What is a Manual S Equipment Sizing Service?
Manual S by the ACCA is method of sizing heating and cooling equipment for your home. The Manual S uses data generated from a Manual J in the for of Sensible Cooling Loads, Latent Cooling Loads, and Heat Gain. It compares these with conditions that are present at the jobsite itself. The specifications of the HVAC units being installed are then compared against the total heat loss and heat gain data to see if the unit is too large, too small, or just right. In regards to SEER rating, COP, HSPF, and EER these are typically specifications required by local codes or building officials and the Manual S has a descriptive place for them, but does not actually incorporate them into the mathematical equation of Manual S equipment sizing.
So when might you need a Manual S?
The first step of any Manual S is to have a Manual J. If you have a Manual J created then your logical next step might be to have a Manual S created. Let’s say that your HVAC unit is not longer performing well and it is 15-20 years old or you are building a new home. A Manual S will help size a system that can provide comfort for you and the occupants of your home in conjunction with the Manual J. Also Manual S will help make sure your HVAC equiptment is sized for the following:
Total Heat Loss
Total Heat Gain
Sensible Heat Gain
Latent Heat Gain
Sensible / Total Ratio
Target Cooling TD
Winter Outdoor Design Temperature
Summer Outdoor Design Temperature
ID Design Relative Humidity Indoor
ID Design Relative Humidity Outdoor
Altitude
It takes all these variables and then compares them against the equipment you are choosing to install.
For equipment selection the Manual S will look at:
Furnace
Boiler
Heat Pump / Air Conditioner
Evaporator
Air Handler
Total Capacity with Altitude Correction
Selected Equipment Size
Total Cooling Capacity at Outdoor Design Temperature
Sensible Cooling Capacity at Outdoor Design Temperature
Latent Cooling Capacity at Outdoor Design Temperature
With all these variables in place the Manual S is then ready to make a determination of typically 3 outcomes:
The HVAC unit chosen is oversized
The HVAC unit chosen is undersized
The HVAC unit chosen is properly sized
It is possible you could have a combination of these outcomes. For example:
A chosen HVAC unit for your new home can cover the sensible cooling load, but the latent load of the structure is too large for specifications of the suggested HVAC unit. This would result in the Manual S showing a properly sized unit for Sensible cooling load, and an undersized unit for Latent cooling load. In this instance choosing a different unit would result in the best possible outcome.
It is important to meet the total demand of your HVAC system is satisfied by the equipment being installed. Cooling Equipment can be oversized but not by more than 15% of the total load on the Manual J. Sensible load is a measure of the amount of cooling a structure needs to maintain 75 degrees
Latent load is the amount of humidity that should be removed. The longer a unit runs the more humidity it can remove and typically the less humidity in a living environment the more comfortable the occupants will be. What this means is that an oversized unit will cool the structure too quickly resulting in more humidity inside and less comfort, also possible moisture problems. An undersized unit will run all the time resulting in drier air, but also higher energy bills. A unit that is sized properly using Manual S will operate enough to cool the room to the indoor design temperature, and remove the latent load or humidity from the structure. This results in more comfortable temperatures and lower energy bills.
Heating equipment on a Manual S can be oversized by not more than 40% of the peak heating loss from Manual J. What this means is that if your Manual J specifies a 100k heat loss you can install up to a 140k heat unit.
Cooling equipment on a Manual S varies depending on whether it is an Air Conditioning cooling unit or a heat pump based cooling unit. You should use the following criteria based on which you choose:
Air Conditioner Cooling Unit can be 95%-115% of the Sensible Heat Gain plus Latent Heat gain of the structure. In other words you can use a unit that is in the range of 5% less or 15% more that the total heat gain listed on your Manual J. You can also choose to use the next official size higher of HVAC unit that satisfies the Manual S equipment selection.
Heat Pump Cooling Units are allowed to be within 100%-125% of the Sensible Heat Gain plus Latent Heat gain of the structure. So with this type of cooling system you can use a unit that covers all of the load or 25% more than the load of your Manual J. The next size up can typically be used as long as it covers the sensible and latent heat gains listed on your Manual J.
Some issues with exact Manual S sizing are that HVAC units are typically sized in increments. Cooling units are sized by the half ton and heating equipment is typically sized by 5k BTU increments. This means that not all sizes of units might available. Allowing the use of the next nominal size that covers the Manual J sensible heat gain, latent heat gain, and total heat loss is the solution. This means that if your Manual S calls for a 3.1 ton cooling unit, you may be allowed to use a 3.5 ton unit because it is the next logical unit size that fully covers all the criteria mentioned in your Manual J. The same thing with heating equipment. The building inspector typically has the final say on these specifications so working closely with the building department and constituents is key. If they have preference, that is typically what the Manual S will default to. A Manual S can specify so much, but the local knowledge of an experience building inspector is always something to take note of and give proper respect on any construction project.
If you need a Manual S we charge $79. We need your Manual J, Order Number, PDF Plans, Jobsite address, and square footage. You can email those plans@manualjs.com and we will get your Manual S taken care of.
