Party Walls


Since 2000, Robb has specialized in home energy systems with Steven Winter Associates in Connecticut. He has researched new technologies, monitored performance of many building systems, and worked with builders and developers across the country to create better, healthier, more efficient homes. Before joining SWA, Robb received a masters degree from the Building Systems Program at the University of Colorado and worked for several years designing, commissioning, and repairing solar electric and solar thermal systems.

Posts by Robb Aldrich

Electrify Everything? Part 2.

Heat Pump Water Heaters in Multifamily Buildings

In Electrify Everything? Part 1 that I wrote several months ago, I mentioned that integrated tank heat pump water heaters (HPWHs) can work well in single family homes — even in colder climates. For example, we see quite a few installed successfully in basements in the Northeast. These devices remove heat from the surrounding air, so there needs to be enough heat in the basement air for them to work effectively. During the winter, a home’s space heating system probably needs to work harder to make up for the HPWH. In the summer, the HPWH provides a bit of extra cooling and dehumidification. We put together some guidelines a few years ago on how to get the most from these systems in single family homes.

Image of heat pump

Some places where I’ve seen problems:

  •   Installing a HPWH in a basement closet. Even if a closet has louvered doors, there’s not enough heat/air for a HPWH to work well.
  • HPWHs are relatively loud. If there’s a finished part of the basement (e.g., bedroom or office), the noise can be disruptive.
  • Sometimes there is trivial heat gain to the basement (from outdoors, mechanical equipment, etc.). When a HPWH removes heat from the air, such a basement can quickly become too cold for the water heater to work efficiently (and too cold for comfort if someone uses the basement).

But overall, HPWHs in single family basements can work effectively.



Everyone pretty much gets that continuous (or very frequent) ventilation is necessary in high-performance homes. And – at least in theory – most people get why balanced, heat recovery ventilation is better (than unbalanced and/or without heat recovery). But the devil’s in the details.

A couple years ago we started an R&D project with funding from DOE’s Building America program, and one of the first steps was interviewing several developers about ventilation (single- and multi-family residential, mostly on the East Coast). For none of these developers were HRVs or ERVs standard.[i] They all had some experience with ERVs, however, and when asked about these experiences the word “nightmare” came up shockingly often.

The ERVs on the market now can certainly work well in the right application, but we see problems more often than not. One of the biggest challenges is trying to add ERVs on to central heating/cooling systems in homes. Most ERVs aren’t really designed for this, and here’s what we see:

  • Ducts connected to the wrong places! Outlet and inlet ducts get reversed, or the supply air from the AHU getting exhausted (sad how often this happens).
  • ERVs are attached to supply and/or return trunks of the AHU. Unless the AHU fan is running constantly (or whenever the ERV is turned on), outdoor air comes into the AHU and is sucked right back out the ERV exhaust.
  • If the AHU fan is turned on, the relatively small fans in the ERV can’t successfully compete with the big AHU fan. People don’t get the ventilation flow rates they want and/or the flows are very unbalanced.
  • AHU fans can use A LOT of electricity. Hundreds of Watts is common – I’ve measured over 1 kW (though this is changing – more below).

Even if installers follow manufacturer instructions for attaching ERVs to AHUs, they could still end up with low flows, unbalanced flows, or high electricity consumption. Through this DOE R&D effort, we’re trying to do better.

Electrify Everything? Part 1

So in utility and policy circles, electrification is all the rage. Grid electricity is getting cleaner (i.e. resulting in lower CO2 emissions), on-site renewables are taking off (sometimes even with storage), and heat pump technologies are getting better. More regional and utility initiatives are encouraging building owners/designers/developers to forego onsite fossil fuels entirely (or at least mostly) to help meet CO2 emission reduction goals. But is electricity really more sustainable than natural gas? Is it cheaper? Which is better, really?


Ventilation Idyll

Residential ventilation is really a tricky topic. But if you’re looking for a practical, cost-effective, holistic solution, go somewhere else. This post offers none.

Hopefully I can dig into practical solutions in future posts, but I think it’s important to be clear about why we ventilate and what an “ideal” ventilation system might look like in a new, efficient home. My ideal system is similar for both single-family or multi-family (though practical issues can be very, very different).

Purpose of ventilation: Remove contaminants that can compromise health, comfort, productivity, durability, etc. I’m sure there are more rigorous definitions out there, but this will work for now. There are other ways to lower contaminant levels:

Shangri La

Shangri-La image via Olga Antonenko

  • Emitting fewer contaminants from materials and activities is obviously good. Do this.
  • Actively filtering, adsorbing, or otherwise removing contaminants from indoor air can also be good. There’s talk about doing more of this, but I’m tabling it for this discussion. This may be something to keep an eye on down the road.

For most new residential buildings, mechanical ventilation is still be the primary means to remove contaminants. Or at least it’s the primary method that designers/developers need to plan for now.

If building a new, efficient home in Shangri-La, my ideal ventilation systems would look like this: (more…)

Power vs. Energy

I can get worked up about units, and this can really annoy people. It especially annoyed students I taught in grad school. I was pretty tyrannical when grading; they always had to include units in their calculations. They could have all the right numbers, but they didn’t get full credit unless all the units were right too. I have no regrets about being such a stickler, because I see tons of confusion about this in the building & energy fields. So here’s a rant about one of my pet peeves: power and energy.

Question: What’s the difference between Power and Energy?

Is this some kind of philosophical question? A koan to meditate upon? No. There’s a real answer (in the engineering world at least). Power is the rate of energy.


Air-Source Heat Pumps in Cold Climates (Part III): Outdoor Units

I’ve talked a little bit about new, air-source heat pumps (ASHPs) in older posts (I, II). There are some newer products that can work really well in cold climates, but proper sizing, installation, and operation are critical for getting good performance. One key factor is proper location of outdoor units.

First, a bit of nomenclature. The part of a split air conditioner that goes outside is often called the “condensing unit.” It usually contains most of the key refrigeration components: the compressor, condenser, expansion device, etc. The only key component located inside is the evaporator coil: where the refrigerant evaporates as it removes heat from the indoor air.


In a heat pump, all this is still true during the summer. During heating season, however, the condenser is indoors (releasing heat to the indoor air stream) and the evaporator is outdoors (removing heat from outdoor air). Because of this, calling the outdoor unit a “condensing unit” isn’t quite correct. People still use this term for a heat pump, but I think more people are simply calling it the “outdoor unit.”


During the winter, the outdoor unit removes heat from air blowing through it. Here then, is the key point to remember:  If the outdoor unit is encased in snow and ice, it is not able to remove heat from the air. Obvious, yes? But it’s amazing how often there are lapses in this.

This image below is of a new, all-electric home, and this heat pump is the primary heating system. If this was simply an air conditioner, there’d be no problem. But this is located directly beneath the gutter-less drip edge of the roof. A lot of rain and melting snow and ice is going to fall on this heat pump. When this moisture hits the evaporator coil, it will freeze. This is a new home in Maine; I expect problems.


Heat pumps have built-in defrost mechanisms, as some coil freezing is to be expected. However, when heat pumps are subject to extraordinary levels of moisture, the systems defrost A LOT. When doing testing for our study, we ran into this problem in several homes. The heat pump below was beneath a deck; it was protected from direct snow, but as snow on the deck melted, water dripped onto the heat pump where it froze. This heat pump only ran for 10 minutes before it needed to defrost again (run for 10 minutes, defrost for 7 minutes, run for 10 minutes, defrost for 7 minutes…). This is not good. Defrost cycles don’t generally use a tremendous amount of energy, but they usually happen only once every hour or so. If the system is in defrost mode ~41% of the time (7 of 17 minutes), it has at least 41% less capacity.


Drip edges from roofs are pretty obvious, but the snow melt from the deck was a less obvious source of moisture. One other source of moisture that has surprised me is other heat pumps. This is obvious in hindsight, but when heat pumps defrost, there’s liquid water that usually just drips out. What happens if there’s another heat pump below? Or three heat pumps? Before some corrective measures were taken in the installation below, the bottom heat pump really had problems – cumulative ice from the three heat pumps above it defrosting.

image009 image011

But this stacked, wall-mounted configuration was really efficient and convenient for this building; what to do? At this building, the owner installed piping to drain away moisture from defrost cycles (pic below). I was concerned that the ice just might freeze and block these pipes, but that hasn’t happened (and this building has been through one very cold, snowy winter).


I think a more simple solution is a cover. The heat pump below had a simple, site built-cover. It worked fine. Observe also that the unit is on a little pad and some blocks to keep it up out of the snow.


The blocks and pad get it ~12” above the ground. What happens if there is more than 12” of snow? Like maybe five feet? The answer is pretty straightforward: either the heat pump stops working or someone needs to do a lot of shoveling. Here they did a lot of shoveling. You may not be able to tell, but the picture above and below are of the same heat pump. Granted, this was during the record-breaking snowfall in Massachusetts two winters ago (2014-15), but there’s no sense in increasing snow shoveling loads.


So below I think is a great solution. These heat pump outdoor units are NOT located beneath a drip edge or other moisture source, but they still have covers on them for good measure. And they’re 4-5 feet off the ground. This home is in Maine where these heat pump “hats” have become pretty common. Some heat pump distributors have contracted with sheet metal fabricators to make hats for common heat pump models.


I think the installation shown above is great, but this may not be appropriate for all buildings. I’ve heard some stories of heat pumps mounted on wall brackets where vibrations from the heat pumps carry through the building. I’ve not seen this in any projects I’ve worked on – in my experience the outdoor units are very quiet and the vibrations are minimal – but others have certainly reported problems. This might be a bigger concern for older, 2×4 framed buildings. The home above has double 2×4 walls with exterior rigid foam – lots of vibration dampening. If vibrations from wall mounting are a concern, try to use stands to keep the outdoor units well above snow height.

Air-Source Heat Pumps in Cold Climates (Part II)

A few months ago I wrote about air-source heat pumps (ASHPs) in cold climates, and I promised more info on how to select the right systems and get the best performance. Below are some things we’ve learned from our work with ASHPs in the Northeast; much of this is based on the results from a study supported by the DOE Building America program. To be clear, we’re talking about inverter-driven (variable-speed) heat pumps in residential applications during heating season. Cooling is certainly important also, but we’ve been more focused on the heating performance, especially at lower temperatures. (more…)

Heat Pumps Are Taking Over

Air-source heat pumps are a booming business. In the Northeast, manufacturers report that sales of residential systems have increased by 25-35% per year over the past 5-10 years. We’ve seen more and more systems being installed in existing homes (to provide cooling while offsetting oil or propane used for heating) and into new homes (often as the sole source of heating and cooling).

We’ve looked into these systems often, and from many perspectives. I’m planning a series of posts, but, for now, here are the answers to some basic questions we receive from clients.

First, the basics: What is an air-source heat pump (ASHP)?

It’s an air conditioner that can operate in reverse. During the summer, it moves heat from indoors to outdoors. In the winter, it moves heat from outdoors to indoors. We helped NEEP (the Northeast Energy Efficiency Partnerships) to put together a market assessment and strategy report on ASHPs. The early sections in this document (see p. 12) outline the different terms and types of heat pumps (ducted/ductless, split/packaged, mini-split, multi-split, central, etc.) Unfortunately, different people can use the same term to mean different things, but hopefully the NEEP Northeast/Mid-Atlantic Air-Source Heat Pump Strategies Report can help clarify things.

Indoor section of heat pump.


Outdoor section of a heat pump.


Recalculating Solar Savings

Ten years ago, seeing a solar electric system on a building was noteworthy. Now they’re popping up everywhere. Lower cost is obviously a big driver of this solar surge; photovoltaic (or PV) system costs have dropped 50-70% in the past 10-15 years. Over the past decade, SWA has helped developers and owners install PV systems on hundreds of buildings. The systems are reliable, they have no moving parts, and they will convert sunlight to electricity for decades.

The cost effectiveness of PV, however, is not always clear. In fact, SWA has seen a concerning trend where the cost benefits of PV are exaggerated. Although costs vary with region and application, installed costs of PV are usually $3,000 – $6,000 per kWSTC.

Then there are incentives, including two key federal programs:

Photovoltaic Panels

  • 30% Federal tax credit
  • Accelerated depreciation (for businesses)

Other incentives vary greatly from region to region:

  • State, local, and utility rebates or credits
  • Sale of Renewable Energy Credits (RECs)

The Database for State Incentives for Renewable Energy ( has a good summary of these regional incentives. Federal and regional incentives can easily lower PV system costs by 50% — often more.

The final piece in assessing cost effectiveness of PV is the electricity savings. With PV generating electricity for your building, you’ll obviously be paying less to the utility. But how much less? (more…)