Calculating a home's heating/cooling loads requires knowledge beyond simply knowing how to enter numbers into canned software. Understanding the "what" and "why" are important. That knowledge goes beyond the level of most HVAC techs. Plus, it takes time to perform the load calculations. Time is money. If you got load calculations for free, you got exactly what you paid for.

i recommend you contact a HERS rater and get the load calculated by a professional. The benefits of right-sizing the equipment will more than pay for the cost of the calculations by a professional.

I agree with the preceding two comments but will also add that if the equipment being installed is a mini split, there's less of a risk (other than added equipment cost) of consequences of slight oversizing than would be the case with any kind of traditional systems, since all or most mini split types modulate their output and adjust to how much or how little cooling/heating is required. Equipment that's a little too big will see less time operating at 90-100% levels and should still do a good job.

As the preceding comments suggest, load calcs and proper sizing are part science and part art/experience. Properly done load assessments are at best are estimates and results will vary within a range. They not fixed amounts. If the variation you're seeing is +/- 10 to 15 percent, I wouldn't be concerned.

Elmer makes a good point. If the calculations differ, but they still land within the range of the same sized equipment, then it should not be a problem.

You can hire a third party to do a load calculation. The problem becomes when the HVAC contractor disagrees with the numbers and walks away from the job.

Following the prescription of an owners' third party "expert" should reduce the HVAC contractor's risk. Under that scenario, the HVAC contractor isn't performing design nor making a performance guarantee to the owner. They are only responsible to furnish and install the equipment, duct work, and associated controls as specified and as designed unless such specifications and design violate codes or the manufacturer's instructions. Fur might fly if the system doesn't perform as intended, but I think the onus will be on the designer--not the installer.

"walks away from the job."

Do you think this could happen? I would understand if a contractor suspecting in good faith that the indicated sizing could be too small (and if disagreeing with his own real calc done honestly) saying "I'll sell you what you want but I won't promise or guarantee that it will be adequate for your needs" but walking away would be something else. Maybe so.

There are still so many who do this work without actually doing real load calcs. I guess I can imagine one of those dinosaurs taking issue with something he himself doesn't do, for that reason if for no other. The "replace same with same" mentality is too well intrenched.

My first thought is why you didn't include the heat load calculations with your question. It would be good for people to know what they are, and if there is a difference that even matters.

No two calculations will come up the same because they depend on so many variables used to calculate them, and how a person measures and determines the variables. If the differences are close by a few thousand BTU it doesn't matter which one is correct and more like horseshoes, close enough.

Heat sources come in Btu's that will will not match your load requirement exactly. There are no 62,450 Btu or 27,015 Btu mini splits. So If one company said 42900 btu loss and another said 46700 btu loss and there is nothing between 40K and 50K you're going to get a 50K.

Thank you everyone. You all bring up a lot of good points.

In response to your questions Mike, I got three different estimates. I should have phrased my question differently because I'm really just assuming that their calculations differed, because their recommendations did. For a single unit in a roughly 12x22 room, one recommended 9000 btus, while two recommended 1500btus. On a whole house system, one recommended 4 units (two 9000 btu units and two 6000 btu units, 30,000 in total), one recommended 3 units (one 15000 btu unit and two 12000 btu units, 39,000 in total), while the third person recommended 2 units (one 15000 unit and one 6000 btu unit, 21000 in total).

As for the second question, only one of them actually measured the rooms, and he sent me the estimate the following day. The one that proposed the smaller units entered info into his computer and gave me his recommendation right away, as well as emailing the proposal later. He also explained his reasoning for his recommendation before recommending it, stating that oversized units would cycle more than desired and would not be as efficient.

Regarding the comparison of current equipment size, I have no idea. I've got oil powered steam heat. I don't know the specs of the boiler.

The OP's dilemma is a common one: contractors are proposing different solutions to the same problem. In this case, the equipment capacities differ by +/- 30% of the average. The intended "apples to apples" comparison is more like an apples to avocados to artichokes comparison. The OP is left to sort out which is the most credible proposal and hope for the best.

A better approach is to have someone with design expertise specify the equipment and have the different contractors quote their cost to furnish and install the specified equipment. You'll need to pay for the expertise invested up front for design, but you won't have to become an HVAC expert in an effort to sort out different proposals and your contractors won't have to fake being system designers.

Elmer,

In today's world where you have to wait for weeks to hire a competent HVAC contractor not taking on the job where the customer is more likely to generate call backs is more likely. But then again I suppose there are people who will install anything you want and then use it as an excuse to not fix any problem.

Katri,

In cases where the heating load value falls just above one size, or just below the next size, the contractor uses his judgement whether to round up or down. You also have to remember that the 9,000 and 15,000 BTUs are nominal sizes. The actual BTU output of is usually a smaller number. The actual outputs differ by manufacture and model, yet they will be labeled with the same nominal size.

The other thing that is also happening is the number of units differ in the quotes. One contractor may say the whole house can achieve a uniform temperature with 3 units, another may say you need 4 units. That is more of a distribution decision. This is the drawback of mini splits versus a central HVAC system. Maintaining an even temperature though out the house is more difficult. You might be forced to over design in sizing.

Mike--Regarding my current system, although I'm not sure about the boiler capacity, it occurred to me that I do have a 350 sq foot, 800 btu window air conditioner. I have it in the bathroom window at the top of the stairs, and it keeps the whole house, 1300 sq ft, comfortable when I have to use it.

Charles--I could also hire someone to do the load calculation and then choose the contractor that A) I feel is most credible, and B) comes closest to that calculation.

When you ask three different contractors to quote a replacement for an existing HVAC system it's not surprising to get three or more alternatives and a range of prices. The exception would be if it's a straightforward equipment swap out in kind (which low-end replacement contractors often manage to make not so straightforward.) If it's not a swap-out, you'll need to wade through every other aspect of each contractor's proposal. One contractor may be providing a zoned system, or a multi-stage system or a variable speed system and others may not. One contractor may include a performance test of existing ductwork and an allowance for additional sealing as necessary. And the controls proposed can range from simple to sophisticated. That's why it's a better approach to design and specify the system to be quoted and get everyone on the same page.

it occurred to me that I do have a 350 sq foot, 800 btu window air conditioner. I have it in the bathroom window at the top of the stairs, and it keeps the whole house, 1300 sq ft, comfortable when I have to use it.

I am not sure I follow this. What area is 350 sq. feet? Do you mean an 8000 BTU window AC? It is located inside the bathroom and can keep the whole house (75 - 77 degrees) comfortable on a 90 degree day?

This might be possible if the house is heavily shaded by trees and the blinds are blocking the sun. Maybe this is the reason your are getting wide variations in the load calculations. Some of the calculators are not including items unique to your house.

I would be curious to know if you can hire someone local to your area who can do a heating and cooling load calculation for your house as unique as yours.

Mike--I think that might be a good idea, getting someone independent of those selling the heat pump to do the calculation. The 350 sq ft figure is from the info on the air conditioner, an 8000 BTU unit, and I guess the 350 sq ft is what it is supposed to be able to handle. And yes, my house is quite shaded--and pretty small.

Charles--Yes, that makes sense.

The heat load calculations for the two different contractors are 30,000 btu and 38,500 btu. A difference of 8,500 seems like quite a difference, but being a lay person, I really don't know. Is that a reasonable difference between the two different calculations? Thanks again.

It suggests a different set of inputs into the manual J software used to calculate heating and cooling loads.