All For One & One For All
I just finished reading about a "dream house" built to showcase the very best in domestic luxury and technology. The owner wisely selected hydronics to supply radiant floor heating, snowmelting and pool heating.
According to the article, the system used a 400,000 Btu/hr. boiler for pool heating, another 200,000 Btu/hr. boiler for radiant floor heating, and a third 600,000 Btu/hr. boiler for snowmelting. All these boilers were located in the same mechanical room, but operated as three independent systems.
The house also used two 60,000 Btu/hr., and one 100,000 Btu/hr. gas-fired furnaces, and several direct-fired domestic water heaters.
By my count, that's six separately-fired space heating appliances with a combined heat output of more than 1.42 million Btu/hr. The space-heating portion of the system alone required the installation of six fuel supply lines, six dedicated electrical circuits, and six exhaust venting systems.
Including the water heaters, the new owner has at least eight heat-generating appliances, all of which require future service. All of which must be provided with combustion and ventilation air. All of which constitute a potential source of carbon monoxide while operating.
If the 200,000 Btu/hr. boiler breaks down, there will be no floor heating even though 1.22 million Btu/hr. of available heat source equipment is still functioning. If the 400,000 Btu/hr. boiler breaks down, there will be no pool heating. And, you guessed it, if the 600,000 boiler breaks down during the winter, out come the snow shovels (or perhaps a new SUV with a previously unscratched power-tilt plow).
Will this system work? Sure it will. It will work just like six separate heating systems in six separate buildings. In this case they're just all under one roof.
Domestic DutiesAlthough not specifically mentioned in the article, I would guess this dream house has at least four bathrooms. One or more of them probably have a whirlpool tub and/or a multiple head walk-in shower. It's also pretty likely the owners don't want to wait 30 minutes for their whirlpool(s) to fill up with warm water. Neither do they expect a drop in water temperature after only five minutes of a much-anticipated tropical downpour showering experience.
Here's a situation where the domestic water heating system needs all the horsepower it can muster. Too bad that 400,000 Btu/hr. boiler is just sitting idle because the pool is already warm enough. Ditto for the 600,000 Btu/hr. boiler since it's not snowing outside. And those three furnaces, well, every time I've tried to use a furnace for heating domestic water they've leaked worse than sources for the National Enquirer.
Wouldn't it be nice to fire-up all that idle heating capacity and direct it toward domestic water heating when those water guzzlers upstairs come on at the same time? Teamwork: There is an alternative to installing all the dedicated, direct-fired equipment described above. It's called a multiple boiler system. The concept behind it is simple: One heating plant supplies ALL the heating loads in the building. This approach has some real advantages in situations like this.
Take domestic water heating, for example. If the system was piped as shown in Figure 1, you'd have one mean, hot-water-making machine ready to go into full afterburner whenever the DHW load gets challenging.
As soon as there's a demand for domestic water heating, the multiple boiler controller receives a setpoint demand and takes aim at warming its supply sensor to 200 degrees F. The number of boilers fired depends on how fast the control sees the supply sensor temperature climb.
However, be assured that a domestic draw of say 15 gallons per minute of 130-degree F water being replaced with 55-degree F water will soon get the attention of all the boilers. That demand translates into 15 x 500 x (130-55) = 562,500 Btu/hr. load. A 120-gallon storage tank could probably provide the first six or seven minutes of this demand. After that, it all comes down to raw heat transfer between the heat source and the DHW tank.
If the space heating or snowmelting loads happen to be on when the call for domestic water heating comes in, they could be temporarily suspended. All the available boiler capacity would be directed to the DHW load. The owner could park himself under those multiple showerheads 'till he looks like a steamed prune, and still not run out of hot water.
The advantages of a multiple boiler system are hard to ignore. Because they're lined up in one mechanical room, all the boilers can be supplied from a common fuel header as well as vented into a common breaching connected to a single chimney. The combustion air supplied to that mechanical room could be isolated away from potentially harmful gases like chlorine vapors from the laundry or a pool enclosure. When it's time to service those boilers, the work takes place in one location instead of throughout the house. In the event carbon monoxide was released, there's certainly a better chance of reducing its spread through the house prior to its detection. The list of advantages goes on.
A multiple boiler system provides redundancy that allows for partial heat output when one boiler is down for service. Better capacity control allows for less standby losses, higher seasonal efficiency, lower emissions, and longer component life. Do these sound like things your customers would desire if they knew they were available?
Load DiversityNo, this isn't some new government mandate requiring wetheads to install one forced-air system for every two boilers. It's the concept that all loads in a multiple load residential system almost never demand full heat input at the same time. Factor in the likelihood of generous boiler oversizing on most high-end homes and you'll probably conclude there's seldom, if ever, a time the dream house we're discussing, even with all its amenities, will require 1.42 million Btu/hr. of heat input. Those dedicated but segregated boilers, furnaces, and water heaters are going to have quite a bit of individual "slack time."
The total installed capacity of the three furnaces and one space heating boiler in this house is 420,000 Btu/hr. The outdoor temperature necessary to make the space heating load reach 420,000 Btu/hr. would make Fairbanks in January seem balmy. If ever it did get that cold outside do you think it would be snowing? Even if it were snowing, would it be wise to run the snowmelting system at such frigid temperatures?
In cold climates, high snow accumulations rarely occur when the outside air is at its lowest temperature. If ever it did get cold enough for this house to require 420,000 Btu/hr. for space heating it's likely the 600,000 Btu/hr. dedicated snowmelting boiler would simply sit idle waiting for snow and/or non-arctic air temperatures before being of use.
Now here's a thought that will probably upset some of you die-hard oversizers: Suppose the three commercial-size boilers and the three furnaces in this house (representing a mere 1.42 million Btu/hr. of total capacity) were replaced by three smaller boilers each rated at around 240,000 Btu/hr.?
How could I even make such a suggestion, you ask?
First of all, remember load diversity itself eliminates the need for part of the 1.42 million Btu/hr. capacity. For those times when 710,000 Btu/hr. (50 percent of 1.42 million Btu/hr.) can't keep up with the load(s), the system invokes prioritized load shedding. Heat input to lower priority loads, like garage floor heating and pool heating, is temporarily interrupted so that heat can be redirected to higher priority loads, like domestic hot water production and space heating. When the high priority loads are satisfied, the multiple boiler system turns its attention to making up the heat deficits of the low priority loads. The large thermal mass of slab-type floor heating and the enormous thermal mass of a pool make boiler sizing more a matter of how much energy can be delivered over a given period - rather than how much instant capacity is available.
If interior temperatures are set back 3 or 4 degrees F at night, the boilers would have a reprieve from space heating as the building's thermal mass slowly cools. During this time, their extra capacity is available for loads like garage floor and pool heating. As morning approaches, a few critical heating zones could be warmed before the owners get out of bed. Capacity would then get shifted into hot water production to keep up with several morning showers. You get the idea.
Secondly, remember priority domestic water heating always has the full 710,000 Btu/hr. at its disposal. That is enough power to heat 19 gallons per minute of water from 55 degrees F to 130 degrees F.
For those who still think 710,000 Btu/hr. of total heating capacity may be a little light for this dream house, consider this: Providing 710,000 Btu/hr. of heat output continuously for 24 hours by burning natural gas costing $0.85 per therm at 85 percent efficiency would cost the homeowner $170 per day in fuel alone. How many people - even affluent people who build big houses - would accept fuel bills like that?
Not to worry, the likelihood of this house needing 17.04 million Btu of heat over a 24-hour period (unless the owners open all the hot water fixtures and leave for the weekend) is virtually zero.
Keep Them BusyBoilers attain their highest efficiency when running "flat out" hour after hour. Multiple boiler systems allow their individual boilers to enjoy longer on-cycles that approach this ideal steady-state operating condition. A multiple boiler system piped so it can serve all heating loads encourages longer, more efficient burner cycles relative to several individual boilers each dedicated to a single load. Not only does your customer save by installing less heating capacity, they'll likely save thousands of dollars over the life of the system as a result of higher seasonal efficiency. Longer duty cycles also mean longer life and reduced maintenance for components like hot surface igniters, oil burners and relays.
The moral of this story is to keep you boiler's appointment book filled whenever possible. Do so by shifting and rescheduling loads so the installed boiler capacity doesn't sit idle for long. Making all loads accessible to one multiple boiler system lays the groundwork for this.
The schematic in Figure 2 shows how I would pipe the system for the house we've discussed using a multiple boiler system.
Each boiler is piped in parallel to a common header. The circulator for each boiler operates only when that boiler is fired, and perhaps for five minutes of thermal purging after the burner shuts down.
All heat from the boilers is handed over to the system piping at the closely spaced (primary/secondary) tees. This eliminates any interference between the small boiler circulators and what are likely to be larger circulators in the distribution system.
The indirect water heater receives the hottest water from the boilers. It's crucial the heat exchanger in this tank can transfer heat into the potable water as fast as the multiple boiler system (with all stages operating) can get the heat to the tank. Don't use a tank with an undersized heat exchanger or you'll "bottleneck" the heat transfer between the boilers and the domestic water. The DHW recovery rate will not be as high as it otherwise could be. If you can't find a tank with enough heat exchanger capacity, consider a generously sized external heat exchanger paired with an unfired hot water storage tank. This is shown as an option in Figure 1.
I would also make domestic water heating the highest priority load. All other loads would be temporarily suspended while the DHW tank is being heated. If, after an hour of priority DHW operation, the tank thermostat were still not satisfied, the controls would reestablish the space heating loads. The latter control strategy simply prevents a sustained space-heating lockout should the DHW control fail in the "on" position.
The snowmelting and pool heat exchangers are also piped to receive the hottest water in the system. A call from either load gives the boiler control a setpoint demand sending the primary loop to 200 degrees F. A third heat exchanger passes heat to the garage floor circuits using a mixing valve for temperature control.
The water temperature to the house floor heating circuits is managed by an injection mixing system and zoned manifolds. The layout shown assumes a single water supply temperature could adequately supply all areas of floor heating. If, after adjusting tube spacing, floor coverings, etc. this is not possible, multiple mixing devices could be used. Each would tap into the primary loop. Those with the higher water temperature requirement would be located upstream on the primary loop. Those supplying lower water temperature subsystems would be tapped in near the end of the primary loop.
The zoned forced-air portion of the house would be handled by zoned air handlers rather than furrrr . . . furrrr . . . furnaces. Sorry, sometimes it's hard to get that word out.
Certainly, there are other ways to pipe this system that could work equally well. The piping options depend on control strategy as well as selected hardware.
Brains Over BrawnThere are several possibilities for load shedding in a system like this. They depend on the relative size of the loads and the owner's priorities.
Here's one possibility. It assumes the boiler control is trying to maintain the water temperature to the primary loop at 180 degrees F under design load conditions.
When a two-stage thermostat in a high priority area of the house experiences a drop in air temperature sufficient to close its second stage contacts, and the primary loop temperature is below 150 degrees F, then the garage floor heat subsystem is temporarily disabled. If the primary loop temperature drops below 140 degrees F, the snowmelt subsystem and the pool heating subsystem are temporarily disabled. As the loads are dropped, the primary loop temperature should recover and thus be able to satisfy the higher priority loads. Following this, the boiler capacity is directed to restoring the shed loads. Think of all this as a sort of "thermal triage."
One could think of endless possibilities for such load shedding. Ideally, it would all be handled by a single integrated control system with the intelligence to learn how best to adapt load shedding to the usage patterns of the house.
The combination of variable speed circulators and advanced control strategies will soon allow modulating - versus simple on/off load shedding. The variable speed pumps would act like individual "clutches" between the boiler plant and the loads. This strategy could smoothen out system operation and increase efficiency by allowing full heat flow to high priority loads as well as partial heat flow to the lower priority loads. This is not "pie-in-the-sky" stuff, it's coming to a hydronic system near you shortly.
Finally, if you do plan to incorporate a prioritized load shedding strategy, be sure the owner understands how it will work and agrees with the proposed operation. With a properly designed system the compromises are minimal. There's a good chance the owner will never even notice them as the system goes through its paces. Still it's always best to avoid any unfulfilled expectations.
When It's Your TurnAlthough I've taken issue with their hardware configuration, I still expect the owners of the dream house we've discussed will enjoy the many benefits of hydronic heating. Their feet will be warm, their driveway and sidewalks will be clear, and their pool will be inviting. Many of their friends and neighbors will also see and feel the difference hydronic heating makes and one day plan to use it in a dream home of their own.
When the plans for that home cross your desk, remember the advantages of linking all those loads to a common boiler plant instead of installing several dedicated and excessively oversized systems.
Explain the advantage of an integrated multi-load system to your prospective customer. What part of lower installation cost, lower operating cost, and lower maintenance cost do you think they'll not desire?
Then, after your superior reasoning has closed the sale, go install a state-of-the-art system that exemplifies the versatility, economy, and comfort hydronic heating has to offer.