Can anyone help with me with J and D calulations? Jsut tell me what to supply and I'll do it. Thank you.

J&D calculations done right require dimensions of every exterior wall, window and door. Wall ceiling and floor insulation values are needed as well.

so a pdf of my prints? Or are you suggesting that's too much work to ask of someone?

Posted By ljcf on 26 Jun 2013 08:50 AM
so a pdf of my prints? Or are you suggesting that's too much work to ask of someone?

I know the answer to that question: Not at all; that should not be too much work for you!

I have been wrestling with that question as well. I've had 5 HVAC "professionals" come out to give me a furnace bid each one quoting a different sized furnace. Curious as I am I created a rudimentary spreadsheet to do the heat loss calculations in order to at least get me in the ballpark. It is crude but if you want it email me.

Crude I=B=R type calculations using spreadsheet tools are fine for sizing lower cost systems like condensing gas furnaces where the the upcharge of oversizing is miniscule, with near-zero efficiency consequences, but for designing GSHP the fuller analysis really IS called for (and worth paying for.) Simple spreadsheet load calculations are also very useful for comparing building envelope upgrade costs & benefits too, or for ballparking it for the purpose vetting GSHP proposals. The range of proposals can be pretty broad, (and sometimes truly nuts, if prior threads on this forum are any indication.)

Dana is correct…good enough hardly ever is for this. We have free heat loss analysis software on our website for ball parking. However, it is certainly not ASHRAE RTS (our preference) or even ACCA Manual J. You can’t really blame these HVAC folks either. You are not paying them to do a proper analysis and they have no guarantee that they will even win the bid. So why would you expect them to expand much of their time and effort to do a proper analysis, assuming they are even competent enough to do one? They are likely just multiplying the square footage by their favorite catalog design factor to estimate the heating load and furnace size.

You can get a full blown Program for one time use as a home owner for $49 here

"Or are you suggesting that's too much work to ask of someone?"
Folks will do it for you for a fee, but generally one does not offer professional services for free. That said you can google how to calculate heat loss based on heating degree days and get a load based on actual usage.

Doing a load calculation is imperative. And it is the case of "pay me now or pay me later". However, I was taken aback from my experiences with the highly recommended professionals. I can accept the fact that maybe they don't want to take the time and effort to do a load calculation. But it only seem to be "good business" to 1) do a GREAT installation job and 2) make sure it is sized right.

The consumer must be informed in order to make a good decision. Should every homeowner hire an engineer when it comes to an HVAC install? Or does their HVAC vender have the definitive solution? Or do they need to buy the $49 software?

Joe, you need to actually do a heat load analysis to first determine the room/building total heat loss BTU/hour-deg F parameter before you can then apply heating degree day data to estimate/forecast monthly/annual heating needs. This is certainly something one would want to do.

Agagent3, when you really need a good heating/cooling load analysis, we recommend hiring an experienced/licensed professional engineer to do it…which is what we are and one of our services, so full disclosure. We also recommend hiring an experienced/licensed HVAC technician to do the installation…which again is what we are although we only mess with passive solar and hydronic radiant heating systems these days, so full disclosure. So the first question is how good an analysis is good enough for what you are doing? Is this new construction needing an initial and accurate heating/cooling load analysis? Or is this an existing building with an existing heating/cooling system that you are just replacing? You might want to play around with our simple DIY software to first get educated about this and ball park your situation:

Borst Heat Loss Analysis Software

Borst Integrated Heating System Performance Software

BTW, we consider HVAC-Calc to be good heat loss calculator. It has many nice features and is easy to use. However, calling it a full blown program would be a very big stretch for us as it is not nearly as accurate as RTS and doesn’t handle passive solar or hydronic radiant very well. Then again, there isn't any analysis software available at any price that does what we need it to do, so we had to develop our own.  Here is a HVAC-Calc review with much interesting info that aligns fairly well with our critique although it is somewhat dated:

Homeowner Review of HVAC-Calc

"Joe, you need to actually do a heat load analysis to first determine the room/building total heat loss BTU/hour-deg F parameter before you can then apply heating degree day data to estimate/forecast monthly/annual heating needs. This is certainly something one would want to do."

Not so. If you have a known annual MCF (or propane or electric) usage (excluding DHW or cooking etc.) you multiply that X's the btu value of each unit (nat gas 1,123,900/MCF, fuel oil 140,00/gal, propane 91,500/gal, electricity 3,413/kwh......ish) which gives you total btu's consumed. Times furnace efficiency (.92 or .80 etc.) and you have actual heating btu total.
Divide that total by heating degree days and you have daily use which divided by 24 equals hourly use per degree delta T (difference between design low and set point around here we use 0F as design temp so 70 is our delta T). Multiply hourly degree use times delta T and you have actual btu/h loss which is more accurate than a manual J load estimate.

Obviously this only works for existing buildings manual J is much better at predicting load of a not yet constructed building.

What Joe said. You can look at a few summer months to get any water heating usage and also run the calculation separately for several Winter months and take the maximum btu/delta T value (this helps account for wind and solar gain effects).

Doing a Manual J is a good way to go obviously. And the only way to go for a new build. However, actual usage has to be the most accurate. Actual useage gets past the "garbage in; garbage out" phenomenon.

I was reminded of the heat calc dilemma when my friend reported to me that the HVAC people are selling him a 45,000 btu propane furnace for his 980 square foot, highly insulated new build home. it seems to me that a business will sell you as much as you can afford.

"I was reminded of the heat calc dilemma when my friend reported to me that the HVAC people are selling him a 45,000 btu propane furnace for his 980 square foot, highly insulated new build home. it seems to me that a business will sell you as much as you can afford."

If your friend happened to work with an Amana or Goodman dealer they did not have a smaller furnace available to them. Also know that the incremental cost difference in furnaces is relatively modest (often less than $100 to size up) so dealers do not up size for profit (generally). Further unlike with heat pumps the larger the furnace (provided the ducts are sized correctly) the lower the gas bill.

I forgot the rest of the story. He is also installing a Pacific Air wood stove. He was open to solutions but received the same paradigm, burn something with old technology. What about a mini split system? Or what about Daikin Altherma system?

"I forgot the rest of the story. He is also installing a Pacific Air wood stove. He was open to solutions but received the same paradigm, burn something with old technology. What about a mini split system? Or what about Daikin Altherma system?"

We pride ourselves on not being a "one solution dealer". Meaning we analyze and try to establish best bang for our customer's buck. If natural gas is available that usually the least expensive solution with an acceptable operating cost. While geo or ASHP might be cheaper to operate much of the time, the up front additional cost may not be recovered within the equipment lifecycle. The 30% tax credits often can get geo there on new construction.

So what I'm saying is if your friend called furnace companies then don't be suprised if all they sell is furnaces. If natural gas is available, don't be suprised if they suggest ASHPs aren't the way to go.
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