A Chat With Henry Gifford
New York City’s premier designer of heating, cooling, and ventilation systems talks about ERVs, thermostats, and LEED certification
Most builders and designers involved with green building have heard of Henry Gifford. Energy efficiency experts admire his deep knowledge of heating systems and his straight talk about the unacceptably high number of HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. problems in run-of-the-mill new buildings in the U.S. At the headquarters of the United States Green Building Council (USGBCUnited States Green Building Council (USGBC). Organization devoted to promoting and certifying green buildings. USGBC created the LEED rating systems.), on the other hand, he is something of a pariah — due in part to his 2010 lawsuit that accused the USGBC of making “deceptive marketing claims.”
Gifford lost his suit. Yet even those who doubt the usefulness of Gifford's litigious tactics usually admit that he has a point: many LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. -certified buildings use so much energy that they don't even meet Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. standards. Due in part to Gifford's needling, the USGBC has implemented changes in the LEED program that attempt to address some of Gifford's concerns.
Gifford knows more about boilers and heating system design than almost any mechanical engineer in the country. I recently reached him by telephone for a one-hour interview.
Q. What recommendations do you have for the design of heating or ventilating systems for single-family homes?
Henry Gifford: A nice successful person, who society rewards with a three-bedroom house in the suburbs, usually gets a master suite with its own bathroom. Usually, the house has a duct system that blows 400 cfm of heating and cooling air into the bedroom all day. The air comes out the bedroom door, down the hallway past the thermostat, back into the central return near the kitchen, through the air handler and back around again.
At night, guess what happens? Somebody closes the bedroom door. Now the air pressure in the bedroom goes up, so now only 300 cfm gets delivered to the bedroom. Maybe 100 cfm goes through the bathroom exhaust duct through the bathroom fan. Don’t worry that the fan is off — the fan in the furnace is plenty big enough to handle the flow.
Maybe 100 cfm is pushed out of the house through the cracks around the windows, leaving 100 cfm to squeeze through the crack under the bedroom door. Then, an hour or two later, people wake up and fiddle with the thermostat.
The air that gets pushed out the bedroom windows and the exhaust fan duct gets replaced by new air pulled in around the sliding glass door in the living room, or maybe from the garage. If my explanation sounds oversimplified, don’t forget to add that the supply ductwork is probably leaking into an attic that is vented to outdoors.
Q. So the typical forced-air system is a disaster. What should we do instead?
Henry Gifford: Move heat to and from each room with pipes containing water or refrigerant, with valves controlling the flow to each room. Put in an ERVEnergy-recovery ventilator. The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV. for ventilation.
Q. Why choose an ERV instead of an HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. ?
Henry Gifford: HRVs need drains, and drains are problematic. When you install an HRV, you have to include the cost of the drain. The drain needs to be indirect, with a gap. If there’s a P-trap, the trap can dry out. You can get mold in the drain pans, because of the water. You end up with a biology experiment in the drain pan. I’ve never used an HRV — it makes no sense to me.
The ERV should be ducted to supply fresh air to the bedrooms. The bedroom register should go on the wall, and it should be directed to skim across the ceiling. The register should not be near the bed. You need to look at the blueprint, guess where the bed will be, and locate the ventilation elsewhere in the room.
You pull the exhaust air from the bathrooms and the kitchen — but not real near the stove. The fresh air goes from the place where the person spends the most time to the place they spend the least time. It goes from cleanest room to the dirtiest, and from the driest room to the wettest, and then out.
I like to put the outside intake and exhaust ports relatively near each other, on the same face of the building, to minimize the effect of wind. The best place for the outdoor air intake is near a second-floor window, so a person can lean out the window and clean the air intake screen. It’s less dusty to pull air from a location that is off the ground.
Q. What about cooling?
Henry Gifford: I generally do cooling with ductless minisplits. Sometimes I include a flat air handler that you can hide above a closet. Unless somebody wears big hats to church, you don’t use the top 12 inches of your closet anyway. For better access to the air handler, skip the ceiling and leave it exposed.
As buildings get to be better insulated and have better glass — and hopefully in the future more shaded glass — cooling loads are dropping, while humidity loads are not dropping. In the humid South I would design a separate dehumidification system with one or two ducts leading far away from the dehumidifier intake, as a way to circulate dryer air from one end of the house to the other. An effective dehumidification system could completely remove the need for mechanical cooling, making real the dream of living without air conditioning. This of course would only be suitable in a strip about one or two thousand miles wide in North America.
Even a dehumidifier that raises the indoor temperature makes people feel cooler. Decreased humidity can cause a wider variety of people to all be comfortable at the same condition, whereas when humidity is higher, people fight over the thermostat. With a dehumidifier, even the in-laws will be comfortable.
For some years now, improved dehumidifiers have been available with a built-in ERV-type exchanger, like the Therma-Stor Santa Fe. Therma-Stor will soon come out with a cooling dehumidifier which rejects heat outdoors — a unit that mostly dehumidifies, with a little cooling — that will be a suitable replacement for cooling systems in most any energy-efficient home.
I’ve done some pretty in-depth research into the coincidence of cooling and dehumidification loads. In New York City, the worst periods we have occur in spring and fall, at night, when we have a dehumidification load associated with ventilation but no cooling load. In the humid South, there are weeks and weeks when they have a high dehumidification load but no cooling load. The charts are scarier than one’s intuition might lead you to think. Remember, sensible cooling loads decrease with better insulation and shading. That means that there are many hours when you simply can’t cool, because the house will get too cool. This is already a problem now, and as buildings get better, it’s only going to get worse. That’s why many homes need a separate dehumidification system controlled by a dehumidistat.
Q. If you used ductless minisplits for cooling, do you worry about even distribution?
Henry Gifford: If you have a McMansion with high cooling loads, you should put an indoor unit in almost every room. But if you have a very well insulated house with good windows and exterior shading and some dehumidification, as appropriate, you can get away with a smaller number of indoor units — certainly fewer than one per room.
If the cooling loads are very low, and you have a dehumidifier, the dehumidification is spreading throughout the house. The dehumidification isn’t spreading by vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. , but by moving air currents which tend to level out the humidity. If you have a dehumidifier in one corner of the basement of a school, it will eventually dry out the whole basement. You’ll get circulation from the fan on the dehumidifier, as well as convection driven by the heat given off by the dehumidifier. It’s the air circulation which does the job.
Q. What type of heating system do you recommend for a single-family home?
Henry Gifford: If you don’t have a thermostat in every room, you are not in the game. You should add or remove heat by moving water or refrigerant only, not by moving air around. I like a sealed-combustion boiler that makes hot water for space heat and (using an indirect tank) domestic hot water.
I have also designed buildings with air-source heat pumps that do heating in addition to cooling, but if you are burning fossil fuel to make domestic hot water, why not use it for heat? As long as there is a combustion appliance heating water for the faucets, we might as well get the advantages of also using it for heat: no air blowing on people in the winter, and lower energy use — because my measurements show that the claimed COPs of air-source heat pumps are greatly exaggerated. Actual COPs are low enough that when you consider the amount of fuel burned by the power company, you are using less fossil fuel if you burn it directly.
Hot water heat is very easy to control with a thermostat in every room, including bathrooms. I’m also scared of everyone around the country heating their house with a ductless minisplit, because then the utilities will have a winter peak problem. The summer peak can get reduced by PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. panels, but a nighttime winter peak will never be reduced by solar.
I love radiant floors, but I got away from them years ago because the heat output required for a modern house is so small — and it continues to get smaller over the years, while the thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. of the floor has not — that they respond ever more slowly over time, to the point where it can take days to warm up or cool down a house. I know of more than one house where the radiant floor has been simply abandoned in favor of radiators.
Q. Recently, some critics of the Passivhaus standard say that the standard is inappropriate for cold climates, because it requires envelope specifications that aren’t cost-effective. What’s your take on this controversy?
Henry Gifford: The envelope isn’t cost-effective when you only consider payback on an energy basis. But the essence of the Passivhaus standard is that you save money because of the avoided cost of installing a heating system separate from the ventilation system, and those savings pay for the last half of the envelope improvements. The standard is great because it gets us just to but not beyond the critical point where we can avoid the cost of the separate heating system.
Q. I’m confused. Didn’t you just explain that your designs have separate ventilation and heating systems?
Henry Gifford: No one I know of — including me — has had the courage to design a building that way yet, but it’s not the fault of the Passivhaus standard.
Q. Why are so many mechanical engineers bad at sizing pumps?
Henry Gifford: I disagree with your assumption. They don’t size pumps — they simply guess.
Q. Why they are lazy?
Henry Gifford: They have become lazy after years of being asked to cut their prices and being told, “We are giving you the architectural drawings this afternoon — can we have the mechanical drawings by the end of the week?” If they decide they will take more time, someone else will bid a lower price and get the work.
Q. In most schools, the ventilation system is basically a black box that the custodian doesn’t understand. Some schools are underventilated, while other schools are overventilated. What do you think of Marc Rosenbaum’s solution of installing an individual residential-size HRV or ERV in each classroom?
Henry Gifford: I agree with Marc more than Marc agrees with himself. I agree with him completely, and I’ll take it a little further. It’s better to put two house-sized ERVs in each classroom: one on a motion sensor and one on a clock timer. From 6 a.m. on, it’s good to introduce a little ventilation air before the kids come in.
This approach saves the cost of the fire dampers through demising assemblies. It avoids the fire hazard of smoke going through the ventilation system from one part of the building to another. It avoids the fan energy use associated with moving the air long distances, or moving the air to somewhere it isn’t needed. It avoids the need for a digital control system in the building. It avoids the need for the ventilation system to be connected to the fire alarm system.
Q. Is the LEED program getting any better?
Henry Gifford: No. Years ago, they promised to require energy measurement and verification, but there is nothing to verify with LEED. It became a requirement that every building, starting in 2010, is required to send actual utility billing data to USGBC so the USGBC can do a study. They promised in 2008 that the results would verify the results of the NBI study. They also promised to make public the energy use of all LEED buildings. However, they still keep secret the energy use of each LEED building. Instead of releasing the data for individual buildings, they will combine all the data together and release that number after an “appropriate analysis methodology.” The data hasn’t been released yet.
Q. If you were invited by the dean of an architecture school to make recommendations for changes to the architecture school curriculum, what would you recommend?
Henry Gifford: I would require every student to spend a summer being paid to work on a construction site, with the hazard of not meeting a course requirement if the student gets fired. They would have to produce work of value that someone is willing to pay for.
Not just one summer — let’s say every summer. One experience they would gain: they would be horrified to see how much of the building is designed by someone holding a saw or holding a phone connected to a material supplier. They would run horrified back to school in September and add many more details to their drawings.
And when they get back to school, they should be required to design a building in a studio class in conjunction with an engineering student from their school or a neighboring school, to force collaboration, which starts with the difficult task of choosing someone appropriate.
Q. What projects are you working on now?
Henry Gifford: With Chris Benedict, I’ve been working on two 24-family Passivhaus apartment buildings that we expect to be the first Passivhaus-certified apartment buildings in the country. The contractors have bid them for the same cost of construction as normal code-minimum junk. Both buildings are holes in the ground now. They are at 803 Knickerbocker Avenue and 442 Melrose Avenue in Brooklyn.
Q. Are there any new approaches to heating and ventilation that excite you?
Henry Gifford: Everything we need to design and build buildings that use a fraction of the energy of buildings commonly built today is already there in the building supply store. What we lack are people who know how to design buildings this way. New equipment is not the answer. The new cooling dehumidifiers are great, but we could still have done the same thing with the equipment we had 20 years ago.
Henry Gifford is a mechanical system designer at Gifford Fuel Saving Inc. and a partner at Chris Benedict, R. A.
Martin Holladay’s previous blog: “Do I Need a Vapor Retarder?”
- Charles Hoxie
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