Heat Pump Helper
How many of you Wet Heads know someone with an all-electric home heated by an air-source heat pump? You know, those marvels of modern engineering sold by all the big-name HVAC companies that refrigerate outside air in winter, then dump what heat they gather into a forced-air distribution system.
Assuming you do have acquaintances with these systems, here are a few things to ask them:
- Are they happy with the comfort their systems provide? Or do family members zip themselves into those little sleeping bags with arms and legs before curling up on the sofa for the evening?
- Are they satisfied with their monthly electric costs? Or do they pay in advance (on a so-called "budget plan") to avoid winter bills that rival their mortgage payments?
- Can their electric water heater keep up with the demands of three or four bathrooms? Or do showers end up being strategically timed so the tank can catch its Btu breath in between?
Ample AfflictionsHere are some "distinguishing characteristics" of a typical all-electric home equipped with an air-source heat pump and an electric water heater. These attributes are willingly accepted every day by homeowners who presume their builder and utility customer service representative know best how to heat their house.
Symptom No. 1 - Cool Air From The Registers. As outdoor temperature drops, so does the refrigerant temperature within the indoor condenser of an air-source heat pump. This shows up in the form of lower discharge air temperatures at the registers.
Even though heat is still being delivered to the rooms, the temperature of the air leaving the registers feels cooler than normal. If the air stream is discharged in occupied areas without complete mixing, it tends to create drafts that further add to the discomfort.
Symptom No. 2 - High Operating Cost In Winter. Obviously all houses cost more to heat during cold weather. But those with air-source heat pump systems have an especially effective way of putting their utility meters into overdrive.
As outdoor temperature drops, the heat pump reaches a point where its refrigeration cycle can no longer meet the increasing heating load. The multistage thermostat responds by turning on electric-strip heat elements in the heat pump.
These elements operate just like the wires in your toaster, converting electricity directly into heat. Although it's a simple way to provide the extra heat, its operating cost far exceeds that of natural gas or fuel oil.
How much more? Here's an example: The operating cost of a 15-kilowatt electric-strip heater in a location where electricity costs $0.12 per kilowatt-hour is $1.80 per hour. This is in addition to the cost of operating the heat pump's compressor. If natural gas were available at $0.75 per therm and burned in a modern boiler, the same heat could be supplied for about $0.45 per hour - 25 percent of the operating cost of the electric-strip heat.
Add up the difference over a few hundred operating hours in a typical heating season and you're talking real savings! Savings that would be even greater for heat pumps in light commercial applications, where utilities charge for the kilowatt demand as well as the energy used.
Symptom No. 3 - Not Enough Domestic Hot Water. The most powerful heating element that can be installed in most residential electric water heaters is rated at 6 kilowatts. That's about 20,500 Btu/hr. of heat output. Many tanks come standard with smaller 3.8- or 4.5-kilowatt elements, which deliver about 13,000 and 15,400 Btu/hr. respectively.
By contrast, a typical residential boiler would be rated from 65,000 to 100,000 Btu/hr. In a properly designed hydronic system, all this heat output can be directed to the domestic water heater when necessary.
This allows domestic hot water to be produced three to five times faster than with a large electric water heater. Your customers will no longer run out of hot water after a couple of showers, or have to slowly fill their whirlpool baths if the washing machine happens to be running at the same time.
Furthermore, the cost of producing domestic hot water using a gas- or oil-fired boiler is far less than with an electric water heater.
Comfort ReclaimedBy combining standard hydronic hardware in creative ways, you can improve the comfort of the forced-air system, provide abundant quantities of domestic hot water and perhaps even pick up a couple other heating loads the owner hadn't even considered. The icing on the cake will be a substantial reduction in the owner's utility bills.
I know some of you "purveyors of the pipe" would rather eat used furnace filters than work with ducting. But think of these systems as a professional challenge. You're going to apply your honed hydronic design skills to give that customer the best comfort system possible given what you're starting with.
So what if it isn't a house full of radiant floor heating. It still has the potential to produce an elated customer who'll likely spread the news of his thawing to his heat-pumpin' neighbors and friends.
The mechanical makeover starts with a conventional gas- or oil-fired boiler as the sole heat source. A hot water coil is added to the discharge ducting of the heat pump's air handler. This will probably require some customized sheet metal transitions.
Placing the coil downstream of the air handler keeps the blower motor and other innards of the unit cool. Be sure the coil is mounted so the air stream will spread out across it rather than be jetted through its center. This may require a splitter vane in the ducting upstream of the coil. A three-row, or even better, a four-row coil, will give higher leaving air temperatures for a given inlet water temperature.
The RunaroundThe simplest approach to heating the coil is to circulate hot boiler water through it whenever the thermostat calls for heat. To prevent an initial blast of cool air into the house, you could install a temperature setpoint control that prevents blower operation until the water in the coil circuit gets up to say 140 degrees F.
Even though this approach is sure to deliver warmer air than the heat pump, it still lacks a provision for accurately controlling the air temperature. If the boiler water is really hot, say after satisfying another load, the delivery temperature of the air will also be higher.
Adding a duct thermostat that starts and stops flow of hot water through the coil is not necessarily a good solution. If the duct thermostat's differential is wide, the swings in air temperature will become even more noticeable due to the fast cycling of the low thermal mass coil and air stream. If the differential is narrow, the thermostat will short-cycle the coil circuit on and off, leading to unnecessary wear and tear on those components.
A better solution is to install a "runaround" piping loop on the coil and feed heat into it using a variable speed injection pump or other mixing device, as shown in Figure 1. This allows the air temperature leaving the coil to be dialed in as necessary to provide good comfort without overheating the air and leading to more pronounced room temperature stratification.
This approach even offers the ability to reset the air delivery temperature based on outdoor temperature. However, if you do this, make sure to set a generous minimum supply air temperature on the control to prevent the same low temperature air delivery you were called in to correct.
Perhaps you're thinking, "Why not just vary the flow rate of hot water through the coil to control air-delivery temperature?" While I won't say it's impossible, I will caution that controlling heat output from a coil by varying flow rate through it is very "non-linear."
With only 10 percent of design flow rate through the coil, it will transfer about 50 percent of its full heat output rating (at a given water temperature). This makes capacity control difficult at the low end of the output spectrum.
Using a runaround loop allows heat output to be controlled by varying the water temperature in the coil, rather than the flow rate through it. Output varies in almost direct proportion to the temperature difference between the entering air temperature and the water temperature in the coil. Control stability is much improved.
The new hydronics controls should include an isolated relay contact that closes to power up the blower relay in the air-handler when heat is called for. A toggle switch can be installed in the control wiring that allows the owner to restore the system to heat-pump mode should the hydronic side ever be down for service. When the thermostat is switched to cooling mode the hot water coil circuit must be disabled. In the cooling mode the heat pump operates as it always has.
The likelihood of the electric-strip heat elements ever being used again is very slight. I favor their removal to compensate for the added air flow resistance of the new hot water coil.
Now That A Boiler's In The HouseAs many of you creative hydronicians can appreciate, the hydro-air conversion is just the opening salvo in a hydronics assault on the all-electric house. The next glaring target is the electric water heater.
An indirect DHW tank heated by the boiler is one obvious solution. Another possibility is retrofitting a small stainless steel heat exchanger and bronze circulator to the existing hot water tank as shown in Figure 2. The latter solution avoids replacing what may be a perfectly good tank and lets the owner "fall back" to electric water heating should the hydronic side be down for some reason.
If the electric tank remains part of the system, it needs to be retrofitted with a set-point control (either electromechanical or electronic). Ideally the sensor for this control can be mounted in a well that projects into the upper portion of the tank. If such mounting isn't possible, make sure the sensor is securely fastened in tight contact with the tank's pressure vessel and fully covered by insulation. If the tank doesn't already have one, now is also a good time to install an anti-scald tempering valve.
Does the house have tile or vinyl floors in the kitchen or bathrooms? How about retrofitting them with a staple-up tube and plate system. Just add another secondary circuit to the boiler loop, along with a simple mixing system, and show those folks what real comfort feels like.
Is there a swimming pool out back? Might there be one in the future? That new boiler has plenty of spare capacity on spring and fall days to heat it.
Another stainless-steel heat exchanger on the primary loop sets the stage. Then it's just a matter of piping the pool water to the heat exchanger. Ideally the pool water could be routed to the location of the heat exchanger via underground piping. All such piping should be pitched so it can be drained when the pool is winterized. Another possibility for retrofit applications, albeit less attractive, is to provide exterior connections for supply and return hoses that run out to the pool piping.
The relatively cool pool water allows the possibility of operating the supply and return piping/hoses with a ³T of say 40 to 50 degrees F. In the latter case, each gallon per minute (gpm) of flow between the heat exchanger and pool piping carries about 25,000 Btu/hr. of heat along for the ride.
'Hydronicize' Those Heat PumpsI'm sure air-source heat pumps will remain the norm for many new homes across the frozen forced-air frontier. Incredibly, most of the eventual owners of these systems will just accept their marginal comfort and high operating costs as normal.
Each fall they'll prepare for the usual interior afflictions of winter in a house that's probably not as well heated as their car. A few may be fortunate enough to cross paths with a savvy hydronics professional who can show them what they've been missing - one who can transform their builder-knows-best heating assemblage into a true comfort system.
The market is rife with opportunity for those who recognize what's at their disposal.