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Heating, Ventilation, & Cooling Systems


Heating & Cooling


This discussion will start by just considering heating and cooling systems, ventilation will be discussed later since it is usually a separate system. Furthermore, only the most common type of system will be discussed, that being a forced air furnace and central air conditioning equipment (if present). Although a heat pump system may have many similar problems, it is not being specifically discussed.

Equipment vs System

Most people, when think of their heating or cooling system, only think of the equipment. Much like we think of our other appliances; washer, dryer, stove, and dishwasher. In reality the heating or cooling systems includes much more stuff; the ducts, grills and registers, filter(s), sealant, as well as the equipment. All of this stuff has to work together well for the system to perform properly. However, according to the D.O.E., over 70% of heating and cooling systems have one or more design or installation errors. These errors include: oversized equipment-equipment that short cycles and does run long enough to get the air to everywhere it is needed, supply or return duct that are undersized-stifling the airflow into or out of the air handler, improper cooling charge-not delivering the designed cooling, improper airflow rate-heat transfer does not occur at the proper rate, and unsealed ducts-air flows wherever it can instead of where it should, just to name a few. Any one of these errors in the system will cause the equipment to operate below its design capabilities, several errors together can cause the system to operate significantly below its design capabilities. In fact, the average system in the U.S. operates at only 57% of it's equipment rated performance. Meaning that the 95% efficient furnace you had installed may only be giving you 60% or less actual performance.

Quick Check - To see if your system was designed or just done without much thought, check the duct sizes going to the various rooms. Obviously, this is easier if your basement is unfinished. If your basement is finished, you can still check by removing the room's register, placing your hand into the duct work, and expanding your hand to estimate the ducts size (exact measurement is not necessary). You then compare the various rooms and their duct sizes. Larger rooms with only one register should have a larger duct feeding it than a smaller room, also with one register. If all the registers, regardless of the room, have the same duct size, then you can be pretty sure your system was NOT DESIGNED by anyone, it was just installed without much thought. Unfortunately, this is the norm for HVAC systems.

What to watch out for:

If you call a typical HVAC contractor, they will be willing to change out your equipment (the box) with higher efficiency equipment. Unfortunately, this will usually NOT fix the issues. This is because the initial design was not correct or the installation was not properly verified. Thus changing the equipment does not fix the real cause of the problem. Additionally, most companies will just replace it with the same size equipment without doing the math (actual a computer program does the math). There are at least three problems with this: 1. They don't know the proper size equipment for YOUR house, 2. Airflow through the new equipment is likely different from the old equipment and they don't know whether it is correct, and 3. Higher efficiency equipment means higher output for the same input. For example; if you have a 100,000 btu furnace that is 60% efficient and replace it with a 100,000 btu furnace that is 93% efficient your potential output has gone from 60,000 btu to 93,000 btu. This means your getting 50% more output than before. Unfortunately, when it comes to heating and cooling, bigger is definitely not better. The new system will short cycle, meaning it runs for very short periods and puts out big puffs of hot air. The result is very uneven heating throughout the house. Thus oversized equipment actually makes the house less comfortable. Additionally, the short cycling will prevent the equipment from reaching its design efficiency because it does not run long enough to reach a steady state (which is where the efficiency is based on), so it costs you more to run while making you less comfortable (talk about a double whammy).

The Right Size Equipment

In contrast, properly sized equipment will run for longer periods at a lower rate, such that all parts of the house can be evenly heated (assumes ducts also sized correctly). You might ask, if my equipment is running longer won't it cost me more to run. The answer is no, because the equipment is smaller it burns less fuel. Additionally, the equipment will actually reach its design efficiency meaning you are actually getting what you paid for. Bottom line: Right Sized Equipment = More comfort at less cost.

Testing

HVAC Airflow & Efficiency Testing is the first step in figuring out how to fix the system, by showing how well the existing system is working. These tests show areas in your house that get too little or too much airflow, duct leakage, and the actual overall efficiency of the system. From this information, a prescription of changes can be made to improve your system, such as increasing certain duct sizes to improve the airflow into a room or balancing dampers to reduce the airflow into other rooms. Even if your furnace or air conditioning equipment is old and needs to be replaced, you should get the testing done so that there is a known starting point from which to begin the new design and installation of the new equipment/system. Additionally, for new equipment, you should have a computerized Manual J, S, & D calculations run to ensure the right sized equipment and duct sizes are determined. (Yes, this is likely to cost some money, but it can save you money in the long run by making sure you get the right sized equipment for YOUR house.) Together these pieces information will allow your new equipment to be designed to perform to its full potential. After the system changes are made, the testing should again be done to verify that the new or upgraded system is actually operating as it was designed.

Quick Check - To see if your equipment has EVER been tested, look for test ports. You should have at least four, although at least one may be hard to find. These test ports should be located on the return air duct before the filter, at the air handler (after the filter), between the heat exchanger and the evaporative coil (this is the hard to find one typically), and after the coil. The test port are typically about 3/8" diameter (they can be smaller or larger) and usually plugged with what looks like a small plastic thimble. It is not uncommon to find a 1/8" hole above the coil (for temperature measurement), but if you cannot find any others, then your system has NEVER been tested.

The Right Equipment - Our Recommendations

Besides installing the right sized equipment, as per a Manual J, we recommend that you install a multi-stage furnace. A modulating furnace is the best, but can be a bit pricey. A 2 or 3 stage furnace will work nearly as well. The multi-stage furnace will run at its' lower burn rate most of the time and provide a nice even heat throughout the house (assumes airsealing and insulation also done). It will fire at a higher rate if the lower rate is insufficient, such as on a really cold day. We also always recommend an Electronically Controlled Motor (ECM). This can come with the furnace or as a retrofit ECM. The retrofit ECM will often save you money vs a factory supplied ECM, in either case an ECM should save you some money on operating costs vs the standard PSC (permanent split capacity) motor that comes with most furnaces. Plus, you get the additional benefit of a quieter system because the blower also runs a lower rate when the furnace fires at a lower rate. This means you will not need to turn up the television's volume, to hear your show, just because the furnace is running.

Should I go for the most efficient equipment?

If you have an unlimited budget, then yes, go for it. For the rest of us, it depends on a number of things. How old is your equipment (equipment should last 15-25 years)? Have you airsealed and insulated to the proper levels (or as close as you can) to minimize the equipment size? Have you had the Manual J, S, & D calculations done to ensure proper sizing? Have you had the airflow testing done to know the shortcomings of your existing duct work before making changes? Have you found a HVAC contractor that will guarantee the performance of the new system?

  • If your equipment is still within its' useful life and you have not completed all the other upgrades and steps, then no, you should complete the other upgrades and steps first.
  • If your equipment is still within its' useful life and all the other upgrades and steps are complete, then you need to evaluate why you want the higher efficiency equipment. If it is strictly to save money then run a payback calculator such as Natural Gas Furnace Savings Comparison. Often it is better to wait until the end of equipment's service life, if you already have a mid-level efficient equipment (for example an 80% efficient furnace). Otherwise, the decision is yours to make.
  • If your equipment is near the end of its' useful life and you have not completed all the other upgrades and steps; doing the other upgrades first can allow your existing equipment to run longer by reducing how hard it works. This will allow you time to finish the others steps to put in a properly sized equipment.
  • If your equipment is near the end of its' useful service life and you have completed all the other upgrades and steps, then yes, put in the higher efficiency equipment.
  • If your equipment needs to be replaced now, attempt to complete as many steps and upgrades possible. However, you need to realize that system won't perform as well as if all the upgrades were completed first. Additionally, the equipment is likely to be oversized (causing less comfortable house vs the right sized equipment) after you complete the upgrades.

Ventilation


The V in HVAC is for ventilation. Unfortunately, it is often the forgotten part. Ventilation is all about Indoor Air Quality. This is expressed with the saying "The Solution to Pollution is Dilution". Ventilation has two primary goals, removal of polluted air and adding fresh air. These two goals are accomplished in two basic ways. First, the use of spot ventilation to remove pollutants at their source. For example, kitchen range hoods remove the water vapor, smoke, oil/grease particles, and odors from cooking. Bath fans remove the water vapor from bathing, as well as the chemicals that are airborne that are used in the bathroom (such as hair spray, deodorants, and cleaners). Plus the other odors that are produced in the bathroom, although in the scheme of things this is actually a very minor use for the bathroom fan. The second way the goals are accomplished is by getting fresh air to come into the house. This will push out more of the pollutants as well as dilute any remaining pollutants. This fresh air ventilation is accomplished in one of three methods: exhaust only ventilation, supply only ventilation, and balanced ventilation.

Exhaust only ventilation is accomplished by using fans that take air out of the house. This depressurizes the house and air is drawn back in which provides the fresh air. The biggest advantage of using this method is expense. You can use your existing bathroom fans for this method, all you need to add a control system to cycle the fan to ensure that it will run enough throughout the day to get you the required amount of ventilation. On the down side, if your bathroom fan is too noisy, you will soon disable the system, resulting in insufficient fresh air being added and IAQ going down. The biggest problem with this method is that we do not know the source of the incoming air. It will be coming in through the leaks in the house envelope, it could be from the garage, crawlspace, attic, or wherever it can find an entrance. Bottom-line is that your fresh air may not be so fresh. In really tight houses, there might not be enough air leakage inward, so the fans will not be able to exhaust air and the IAQ will go down. This will likely cause the water heater or furnace to back-draft, putting combustion products (including carbon monoxide) into the house further reducing IAQ. Another potential problem is radon, by depressurizing the house we will be drawing in more radon than if the house was at a neutral pressure. The exhaust only method has been typically used in a heating dominated climate, such as Utah, but the IAQ issues are just too significant to overlook.

Supply only ventilation is accomplished by using a fan to pull air into the house. This pressurizes the house and the air leaks out wherever it can find a hole (spot ventilation fans have dampers that should open allowing some air to have a controlled exit path). This method is typically done by adding a duct connected from the outside to the return air duct going the air handler/furnace, plus a control system and damper to regulate the amount of incoming air. The advantages are that: 1) we know the source of the air and can add filters as necessary. 2) By using the existing ducts, the fresh air will be distributed throughout the house. 3) The supply only system should be relatively easy to add to most houses. 4) It may reduce the amount of radon entering the house because the positive pressure on cracks in the basement will make it harder for the radon to enter (note: it will not stop the radon, just slow it down). On the down side, unless you have an ECM (electronically controlled motor) on your air handler/furnace, your electric bill will be higher, particularly during the non-heating and non-cooling parts of the year. Although there are other ways to accomplish a supply only system that eliminate this problem. Next, the supply air will be at whatever the temperature is outside and it will cost money to heat or cool the air to the proper temperature. The supply only systems have typically been used in cooling dominated climates, but it is worth looking at because of the advantage of better IAQ.

Balanced ventilation is where air is exhausted and taken in at the same rate and is considered the best method of ventilation for fresh air. This is typically done in Utah by using a HRV (heat recovery ventilator) in a standalone system. It has the advantage of keeping the house at a neutral pressure and reducing your energy bill by transferring heat between the incoming and outgoing air streams, plus you do not need to run your air handler/furnace. This is what should be installed in new homes, since it can be designed and installed to optimize its’ performance. However, it by far the most expensive option, especially to retrofit into an existing home, since it needs its’ own ductwork. The HRV can be tied into the existing duct work, but this somewhat compromises its’ advantage. A balanced system can also be done by using both an exhaust and a supply system that has been match for flow, but this loses the heat exchange and relate energy savings.

Spot ventilation, which include bathrooms’ fans, range hood, and the dryer, will also affect the house. All spot ventilation systems are exhaust only systems and can easily put the house into a depressurized state with all of its’ associated problems. Regardless of which method is used for the ventilation, the spot ventilation system will need make up air to reduce or eliminate this depressurization. Balanced systems, such as a HRV, cannot be adjusted, so a supply air intake must be added for the makeup air to keep the house at a neutral pressure. For supply only ventilation, it must be sized so as to not only provide proper fresh air, but can have increased flow to account for all spot ventilation exhaust. Exhaust only systems need the makeup air the most and it ends up being the same as a supply only ventilation once enough make up air is added.

How much ventilation should you have? There is a standard (ASHRAE 62.2-2013) that specifies a minimum based on the size of the house and number of occupants, plus a couple of other considerations. We can determine how much ventilation you should have by having us perform a Home Performance Diagnostics on your house.


Interesting Links



Dept of Energy - Home Cooling

Dept of Energy - Home Heating