced ballpark.

But it takes someone who understands the numbers to know where on the "precision continuum" either approach is likely to get you.

Beyond this, zone design is simply a matter of experience since you have to have gotten a few complaints to appreciate how thermostat placement, and to a lesser degree, specification can effect efficiency and comfort.

We added a radiant floors and a thermostat to an always cold sun room and had to move the main floor thermostat 15 feet to avoid the heat generated by a tile floor directly above the main house furnace plenum. After 10 years our new client is comfortable.

Manual 'J' can model new, old or a combination thereof, considering all aspects of the envelope and using real climate history for real world loads at 99 percentile rather than 100 years loads from word-of-mouth accounts. Working with a good Manual 'J' program also gives us perspective and makes ROI and payback easier to estimate by accounting for changes in insulation, window quality and efficiency. When you are building an addition you have on model for the new, another for the old. Some of our larger Victorians may take a full day to model with major remodels or additions to be lay out.

The need to properly size equipment can't be overstated and is most critical is air conditioning since an over-sized RX system will no effectively remove latent heat.

Also note that when it's 80F and humid outside, even a "correctly sized" AC system is massively over-sized and won't effectively remove humidity. Variable speeds help, as does a tight house.

We are back to the Manual 'J' . We use a lot of Unico high pressure systems for old and new construction extracting more moisture regardless.

well this post got way off topic lol :D.. Anywho, to update everyone, my home is only 800 sqft. My quote was for a 60k btu furnace. Also i already kno they quoted me high because i should qualify for a rebate here in New York, that pays for 50%!!! of the cost.. so regardless that brings my price for the high end furnace to $3250 with installation.. Also through my energy supplier i get another $1100 rebate!! which brings the cost down to around $2000.. Im not sure if you guys do conversions, but i used roughly 450 gallons of oil this winter, at $2.54 per gallon. Now im not sure what that converts to therms used, but based on what my gas supplier is charging https://www.nationalgridus.com/niagaramohawk/home/rates/4_gas.asp which is a $20 delivery + $0.38 per therm for the first 47 therms and $0.63 for over 50 therms... can anyone give me a rough estimate how much i would be saving just in Gas heating versus oil.

you have a huge amount of potential savings in your home by tightening it up and improving your "envelope". To compare, many area builders could and have built homes of that size that are comfortably heated and air conditioned with a 9,000 - 12,000 btu mini split. you should start with an energy audit, which you might be able to get through the state.

Posted By Bob I on 08 Apr 2015 07:24 PM
you have a huge amount of potential savings in your home by tightening it up and improving your "envelope". To compare, many area builders could and have built homes of that size that are comfortably heated and air conditioned with a 9,000 - 12,000 btu mini split. you should start with an energy audit, which you might be able to get through the state.

As a matter of fact, i have a insulation company coming out friday to give me a quote on some doing some blown insulation in my attic (im assuming roughly 12-24in across the entire attic) as well as spray foaming my basement. I have been approved for a home energy audit as well, but they need/want to do a estimate first, then they will do the audit.

A mini-split indeed and good on ya!

"If you are unsure about sizing the equipment hire an engineer. They have nothing to sell."

Oh the do indeed have things to sell. I also suggest one leaves the design to someone who owns the results vs having multiple fingers in the pie anstarting the blame game if something goes awry.

"So with a 96% efficient furnace, reducing a 1.7x oversizing down to a riskier 1.25x can't possibly save more than 4% (I'll guess more like 1%)."

Operating cost calculators suggest the larger units are less expensive to operate. If required btus are fixed, then delivering them more quickly (shorter vent and circulator fan run times) may indeed be more cost effective.

"I've looked at many entries on this site about sizing HVAC equipment and find in variable the phrase "do a complete room by room heat loss analysis".

Room by room loads would be uncommon in a retrofit where duct system is already in place.

"If there is that much slop in the home system I guess flying by the seat of your pants is okay. Being somewhat facetious, I guess my balloon has been burst and all this mambo jumbo of ACCA and Manuel J is just another money making ploy."

Not sure your conclusion follows. We typically do a "block load" for retrofits and it is part of the estimate which is not charged for.

the parasitic energy draw difference between a larger and smaller furnace or boiler should in no way shape or form outweigh the efficiency hit of oversizing... if that's happening there are other more severe problems with the design.

Posted By NRT.Rob on 08 Apr 2015 11:08 AM
I disagree with your commentary on fuel use analysis there dana. *In some cases* it's better, but it's all about the quality and purity of the knowledge you have access to for either calculation or measurement. Even if you know the house is left at 68 24/7: is it running at 95%, 90% , 85%, or 80% efficiency, including distribution losses? Duct systems with leakage can be even worse than that. Should you use base 65, 60, or 55 for the HDD or even lower? Is DHW included and what impact does it have, is there uninsulated recirc on an indirect? atmospheric water heater using the same fuel? tankless coil? etc, etc etc.

It's very easy to be off by 25% or more with fuel use analysis. The quality of the available information is critical for either MJ or Fuel use analysis, and both often involve significant judgement calls for various unknown factors. In some.. dare I say many... cases, evaluation is simply not possible at a degree of precision greater than an experienced ballpark.

But it takes someone who understands the numbers to know where on the "precision continuum" either approach is likely to get you.

If they're using the same ducts (or other distribution system) all of those potential system efficiency factors are already built into the fuel-use numbers.  The new equipment using the same distribution will suffer the same losses, but will be sized correctly to compensate for that loss.


If they're heating hot water with the same fuel the fuel use numbers will skew the load numbers to the high side, but it would require a major hot water (ab)user to turn that alone into a double-digit error.  The falling efficiency of older equipment will also skew the numbers to the high side. The error factors in a fuel-use analysis in most cases will overshoot reality by a bit, when re-measured with brand new more right-sized equipment, which is why upsizing by 25% would be "safe".   Of COURSE it's important to ask about thermostat settings, auxillary heat sources, plug loads, etc, and use some judgment.  But unless there are some red flags on the front, up-sizing by 25% from the implied heat load based on fuel use, nameplate equipment efficiency, and heating degree days won't leave you cold, even in those rare instances where the fuel use numbers might undershoot reality.

It takes a VERY high performance house to skew the heating/cooling balance point base down to 55F, but we don't see very many of those with heat loads even half the output of most fossil burners. Special houses are special cases.  But for most base 65F is close enough, even though for very tight houses built to IRC 2012 code min it might be more accurate if using base 60F.  In those cases using base 65F won't change the numbers enough to matter-  the loads in those houses are usually smaller than even the smallest condensing gas furnaces, and if you upsized it exactly 1.25x from the fuel use numbers the subtle change in slope doesn't diverge sufficiently to leave you cold on design day or colder.

It's always good to sanity check a load calc using other methods. If an aggressive manual-J was delivering a number greater than 1.25x the fuel-use derived number (or 0.75x that number) it's worth looking for the reasons why that might be so.  But given that most don't bother with a Manual-J or I=B=R, a fuel use derived number is still a  legitimate stake to put in the ground, and it takes very little time, and won't lead to the ridiculous oversizing that is so rampant in the industry, even if you upsized by 1.5x "just to be sure...".

Fuel use based heat loads are certainly valid and should be part of any retrofit heating appliance sizing excercise. It is also true that many, perhaps the majority of, professional HVAC contractors neglect to us HDD, peak load or any valid heat load software, in spite of the fact that most are readily available from heating appliance producers.

The fact remains, Manual 'J' addresses all of the valid factors related to accurate heat load modeling. The extraordinary, e.g. Dana, may perform such calculations faster than my Wrightsoft program, but this is an extraordinary circumstance.

Most mere humans will need, and much benefit from, a good software program. The more complex the structure and particular where loads are high demand, low load a good program will help in control strategy.

If we are doing a straight up replacement the bar is not that high for the minimally informed, i.e. contractors that recommend equipment with a higher output than the adequate output of the equipment they are replacing are frankly not bright. So, don't use them. Those who produces a sample of their Manual 'J' work should be considered seriously. It is the standard for the US HVAC industry and qualified HVAC contractors know it.

We could not design and install HVAC and hydronic heating systems for challenging homes, new and old, without Manual 'J'. The time and risk of serious mistakes would put us out of business.

It would be interesting to see a study that compared pre-construction manual J calculations by typical (and also careful) contractors to measured peak and seasonal loads.

One thing is clear- if you put only 10 kbtu of hvac into a house that needs 40 kbtu, you will save energy :-).

I would read that study if it included the alternatives. Well done Jon.

fair points Dana, I'm not trying to say fuel use shouldn't ever be done. Just that it's not a panacea: there are many cases where the reality is not easily reverse engineered from fuel use for a home that might change occupancy and usage habits, etc. At least, not to a degree of precision greater than a decent ballpark or even a half-assed block load in Manual J. IN SOME CASES IT IS BETTER, though, I want to make sure to say that in big block letters. It's definitely a case by case thing is all and I run into ambiguous cases pretty regularly... then it becomes a question of figuring out what path will get you closer to reality with the available information and time constraints: MJ, FUA, or SWAG .

It's a quick sanity checker, but to be honest I can probably get within the same degree of precision by guessing most of the time with the kind of missing, ambiguous, or changing data set I usually have access to in a retrofit situation. If we're setting the bar at 1.25-1.5x oversized, that's not a hard bar to hit. Personally I disagree with the oversizing philosophy, this last feb notwithstanding, a straight heat load already discounts anything that could contribute to the load internally and boiler sizing is necessarily oversized to some degree by limitations of available model sizes.. I think a "right sized" boiler will already have the excess capacity you need for basically any situation anyone is likely to see with any regularity. If a record setting winter means that rooms are a few degrees cooler periodically, putting a price tag on that experience would, I think in most cases, be a price that most but not all clients would be willing to pay.

That's a fine conversation to have with a client. Setting expectations is very important in any case. Some people don't even own sweaters

Jonr: I don't know anyone who routinely generates loads lower than I did with a basic homebrew load calculator based off of seigenthaler's load recommendations. To my knowledge I have never undersized a heat source for a house (missed a couple of times on room loads...). So at some level I'm probably still oversizing myself... They might be slow to recover from setback but then, using reset, they are going to be slow for that anyway. So if I'm in that boat I'm pretty confident in saying that the vast majority of other loads I've seen, when done at all, are at least 25% high and a distressing number are even higher.

Getting back to Ed's question...a 60,000 Btu unit for an 800 sq. ft. house seems like a lot! To be that energy inefficient it would have to have single pane windows and sorry for the wording but like a tar paper shack.

Fuel use gets to be more informative as one accumulates years of data. Trend analysis and the leveling out effect would make the data more reliable.

Frankly at 800 sq.ft. saving fuel is a non-issue. A proper Manual 'J' would then serve to choose the best heat emitter and prove that said emitter would meet the heat load of the rooms in question.

We are now down to providing comfort.

These loads disqualify all conventional gas-fired appliances, save storage water heaters, which is the direction I go on nearly all small radiant floor loads e.g. basements and "cabins". The built-in buffer feature of a storage water heater, especially a condensing storage water heater absolve all sins, i.e. micro-zones and short-cycling phobias. Not to mention the small footprint in a close space.

Posted By agagent3 on 07 Apr 2015 09:31 AM
We heated with wood for 30 years. At retirement the wife wanted to be able to get away so we had 4 professionals come out to size and replace the old propane furnace. I made sure to give them all the R-values and U-values since we had just remodeled. Every professional over sized the unit and wanted $4000 to $5000 to replace it. I did my home work and asked if a 45,000 Btu furnace would work. Everyone to a person were extremely reluctant to go with a 45,000 Btu unit. Dismayed with the professionals I purchased a 2 stage, high efficiency Goodman and installed it myself. I fired it up on January 9th. The furnace always kicks in on high stage then goes to low stage (35,000 Btu). It will go to high stage if it can't keep up. Not ever during even the coldest days did it ever need to go to high stage. The moral of the story is; educate yourself and be wary of professionals after all they are salesmen first and foremost. If you are unsure about sizing the equipment hire an engineer. They have nothing to sell.

So very true… The instructions for the existing building energy usage calculator that Borst provided a link earlier explains this very well:

"If you are just looking to replace the existing cooling/heating system with a similar system and you only want to ensure that the replacement system will provide the appropriate cooling/heating capacity during the hottest/coldest expected climatic design conditions where the building is located, this calculator will provide a very simple and very accurate way to do this. This calculator allows you to use the existing cooling/heating system to remove/add heat in a quantifiable manner to allow accurate determination of the actual heat transfer characteristics of the existing building. However, if you are designing a cooling/heating system for a new building that does NOT yet exist or if you are designing a totally different type of heating system for an existing building (e.g., replacing an existing furnace forced air heating system with a hydronic radiant floor heating system), you will need to exercise our Cooling Load Analysis Calculator and Heat Loss Analysis Calculator, and you will likely also need to accomplish a room-by-room analysis as well.

It is entirely possible that the existing cooling/heating system is significantly under or over-sized for the existing building. If a system is significantly under-sized, the system may not provide adequate cooling/heating capacity for the existing building during the hottest/coldest expected climatic design conditions. If a system is significantly over-sized, the system may operate inefficiently and the system may even short cycle resulting in increased operational and maintenance expense. Unfortunately, many HVAC contractors are often not capable of performing a proper analysis or simply do not want to expend the effort/time required to perform a proper analysis. Conversely, you may also find yourself in a situation where the HVAC contractor recommends doing a more expensive standard cooling load analysis or heat loss analysis when only this relatively simple analysis of the existing building energy usage is really required. Please see our Hydronic Radiant Floor Heating Design Calculator instructions for more information about HVAC contractor problems.

Performing this existing building energy usage analysis is also often much more accurate than performing a standard cooling load analysis or heat loss analysis. The reason for this is because a standard cooling load analysis or heat loss analysis requires making assumptions about the building material R-values and guessing the building outdoor infiltration rate. There is also a lot of opportunity to make errors when doing the standard cooling load analysis or heat loss analysis and making these errors is unfortunately very common in the HVAC industry. The only factor that limits the accuracy this existing building energy usage analysis is measuring the fuel used during the measurement period and this can normally be accomplished within a couple percent accuracy."

So an existing building energy analysis will likely be more accurate than a Manual J. The only reason this would not be the case is if the person doing the analysis was grossly incompetent.

can anyone give me a rough estimate how much i would be saving just in Gas heating versus oil.

Nat gas will be about 1/3 the cost, plus whatever you gain from increased furnace efficiency. In my very non-expert guess, you should be able to get a decent furnace installed for $4k and have it pay for itself in < 5 years.

No, it's not just about incompetence. That explanation of the accuracy of fuel use analysis is more than a little misleading and it's nearly impossible to get within "a couple percent accuracy" because you can't even define the HDD load seen by the building in any given timeframe... modified for solar gain, plug loads, or building mass... to that degree of accuracy. And that HDD assumption is the fundamental basis of the calculation.

You have to have the right information to make fuel use analysis accurate, and that information is not always available either, no matter how competent you are. And then you end up having to make your educated assumptions, just like you do for a manual J. The assumptions that you make in an MJ calc are infiltration on most projects... which might swing the final load number 10-15% in most cases, though it can be higher for people who have no idea what reasonable infiltration is... and in more extreme cases overall insulation quality/consistence may be severe enough to move the needle significantly from a blanket assumption. People screwing up calculations happen in any case. Including, oddly enough, people using free calculators reflecting factors they have an experience dataset of 1 project for or, if we're being generous, even a few dozen projects. But it's just as easy to screw up a FUA calculation you don't fully understand as it is to screw up an MJ calculation you don't fully understand.

Go back to the HDD assumption, the fundamental basis of a fuel use analysis. "typical" housing stock uses base 65 HDD... the house is presumed (<-- GUESSED) to need heat whenever the temp falls below 65 degree outdoor temp. But good modern construction could be more accurately reflected as base 60 in many cases, or even better. How's the mass? Solar gain profile? plug and internal load situations? Even if you "nail" the HDD assumption, that number itself is an abstraction that is not perfectly accurate, and the difference in HDD between base 65 and base 60 in my area is a spread of 22%... much wider than the swing I'm likely to get with an educated guess at an infiltration rate. And that's assuming all the rest of the info I need for a FUA is right there and perfectly accurate, which it rarely is.

OFTEN, fuel use analysis is better, that's undeniably true. But OFTEN, Manual J is better. OFTEN, neither will get you to a high degree of precision in a retrofit situation. OFTEN, like in the 800sq ft guy here's case... it doesn't matter even a little bit for any likely heat source he's going to use and you can skip the whole thing and just throw a reasonable ballpark at it (not so for cooling, which almost always must be calculated because of the huge impact solar gain plays and how hard it is to guesstimate solar gain by looking, unless you're in a low/no solar gain situation or it's overwhelmed by other factors).

Here in maine many, if not most people have wood or pellet stoves in their homes and use them regularly. That makes fuel use analysis for heating systems almost completely useless for something like half the houses in this state right at the starting gate (unless, of course, you can quantify the wood, moisture content, stove efficiency, wood species, or ensure the house will continue to have that much wood heat added every year from now on forever...). People change needs, expectations, occupants, thermostat settings, insulation/windows... all of which sacrifice precision in this analysis. Dana's right that it can give you a "stake in the ground" but it's often simply not a method of determining a particularly precise load.

MJ and FUA are tools. You use the right tool for the job that is in front of you. Competence is knowing which tool is right for that job.

IF usage is consistent, IF temperatures indoors are fairly consistent, IF the user can make reasonable HDD assumptions for a given project, and no other sources of heat or significant changes in the envelope or distribution system are occurring, then FUA is probably the fastest and most accurate way to get to a reasonably solid number. But that's a lot of Ifs.

Block loads in MJ are not very time intensive either, and if the envelope information is known more precisely than the information needed for a clean FUA then the sizer should use that instead.

If any improvement to the distribution/emitter system is desired, then room by room analysis is the only game in town.