An Ideal Heating/Cooling System for Your Small Addition

Last week I explained why the use of a ductless mini-split heat pump system as the sole source of heating and cooling is not the right application for the small (less than 1000 sq. ft.) addition. In order to properly size for the heating load on colder days, you’d be left with grossly oversized cooling capacity. And that will cause the heat pump to short-cycle when cooling on any day except for the very warmest. This short-cycling will result in less comfort, less efficiency, increased maintenance and shorter equipment life. Not good. After all, who wants to stress about their heating/cooling system.

You want to be comfortable all year long. If you want to expand your home with a small addition, I recommend ductless mini-split heat pumps for cooling in applications where the primary heat is supplied by another source — like radiant floor, finned-tube baseboard, radiant panel or hydro-air. That way I can size the cooling portion for ideal comfort and the heating portion of the heat pump can be used as a backup heat source if the main heat source is down for maintenance.

But now there’s an exciting new technology coming to the market. It combines the best of variable-speed compressor heat pump technology with hydronics to provide super high-efficiency performance with the awesome comfort of radiant heat and central cooling.

An air-to-water heat pump is the heart of this technology. During heating season it extracts heat from the outdoor air and transfers it to water using an indoor heat exchanger. In the cooling season, the process is reversed — indoor heat is transferred (via the same heat exchanger) to a refrigerant and expelled outdoors by the heat pump.

Since the heat pump has a maximum heating output temperature of approximately 120˚F, it’s a perfect match for low-temperature radiant — thin-slab, above-floor tube and plate, walls, ceilings, panel radiators and some types of finned-tube baseboard. This is the most comfortable heat around.

To cool the air, a pump circulates water (chilled by the heat pump) from the heat exchanger through a cooling coil located in an air handler. This distributes cooled and dehumidified air throughout your addition. If the design of the addition permits it, a standard air handler with the familiar ductwork can be used. But if equipment space is at a premium, a high-velocity mini-duct system with its 2” diameter ducts may be a better fit.

The efficiency of an air-to-water heat pump is rated in terms of its Coefficient of Performance (COP). COP is a ratio of the amount of heating (or cooling) produced to energy consumed. It’s not unusual for a variable-speed air-to-water heat pump to have a published COP of over 4.0 and an average COP of 2.7 to 3.0. In simple terms, for every one unit of energy consumed, the heat pump can produce almost three units (annual average) of heating or cooling. The only system more efficient than that is a geothermal system. (More on that comparison in future installments.)

Even though some of these heat pumps are advertised to operate in outdoor temperatures down to -4˚F, when the outdoor temperature drops to about 20˚F, the cost of energy input increases to the point where an alternate heat source is more efficient. A separate gas- or oil-fired boiler can provide a backup heat source for the coldest days as well as domestic hot water (DHW) production year-round. And if the main part of the house is hydronically heated, the air-to-water heat pump and the existing hydronic system are a match made in heaven!

But even if you need a backup/DHW boiler, the incredible efficiency of the air-to-water heat pump will offset the higher initial equipment cost with fuel savings in just a few years.

If there’s a small addition in your future, consider an air-to-water heat pump with radiant heat and chilled-water cooling as a renewable-energy alternative that pays for itself.

Heidronically yours,

Wayne

Ductless Mini-Split Heat Pumps and the Small Addition

The popularity of ductless mini-split heat pumps has grown tremendously in recent years. They’re a great way to add cooling to a hydronically heated home because they don’t need bulky ductwork. But like many new things, there’s a tendency to apply them to as many situations as possible, including some that they may not be well suited for. One such misapplication is as the sole heating and cooling source for a small addition.

Ductless mini-split heat pumps are usually an air-to-air heat pump — meaning it takes outside air and strips it of its heat value and transfers that heat to your home to provide space heating. For cooling, the cycle is reversed — it pulls the heat out of your house and expels it to the outdoors. You could think of a heat pump as an air conditioner that’s capable of working in reverse.

Heat pumps are nothing new, but the configuration of the ductless mini-split is. The condensing unit is located outdoors and a refrigeration lineset, small drain and wiring are run into your home through a 3˝ opening in an outside wall. They supply the indoor unit, which is usually mounted high on a wall and contains the blower and indoor controls. Ductless mini-splits are incredibly quiet (inside and out) and efficient.

I’m often asked to design a heating and cooling system for a small addition to an existing home (less than 1000 sq. ft). The first thing I look at is the capacity of the existing system to handle the addition’s extra heat and cooling load. More often than not, especially with forced air, the existing system can’t do the job. The system in a hydronically heated home can almost always handle the additional heating load but it obviously can’t provide cooling.

That usually leads to someone suggesting a ductless mini-split heat pump for heating and cooling the addition. It’s tempting, because it’s a relatively easy, quick and inexpensive installation. And here’s where the misapplication comes in. As an example, let’s apply a ductless mini-split heat pump to a typical 700 sq. ft. master bedroom, bath and laundry addition.

An addition like this would typically have about a 14,000 Btu/hr heat load on the coldest day of the year (considered 0˚F in the Rochester, NY area). It would also require just under 1 Ton (12,000 Btu/hr) of cooling on the warmest day of the year (considered 90˚F in this area). Both design loads would keep the indoor temperature at 70˚F.

Now, when sizing a heat pump, you size for the greatest load (heating or cooling) so you can be sure there’s enough capacity for both seasons. In our example case, as with most applications in this climate, the largest load is the heating load. So wouldn’t logic dictate that we’d need a heat pump rated for 14,000 Btu/hr? Not so fast.

We also need to consider the fact that as the outdoor temperature drops, so does the efficiency of the heat pump. In fact, even though some of the newer models are capable of providing heat down to an outdoor temperature of -4˚F, at those temperatures their heat output drops to near 50% of rated capacity. So now we realize that we need to DOUBLE the capacity of the heat pump to have any chance of maintaining our 70˚F indoor temperature on a 0˚F day.

That means we’re looking at installing a heat pump with a 28,000 Btu/hr minimum capacity, which actually works out fairly well, because heat pumps come in a 2-1/2-Ton size (30,000 Btu/hr). So now that we’ve decided that we need a 30,000 Btu/hr unit for heating, let’s see how that works for the cooling side.

Remember, the cooling load is 12,000 Btu/hr on the warmest days (90˚F). And there’s a 30,000 Btu/hr capacity. Simple math tells us that on even the warmest days, our heat pump is oversized (for cooling) by 250%. And, as you’ve heard me preach before, in cooling (and heating), bigger is not necessarily better.

Most of the better ductless mini-split heat pumps these days use inverter technology to modulate the compressor speed, which tailors the output to the load. With our example, the compressor would modulate down to 40% of capacity on the WARMEST day. That means that on a milder day it may need to be operating in the single-digit-capacity numbers. The problem arises when the compressor is only capable of modulating down to 30% of capacity, meaning that anything less than 75% of the maximum cooling load (in our example) will be asking the heat pump to work below its minimum capacity — which will be the bulk of the cooling season!

When a heat pump is asked to work in a range below its minimum capacity, it will short-cycle and, as a consequence, fail to properly dehumidify. We’ve discussed short-cycling and its consequences before — less comfort, less efficiency, increased maintenance and shorter equipment life.

For this example addition, the ductless mini-split “kind of” does the job. It can either do an acceptable job of heating with not-so-good cooling, or an awesome job of cooling with unacceptable heating performance.

So if a ductless mini-split isn’t the answer to heating and cooling your small addition, what is? Be sure to check next week’s Heidronics blog post for the answer.

Heidronically yours,

Wayne

When More Power Isn’t Always the Winner

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The Daytona 500 is all about speed and power and getting to the finish line first. Hydronics is basically the opposite — but you’d never know that from the way most hot water heating systems are installed these days.

Historically, North American hydronic designers and installers have specified and installed circulating pumps that pump more water and use more power than what is actually needed. It’s called “over-pumping.” And if a system underperforms, the first reaction of many technicians is to install a bigger, more powerful pump. But this almost never solves the problem.

It’s a chicken vs. egg thing. Installers either don’t have the knowledge or won’t take the time to calculate the pumping requirements for the system, and wholesalers don’t stock more than a few different pump models. I’ve heard installers justify their pump choice by the “bigger is better” mentality. And wholesalers have told me that they’d stock a wider variety of pumps but the installers aren’t asking for them. That’s a shame.

In hydronics, like stock car racing, the object is to go round and round until you cross the finish line and meet your goal. But unlike stock car racing, the winner in hydronics gets there with as little effort and speed as possible. The goal is delivering the right amount of heat from the boiler to the heat emitter (radiator, radiant floor panel or baseboard heater, for example). Pushing the water faster doesn’t make that happen any better. It just wastes energy!

Over-pumping can also create a condition known as velocity noise, which is caused by the water traveling too fast through the pipe and fittings. It can also cause erosion corrosion — a wearing away, or eroding, of the pipe wall due to the scouring action of high-velocity water flow.

But there’s hope. A new generation of circulators uses variable-speed technology and highly efficient electronically commutated (ECM) motors to vary their output to the specific needs of your system. If a zone valve closes, the pump slows down. If another opens up, the pump speeds up. Some are

Variable-speed ECM circulator

designed to operate on a pressure difference. Others operate on a temperature difference. But either type delivers just the flow necessary to heat the space, and either will consume much less electricity to accomplish the same results as compared to a bigger pump.

I’ve been using these variable-speed ECM pumps for several years now and have found them to be incredibly energy-efficient and versatile — especially for systems subject to changing flow-rate requirements. But they’re not the answer for a poorly designed system. While these pumps are capable of responding to a wider range of conditions, they still have their limitations. The application of solid design principles will determine the best application for these new-generation pumps.

You could compare great hydronic pumping to the tortoise and the hare. A bigger, faster pump will just wear out your system while a slower, steadier, variable-speed pump, like the tortoise, will win the race — every time.

Heidronically yours,

Wayne

It's a Numbers Game

A heat-loss calculation is where it all starts. It’s the basis for sizing any new or replacement system. It’s a roadmap to a well-designed, high-performing and comfortable heating system. And it’s not hard to do. It just takes some time and a little patience.

First, I measure each room ­— length, width and height. I also measure windows and doors and categorized them by construction type. Then I check the quantities and location of insulation. Usually I make a sketch to scale. For the average home, it takes about two hours to accomplish.

Next, I enter those measurements into a worksheet or use specialized software to produce a room-by-room and whole-house heat-loss calculation. The resulting numbers are the amount of heat lost by your house on the coldest days of the heating season.

These calculations tell me (or another heating designer) what size boiler or furnace is needed. It gives me the information I need to determine how much heating element or how many ducts are needed in each room — or how hot the water needs to be. Or what the flow rates need to be. And the pump sizes, pipe diameters, tubing spacing, panel size, and on and on and on.

Without a heat-loss calculation, it’s all guesswork. Luck. A wing and a prayer.

This is your heating system. The one you’ll be living with and fueling for the next 15, 20 — even 30 years. Ask for it. Demand it. Accept no shortcuts.

Heidronically yours,

Wayne

How to kill a workhorse

The vast majority of residential hot water boilers in service today are cast-iron mid-efficiency boilers. These are the workhorses of the hydronic industry and have been for many years. Installed and maintained properly, they can provide reliable service for 30 years or more. They typically have efficiency ratings in the low 80% range, meaning about 80 cents of every fuel dollar spent is converted to useable heat. The rest is lost up the chimney.

Older cast-iron water boilers from the early part of the 1900s were larger and held more water volume than today’s models. While this larger volume of water acted as a buffer and helped to smooth out some of the variability in water temperatures, it was at the expense of some efficiency. Today’s cast-iron boilers are smaller, and consequentially more efficient, but are less forgiving when it comes to handling low water temperatures.

Sustained low water temperatures can cause flue gasses inside the boiler to cool to the point that they condense on the relatively cool cast-iron heat-exchanger surfaces. This condensate is corrosive and will attack the bare metal surfaces of the boiler, creating rust and scale that can plug flue passageways and interfere with the operation of the burner. At its worst, this condition will allow dangerous products of combustion to enter your home. But at a minimum, it will shorten the useful life of your boiler. Today’s cast-iron boilers need to maintain water temperatures above the 130° - 140°F temperature range to prevent flue gas condensation.

The key to maintaining these safe water temperatures lies in your boiler's ability to produce heat at a faster rate than your house can use it.

Copper finned-tube baseboard

An example of a system that would work well is a home with copper finned-tube baseboard and small copper distribution piping. Many homes built in the 50s and 60s fit this description. Considered a “low-mass” distribution system, its copper tubing and light-weight baseboard emitters heat up quickly. These systems are usually designed for fairly high operating temperatures—typically 180°F. Assuming the boiler is sized properly to the home’s heat loss, it can come up to temperature quickly and has no trouble staying ahead of the home’s heating load. Water returning to the boiler will remain above the 130° - 140°F range for most of each heating cycle.

Where flue gas condensation problems start to develop are in high-temperature/high-mass systems, or low-temperature/high- or low-mass systems.

Cast-iron radiator.

A very common high-temperature/high-mass system where sustained flue gas condensation needs to be considered is an older (early 1900s) home with cast-iron radiators and large steel distribution piping. There are literally tons of cast iron and steel, and hundreds of gallons of cold water that need to come up to temperature before the radiators can start heating your rooms. This can easily overwhelm a properly sized boiler and cause it to run at sub-130° temperatures for long periods of time.

Another type of system that can overwhelm a boiler is a radiant in-floor system of tubes in concrete. This one is a one-two punch for your boiler. Not only are these systems designed to run at low water temperatures (110°F is typical) but the entire concrete slab must be heated before it can start delivering room heat. Some of these systems take days to recover from set-back. And the flue gasses are condensing the whole time. It’s a recipe for disaster.

One recent trend I’ve been seeing is for radiant in-floor tubes to be stapled to the underside of the subfloor and connected directly to a cast-iron boiler. This type of installation would typically run at 100° - 130°F water temperatures. The installer simply turns the boiler aquastat, or temperature setting, down to 120° and walks away. This system will likely condense for its entire—albeit short—life.

The effects of flue-gas condensation.

I’ve serviced boilers subjected to these conditions, and believe me, they’re not pretty. Sometimes there are piles of rust on top of the burners.

The good news is there are ways to protect your cast-iron boiler from low return water temperatures, extend its life, improve comfort and reduce your fuel consumption. Next week, I'll tell you my favorite solution to this problem.

Heidronically yours,

Wayne

What every boiler owner should know about indirect water heaters

Indirect water heaters have been around since the 1970’s in this country, but somehow, even after all this time, they don’t seem to be very well understood. They get their name from the fact that they’re heated “indirectly” by your boiler. They’re connected via piping to your boiler and circulate relatively hot (usually 180 to 200°F) water from your boiler to a heat exchanger within the water heater. The water surrounds the coils of the heat exchanger to produce your domestic hot water. This is in contrast to the typical gas- or oil-fired water heater that heats water through the use of a “direct” flame or heat source within the water heater.

If your home is heated by a boiler — any boiler (hot water or steam, oil or gas, mid- or high-efficiency) — you have every reason to heat your domestic hot water with an indirect water heater. Some of the benefits include:

• High efficiency — When you use your boiler as a heat source to produce your hot water, the water is heated at the same efficiency as your boiler. Some high-efficiency boilers reach 96%. The average gas water heater is about 60 - 70% efficient.
• Reduced heat loss — An indirect water heater is very well insulated and loses very little heat during long stand-by periods (at night or while you’re away). A gas water heater is always losing energy up the chimney.
• High performance — The performance of an indirect water heater is a direct result of the boiler it’s connected to. Given the size of most residential boilers, it’s not unusual for an indirect water heater to produce two to three times the amount of hot water as a standard gas water heater.
• Longer life — Most indirect water heaters have a lifetime tank warranty. It will likely be the last water heater you’ll ever need. The manufacturers can offer this warranty because their tanks are not subject to the abuses that a direct-fired gas water heater is. And many of them are made from stainless steel to prevent corrosion.
• Hot water storage. Available in sizes ranging from 30 - 200 gallons or more, hot water can be stored for high-volume flow-rate usage when you need it.

 The photo at right shows a cut-away view of a typical indirect water heater, with the coil located at the bottom. Boiler water is circulated through the coil and surrounded by the domestic hot water in the tank.

“But Wayne,” you may ask, “doesn’t this mean I need to run my boiler all summer now, too? Won’t that cost me a bundle?” Au contraire, my energy-conscious friend. The only time the boiler fires is when you need hot water. And when it does fire, it’s producing hot water at a higher efficiency rate than a typical water heater. Other than that, it sits quietly and waits. The belief that the boiler is running all summer may be a throw-back to the days of tankless coils in older boilers. They had no storage capacity, so the boiler needed to stay hot all the time to produce hot water on demand.

“This is all well and good,” says the the intrepid Internet traveler, “but I’ve been hearing that the new tankless heaters are the greatest thing since Apple went public. Can’t I save a lot more money with one of those?”

Well, speaking of apples, this is really an apples vs. oranges comparison. A tankless heater usually has little or no storage capacity, meaning it needs to heat the water instantaneously. And it can do that — with some limitations. Older tankless heaters drop the temperature at high flow rates (two fixtures running). Newer models limit the flow rate to maintain the delivery temperature. An indirect heater can handle that high flow rate without reducing the temperature or flow. And, depending on the size of your boiler, it can do it while delivering almost limitless hot water.

Better tankless heaters have efficiency ratings in the 80 - 95% range but typically last just 15 - 20 years. An indirect water heater connected to a 15-year-old cast-iron boiler will deliver hot water at 80 - 82% efficiency. And one connected to a modern modulating/condensing boiler can deliver 96% efficiency. So while tankless water heaters are efficient, they can't beat indirect water heaters.

Indirect water heaters can also reduce your maintenance costs. Having only one gas- or oil-fired appliance means less service. A tankless heater needs to be serviced every year. If not, you can count on reduced hot water production, possible heat exchanger failure and loss of warranty coverage. An indirect water heater requires little additional maintenance beyond your annual boiler safety and performance check-up.

The next time you’re in the basement, take a look at your existing standard water heater. If it's a little worse for wear, you may want to remember that the typical water heater life is 12 years. Wouldn't it be nice to replace it — once and for all?

Heidronically yours,

Wayne