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Make Smoke, Boil Water!
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I just finished having our 24-year-old heat pump system changed out, and got to thinking that maybe all the research that went into the new system, plus a description of the various kinds of systems and their efficiencies, might make a good article for the Tech Exchange. The idea is to give folks some background on these modern systems and to take some mystery out of them.

A little background:

We'd had a succession of service calls on our old heat pump, and in the middle of the last one of those, it went BANG and quit completely.
We got lucky. We were both out of the way when it failed catastrophically. But I turned to the service guy and said, "I think this might warrant a discount on the call."

We were also lucky that as a result of this failure none of the old ozone-depleting refrigerant was lost.

We have a pellet stove, and used that full-time to stay warm while I did the research and called various dealers for bids. It took about three weeks total of research, bids, and bid modifications before we selected a system type, and then a dealer to install it. Here are the results.

Basic System Types: Air-to-air, and Geothermal

Almost all heat pump systems sold today are air-to-air heat pumps. They work just like your refrigerator in that they use a working fluid to move heat from one point to another. So in winter, they capture heat from outside, and move it to the coils in the air handler, where a blower moves the heat through the house; and in summer, capture heat from the air handler's coils and move it outside.

Mini-split systems are a new wrinkle. They work on the same principle, but have the indoor blower and coil assembly all in one box, and so can be placed wherever the need for heating (or air conditioning) may be. They're a convenient way to provide heating and cooling in an area without having to install ductwork. Such systems tend to be customized for the place of use.

Geothermal systems are based on the way temperature doesn't vary much underground. These systems substitute a well and a circulating fluid with anti-freeze qualities to do the job of heat exchange, instead of an air-to-air outdoor unit. The drawback to these systems is they are expensive to install, and their efficiency gain versus payback time only makes sense in very cold or very warm climates. Years ago I was told by a system designer that they're best considered when the temperature is below 30°F or above 85°F for eight months of the year. These numbers may have changed in the meantime, but only you can determine if such a system fits your needs, as you also have to consider the cost of drilling a separate well for the geothermal loops.

This article will focus on the more common air-to-air systems rather than mini-split systems, which are more of a custom install; or geothermal systems, which tend to be a 'super-premium' system (at least in my area), and thus the cost tends to go out of sight for the average guy pretty quickly.
As you consider any system, the best yardstick is the 'payback' time - that time when any of these systems has paid for itself. More efficient systems may cost more, but in many cases can surprise you with a shorter payback period.

Defining Terms

There are two measures of system efficiency. SEER (Seasonal Energy Efficiency Rating) is a measure of cooling performance, and HSPF (Heating System Performance Factor) is just what it sounds like - a measure of heating performance. When comparing systems, these two measures become important.

I'll do a direct comparison with my old system to give you an idea of the efficiency gains which have been made in the last 24 years. However, these numbers will be somewhat approximate since my system was built two years before the Internet became popular. (And because it was so darn old!) The closest numbers I can currently find suggest that the system was 8.5 SEER (Seasonal Energy Efficiency Rating) and 5 HSPF (Heating System Performance Factor).

System Types: Single-stage

Today there are essentially three types of air-to-air systems. Single-stage systems, (our old one was this type) works like any other furnace: the thermostat (or 'stat' as it's called in the industry) calls for heat, it turns on. The stat hits set point, the system turns off, and the blower runs for a few minutes to cool the coils in the air handler and spread the heat around. Sometime later, the temperature in the house drops below the set point, and the cycle repeats. These systems are the least expensive of all systems, and by far the most common.

In my particular case, my old system was this same kind - a single-stage system - and just changing out for a modern single-stage system would save me approximately 40% in power costs. (From 8.5 SEER, 5 HSPF, to 13 SEER, 7.7 HSPF.)

Drawbacks to single-stage systems

From a comfort standpoint, there will be warm and cool spots in the house, often due to the air not circulating long enough to reach those places, and convection currents not keeping up with local heat loss. The air circulates only long enough for the stat to reach set point. As an example, one part of our house always ran 3° cooler than the rest.

From an efficiency standpoint, they can be lower-efficiency than other more sophisticated systems; but even though the sophistication has a price, the cost increase for a more sophisticated system shouldn't be dismissed out of hand. Everyone's case is different.

Especially when considering a single-stage system, it's important to know where the parts and components are coming from. Information I've learned from various technicians suggests that the buyer needs to ask about where they’re made, as some are built better than others. One measure is the warranty period, with ranges that go from only a few years all the way to ten years.

From a longevity standpoint, the system is operated in a 'start-stop' mode. This means the compressor is spun straight to full speed on startup. The techs I spoke with gave me the impression that this duty tends to shorten the lifetime, unless the compressor is built quite heavily.

System Types: Multi-stage

The next step up is to a multi-stage system, where the mechanicals and electronics (including the stat) are more sophisticated, with the stat interacting with both the heat pump and air handler. This type of stat allows for some anticipation of the need for heat. The principle is that the stat measures the speed of the temperature drop and calls for SOME heat, so the system turns on at a lower level, then like the single-stage system, the stat hits set point, and it turns off, with the blower running for a bit to cool the coils in the air handler. The idea is that the system doesn't have to run at full blast all the time.

In practice, it tends to operate a bit differently. The system will come on at the lower level of operation, then some time into the heat cycle (as the need is sensed by the stat), the system shifts to the higher stage of operation; then as the temperature begins to approach set point, the system will shift back down to the lower stage. The advantage is that the system is more efficient in the lower level, but if the system sees the need to shift to the higher stage, then the efficiency advantage over a single-stage system is mostly lost during that time.

Longevity-wise, the techs tell me that this system will tend to last longer, not only for the gentler start sequence for the compressor, but also the compressor type tends to last longer. Multi-stage systems use scroll-type compressors as compared to piston-type compressors in single-stage systems.

Multi-stage systems have few drawbacks, but they do cost more; in some cases not all that much. Remembering that my old system was a single-stage system, so changing out for a modern multi-stage system would save me approximately 50% in power costs. (From 8.5 SEER, 5 HSPF, to 16 SEER, 9 HSPF.)

System types: Intelligently-Variable

The third type of system is what I'll call an 'intelligently variable' system, where the system can maintain the temperature of the house within 0.5° and keep the house more comfortable and with no cold spots. One manufacturer calls this "Precise Comfort", another manufacturer calls it "Greenspeed".

Heat is added in small increments, only when needed. Lennox has a pretty good explanatory video that covers all three types of systems on their website, here:

The efficiency gains come from several places: using the thermal inertia (the heat that's existing in the house) periodic leveling that heat throughout the entire house, the intelligent electronic controls, and inverter-type motor control systems. This last item is the major innovation, and deserves a bit more discussion.

Using electronic motor control technology allows you to drive the AC motor windings with any current or phase, regardless of the input frequency or phase. There's a pretty good schematic on Renesas' website, here:

In this illustration, you can see that the 3-phase motor is completely decoupled from the AC mains and is being driven independently. This allows you to precisely match the amount of motor drive to its load, and vary its output instantly as needed. So if the thermostat needs 'just a little more heat', the system can provide it without having to go to a high-power mode.

The intelligently-variable system also adds this precision motor control to the air handler's blower. This allows the system to circulate air in the house (without adding heat) using as little as 60 watts.

Longevity-wise, this carries even farther forward the idea of a 'soft-start' for the compressor and even the blower motor.

The main drawback to the system is its cost; as I was researching and getting bids, I found that there was only about a 12% cost premium over a multi-stage system.

Comparing operating cost with my old single-stage system, changing out for an intelligently-variable system would save me approximately 70% in power costs. (From 8.5 SEER, 5 HSPF, to 21.5 SEER, 10.0 HSPF.)

Calculating Payback

Payback is calculated by taking the system cost and dividing it by the savings per year. Let's put some general numbers to this: Say that the old heat pump was costing us about $150 a month to heat and cool the house. And let's say we have a quote for a new system that uses 50% of the power of the old unit, and the installed cost will be $8000.

We start by figuring the yearly cost of the old system. That's $150 × 12 months = $1800/year. Let's say that a new system costs $8000 installed, and since it uses 50% of the power, that makes its operating cost $900 a year.

So $8000 (installed cost) / $900 (what it now costs to run) = 8 years and about 9 months which gives us our payback time. We have to recognize that this is a ballpark estimate, because we will have winters which are more severe, and others which are milder; same with summers.

System Options

Several manufacturers offer electrostatic-type air cleaners, which do a great job, but can have a cost premium. The air cleaner consists of a charged grid and a conductive filter, which is cleanable and reusable. Another option is HEPA filtering, with serviceable parts; these filters typically are only replaced once per year. A new type of filter is a hybrid type with an activated carbon element, a MERV 16 filter, and germicidal ultraviolet lights; this type of filter provides hospital-grade air throughout the house, and according to the manufacturer, only requires replacement once a year.

A little more about MERV numbers: there's a good chart on the Mechanical Reps Incorporated site, here:
You can see what the target is for each rating of filter.

Some systems offer dehumidification as part of the overall operating characteristic, even in winter. If you live in an area where humidity is a problem, then you'll want to check into this option. And of course the reverse is true with humidifiers. A note here: some humidifiers can work perfectly fine with a supply of non-chlorinated water, like well water. But you'll want to make sure if you have a well and are choosing this option.

An air-to-air heat exchanger is a way to cut down on the need to heat or cool the air being drawn into a tight building. My particular area is more temperate, so they do not sell well here. Again, it's a balance against how much make-up air you'll need for your house, versus how much heating or cooling you have to do. In my case, I have an outside air inlet with just a barometric damper on it. When someone switches on an exhaust fan, the damper opens just enough to allow makeup air to enter the house.

Hope you found the article useful.

The next post in the series has photos of the changeout process.
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