Heat loss FAQ

What is a Heat Loss or Heat Gain Calculation?

Heat Loss is the amount of heating BTU's (size of heating unit) needed to replace the heat lost from your home per hour during the winter months at a given OD design temperature

Gain is the cooling BTU requirement (size of air conditioner) needed to overcome the heat of summer entering your house in btu's per hour.

How are heat gains and heat losses done?

The whole house is measured room by room, or just possible the home perimeter, making note of window size, direction the windows face, type of construction, wall insulation, length and direction of exposed wall. Any unconditioned walls, which are walls not exposed to the outside, but to unheated areas like a wall with a garage or sunroof on the other side. The amount of attic and floor insulation and size of all exterior doors. All these measurements will affect the Heat Loss or Heat Gain due to prevailing winds and direct sunlight. The data is calculated, the result is generated in total home Btu requirements or room-by-room requirements and equipment sizes.


Does the heat loss or heat gain change with outdoor temperature changes?

Yes, the heat loss or heat gain will change with outdoor temperature changes. The calculations are done for the average minimum and maximum temperature of the area you live in. When a heat loss is performed, let's use for an example 13ºf, we will achieve a given heat loss for the home.  Let's use a heat loss of 45,000 btu/h. In other words, your heat loss at 13ºf is 45,000 per hour. Let's say today it has warmed up to 32ºF. The heat loss is less than 45,000 it may only be 30,000. Your heat loss does the opposite of outside temperatures. As the temperature outside goes down the heat loss goes up. When the outdoor temperature goes up the heat loss goes down.

Why do a heat loss or heat gain calculation?

Properly sized equipment runs more efficiently by cycling properly thus saving fuel. Properly sized units will reduce maintenance costs by not starting and stopping as often. If a unit cycles too often (heating up too quickly and shutting off again while thermostat is still calling) due to being oversized the rated efficiency of the equipment may never be met. The unit does not start at peak efficiency. It will start out lower and work its way up to the efficiency rating of the manufacturer. In short, the larger the unit above the heat loss the less efficient it will become.
If the unit is undersized, it will run a lot more than needed and may never heat the home but using excess fuel. This is not as bad mechanically as cycling too often but it will result in higher fuel bills.
Proper sized equipment is less expensive than larger oversized equipment, so you start saving money right away. Any upgrades done to the home since it was built will reduce the heat loss, which is why we should do one anytime a heating or cooling appliance is replaced.

Is it common that my new heating or cooling appliance is over sized?

Yes, this is the very common and is the number one problem in the heating and cooling industry today. The first step in efficient operation of the new heating or cooling appliance is proper sizing. The existing unit in your home is normally 100% or larger than
My contractor wants to install the same size appliance as the old one is that OK? It heated the house fine for many years.
More than 85% of the old equipment was oversized. It was the thing to do years ago when fuel was cheap. Most people have tightened up their homes by installing energy efficient windows and doors. Updating insulation values and adding weatherstripping to reduce heating costs and improve comfort. This will reduce the heat loss and require a smaller unit. Larger than needed unit will cycle too often and cause higher fuel bills and less comfort as you will feel more temperature swings.
There are three ratings on gas and oil equipment. The first is input BTU's and the second is DOE Output or Gross Output and the last is net ratings. If the contractor would size from the old unit and read the input and size from this figure, he would be installing even a larger unit then the original was. Here is an example: 140,000 btu's on the old unit running at 70% efficiency would be an output of about 98,000 btu's. The new boiler rated at 140,000 and running at 83% efficiency is going to be 116,200 btu's. The new boiler would then be larger then the old and some fuel savings would be lost. Size it properly with a Heat Loss or Heat Gain calculation. Any new piece of equipment should save you money. Size it properly and save the most money.

My contractor wants to size the new boiler by measuring the radiation.

As stated above, the same is true for the hot water heating units. The baseboard was usually run on all exterior walls, which were more than needed when installed and definitely more then you need today. The plus side to excessive baseboard and properly sizing the boiler is the temperature of the water going through the baseboard could be a much lower temperature which requires less fuel and improved comfort. At one time heating systems were sized with a window cracked open at night because people thought it was healthier. Sizing off the existing radiation will oversize the boiler and cost more to install, operate and maintain.
The only time a contractor should measure the radiators and size a boiler with these factors would be a steam system. Steam boilers are sized according to square feet of steam and not so much on btu's. We also will do sizing for steam boilers. This requires different forms. (See pricing page)

You keep mentioning oversized equipment. Isn't it better to size the appliance larger?

No, definitely not! As I stated earlier in regards to cycling times. It is particularly important to size a unit properly. A central air conditioning unit oversized will run and cool the air, but the run time will be too short to remove the humidity properly. A unit too small will remove humidity well but may not reduce the temperature of the house enough for comfort. An oversized heating appliance will heat the home OK but cost more to run and maintain caused by shorter run cycles. With oversized heating equipment you may feel the temperature swing as the unit starts and stops.

What does oversizing an appliance hurt?

Over sizing causes the heating appliance to run shorter cycles. This causes the efficiency to be reduced. The shorter the cycle the less efficient the appliance becomes. It could also run too long. The unit is efficient but may be too small for the heat loss. Properly sized units will run 7 plus minutes. This saves fuel, increases comfort levels and creates less maintenance. The starting and stopping of the equipment sucks life out of the controls and motors. It is better to run solid-state controls than starting and stopping. The constant in rush of power will shorten their life.

If I want to maximize savings should I just get multiple prices and make wise choices?

Unless it is an emergency installation the answer would be no. You must give this some thought and prepare for the appliance replacement. To just replace the appliance is fuelish! Plan a year or two out if you do not need the product right now. It would be a lot wiser to have the heat loss done and see which improvements can be a better choice for your dollars.

What this means is tighten up the home first. Let's get the heat loss lower than replace the heating appliance with a smaller on later. The smaller appliance will cost less, burn less fuel and create more comfort. If you have 3" of attic insulation and add 6 more inches is that a good move? Usually yes. When we do the Premium heat loss we can make suggestions on where your money may be better spent. It may be wise to add insulation, new windows, and doors etc. before replacing the heating appliance. Replacing the heating appliance now may not be the best financial decision. If the investment will be larger for a smaller savings or may not change the heating appliance size it can wait. With attic insulation there is a point adding more will not show a significant fuel savings. A heat loss can show this information. Just let us know and we will breakdown the possible changes and show the change in heat loss when they are done.

What is a modulating condensing boiler?

These are newer higher efficient products. The gas input changes as the heating load changes. The appliance tries to match the heat loss as the outside temperature changes (see Does the heat loss or heat gain change above). This will help the product run longer, maintain a higher efficiency, and increase the comfort levels within the home.

There is a gallon of water produced for every 100,000 btu's of natural gas burned. This is a benefit of the condensing boiler. Operating at lower system water temperatures causes condensing to take place in the heat exchanger of the mod/con Boiler.

When fuel is burned, there is a chemistry change and heat is produced. To keep it simple mod/con's will remove more of the heat from the flue gases and cause the flue gases to hit dew point. They then remove latent heat which is about 9.6% of the vent temperature. The water runs out a drain line. This happens when the boiler water is about 132ºf or slightly less dependent on O2 or Co2 adjustment.
When the water temperature is above that in the boiler the efficiency will decrease as the boiler water temperature increases the efficiency will increase as the boiler water decreases. These mod/con efficiencies are normally in the 92% - 97% AFUE range. When the water temperature entering the boiler at 120ºf and leaving at 140ºf. These are the AFUE testing temperatures. When the entering water gets colder than 120ºf the efficiency could go into the 98% to 99% range. When the entering water temperature gets hotter the efficiency rating will go down maybe as far as 85% or 86%. The cooler these boilers operate the more efficient they are. Lower flue gas temperatures at lower water temperature, they have less pollutants in their flue gasses that make them more of a green product. These products will normally supply you with increased comfort. The best application for these products is large water volume systems like cast iron radiators and in floor radiant heat. Copper tube baseboard and hydro-air systems are less desirable applications although they will still work well and save more fuel.

With a properly sized appliance, and home upgrades, is there anything else I can do for fuel savings?

Yes, we could suggest control strategies, which aid in reducing fuel consumption. These suggestions may include optional controls but not limited to simple outdoor reset, short cycle reduction controls, indoor sensors in addition to outdoor reset, buffer tanks, plenum heaters, etc. We may even suggest multiple appliances dependent on the application.

What about Air Conditioning?

Again, the key is to avoid short cycling of the air conditioner or heat pump. To start a compressor is expensive. The less times you start the better off your wallet is. As far as air conditioning goes you need to run long enough to dehumidify the home. If the unit is oversized, it will run too short of a cycle to proper dehumidify and you end up with the cool clammy feeling. If you walk into a home and it feels good getting out of the heat of the day and after a short time you feel cool but still warm like a humid cool day the a/c is oversized. In a commercial building this may be normal in the morning as the a/c is sized for many people and a hot day. As the day gets hotter and people start showing up to shop or for work everything equals out.

Does Comfort-Cacl have a sample of completed forms we will receive

  Each Component,   selections & SQ.Ft.     Condensed Form

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website.