Heat Load Calculator Canada

[Katrine Freggiaro]

Acopy of the load calculation is part of the required documentation. The load calculation can be documented by a submittal sheet from compliant software or by a load calculation worksheet from TECA, HRAI, ACCA or the CSA F280 standard.

An EnerGuide Rating System HOT2000 Full House Report is provided by a registered Energy Advisor working with a licensed Service Organization. The EnerGuide report must have been submitted to Natural Resources Canada.

The Heating, Refrigeration and Air Conditioning Institute of Canada (HRAI) and the Thermal Environment Comfort Association (TECA) both offer training courses on CAN/CSA F280-12 load calculations. HRAI offers a 4-day course in Victoria and Vancouver. TECA is currently updating its Forced Air Guidelines Course with CAN/CSA F280-12 material and will be offering it throughout BC. Both organisations also offer technical manuals on residential heat loss and heat gain load calculations. There are a large number of available software solutions and mobile apps that allow you to do Manual J calculations.

All I have are the inputs to whatever their modeling program was. I'll post it here, but I don't know if it will be very useful. Its hard to determine exactly what boundaries were used, but most of it seems reasonable. As far as I can tell.

You won't find information about fuel usage based load calculation as it doesn't exist out there. Man J is intended for new building, as long as the inputs are correct, the result is a reasonable accurate load. The issue when you do a Man J for an existing structure is that lot of these design parameters are not known, it is very easy to guess wrong and end up with a man J 2x or 3x reality. Plus lot of times HVAC techs put their finger on the scale when inputting data to come up with a number they want.

In your case if the fuel based calc is 20k, I would add on the heat loss for the 3rd floor from the Man J and call it a day. I would guess you'll end up somewhere around 12 to 14 BTU/sqft which is a hell of a lot closer to reality than 30BTU/sqft.

I don't think it's correct to add the 3rd floor Man J. The third floor is getting heated by heat that is escaping from the second floor. The OP said the third floor is within three degrees of the rest of the house when the heat is off, so the heat from the second floor must be significant. If the third floor were being heated independently the the heat flow from the second floor would be reduced by an equivalent amount.

You are right that using the full upstairs heat loss is overly conservative. But on the other hand, counting only the difference in heat attributable to the three degrees isn't right either. The amount of heat the flows from downstairs to upstairs is more when the upstairs is colder. When the upstairs is at the same temperature as downstairs, that upward heat flow is reduced, theoretically reduced to zero.

The theoretical model underlying both the Manual J and Dana's article is the same: that the heat flowing through an assembly is equal to the temperature difference times the thermal constant of that assembly; the thermal constant is the area of the assembly divided by the r-value. The design heating load is room temperature minus the design temperature times the thermal constant.

Both techniques are attempts to estimate the thermal constant, and thus calculate the design heating load. Manual J attempts to estimate it by measurement and analysis of the building. Dana's method attempts to estimate it by measuring the actual amount of heat put into the building.

Note that the underlying assumption -- that heat loss is linear and directly proportional to the temperature difference -- is probably not valid. However, since both methods use the same assumption both methods are equally affected by this inaccuracy. In practice the inaccuracy seems to be small enough that the results given are still useful. As economists like to say, all models are wrong, some are useful.

ok, feeling more confident about this. Might be a tough sell convincing folks they can get away with that small a heat pump. But saving $10k in electrical upgrades is a pretty good carrot.

Just to confirm, the graphs are from when you are heating with a heat pump?

What is the heat pump rated to?

Yes, the graphs are with the heat pump. Since the graphs include all electricity, it's conservative. It's a 24kbtu ducted Mitsubishi hyper heat, so rated to -14F or something I'll never see. There's a separate 9kbtu unit in a small room without ductwork which runs some but probably doesn't contribute much heat to the rest of the house.

Your load is also small enough that even if you keep in some resistance baseboard heaters to even out temperature, it won't change the overall energy usage much. Much simpler and cheaper than trying to run ducts to the 3rd floor if not already there although this won't get you cooling up there. If there are no ducts and cooling is a must, you can also look at installing a single wall mount in the 3rd floor hallway.

Ok so with respect to equipment sizing, you didn't go with a 16 or 20kBTU unit. Why was that? Having lived with the 24kbtu unit for some time would you have chosen the same size again, or gone with something smaller?

Thanks again for the power use graph. The weather is close to what are record lows in my area and you aren't even cracking 2500kW. Power constraints are significant in my complex so accurate power use information is critical.

I think the only two options were 18kbtu or 24kbtu. The 24kbtu is oversized for sure, but the moisture removal capacity for cooling is 4x the 18kbtu unit and it's an area with humid summers. The minimum heating and cooling capacities are actually pretty similar.

Your water heater likely uses a bit more gas in the winter than in the spring/summer/fall non-heating season, because the water temp comes in colder, so slightly more energy is needed to heat it to the same temp.

Ok, thanks again Folks. What I am hearing is that I am probably closer to the mark using the fuel consumption, given that that far fewer assumptions are required for the calculation. I also like this as it is a pretty simple tool my neighbors can apply to see what their units work out to. Even if they do use a gas fireplace some times, they could run their own experiment and heat their home for a billing cycle without using it. Or even over a week or so if they decide to just read their meter.

If you want to properly size an HVAC unit for a residential building, you should use the technique designed by the ACCA Association (Air Conditioning Contractors of America), the Manual J Residential Calculation.

In short, Manual J is the protocol that is used in order to determine the correct amount of heat that is needed to keep a house warm for its occupants, and the amount of cold air required in order to cool it when needed.

The Manual J Calculation gets complicated oftentimes and requires good knowledge of the technique. This is why contractors developed rule of thumb methods like the simple BTU calculator which you can use above.

The above formulas and calculations are estimated in good faith and are intended for generic, informative purposes. We do not guarantee the accuracy of this information. There are also other external factors that may affect or falsify the recommended BTUs. For accurate values, please consult a licensed HVAC company or engineer.

Note: This HVAC calculator is provided strictly as a quick method of computing general size and value conditions. Square foot methods are considered rule of thumb for use in quick calculations. The exact thermal load can be determined by using a full heat load analysis.

The recommended BTU loads were determined in good faith and are intended for general informative purposes only. We do not take responsibility for or guarantee any completeness, reliability, or accuracy of this information. There can be several other unique factors in certain applications that significantly affect and even falsify these values. You should always consult a licensed design engineer for the most accurate measurements and values, which can only be truly obtained after a thorough inspection of the job site is performed and all related factors are determined.

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An important aspect to properly planning a central air installation is the inclusion of a BTU calculation to ensure that your HVAC system can adequately heat and cool your home or office. Before we explain how to calculate heat load, we must answer an important question:

For an accurate measurement, we recommend contacting an HVAC professional, because there are a variety of factors that can come into play. These factors include insulation, building materials, number of windows, size and positioning of windows, appliances, electronics (computers, printers, etc. all put-off heat), how many people tend to occupy the home, and more. Heat load is measured in BTUs (British thermal units). One BTU is approximately 1055 joules and is defined by the amount of energy required for heating or cooling a single pound of water by one degree. Here is a simple to use formula. It is not intended to be the standard of truth, but it will definitely give you an idea of what direction to take in planning your HVAC system:

In order to illustrate the point further, here is a sample calculation: if you face 30-degree temperatures in your region and you want it to be 70 degrees in a 3,000 sq foot home with 8-foot ceilings, your calculation would look like this: 3000 x 8 x 40 x .135 = 129,600 BTUs Keep in mind that this is a very conservative estimate, meaning you probably will not need an HVAC system that puts out 129K BTUs. When you calculate heat load rather than turning to a professional you will get a less exact number. For reference sake, it seems that professional calculations tend to be in a range between 65-80% of what is calculated by the above formula. Example: a professional will likely find this home to require between 80,000-100,000 BTUs. As the saying goes, it is better to err on the side of caution. As mentioned, for proper planning we urge you to get a professional measurement of your heat load.

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