HRV vs ERV: Which One Makes Sense in a Dry Climate?

Search for HRV versus ERV and almost everything you find is written for Toronto, Minneapolis or Calgary. That is not an accident — balanced ventilation with heat recovery became standard practice in cold climates first, because the energy penalty of throwing away heated air is most obvious when it is below freezing outside.

The result is that the standard advice is calibrated for a problem Los Angeles does not have. Worth working through what actually changes here.

The mechanical difference, briefly

Both are balanced systems: they supply and extract equal volumes of air simultaneously, through ducts, passing both streams through a heat exchanger so they exchange energy without ever mixing.

The difference is what gets exchanged.

	Transfers	Core material
HRV — heat recovery	Heat only	Typically polystyrene
ERV — energy recovery	Heat and moisture	A polymer membrane, commonly a polyethylene-polyether copolymer

An ERV’s core is engineered to be permeable to water vapour but not to air. Humidity migrates across it; the two airstreams do not.

Good residential units recover in the region of ninety percent of the thermal energy, measured to the relevant European test standard. That is the proportion of energy in the outgoing air handed to the incoming air rather than being exhausted.

Why the cold-climate answer does not transfer

In a cold, damp climate the reasoning runs like this: winter indoor air is humid from occupancy, outdoor air is cold, and the concern is condensation forming inside the building. An HRV that removes humidity along with the stale air is doing you a favour.

Turn that around for a hot, dry climate and the logic inverts:

• In summer, outdoor air is often warmer than indoor air. An ERV limits how much of the outdoor moisture load comes in with it, and some of the sensible heat besides.
• In winter, indoor air here is frequently drier than is comfortable rather than damper. An HRV would exhaust that modest humidity and replace it with dry outdoor air, making the house drier still.

So in most Los Angeles homes, an ERV is the better default. Not universally — a house with a genuine indoor humidity problem, a pool room, or unusual occupancy might reverse it. But the cold-climate default of “HRV unless you have a reason” is the wrong starting point here.

The practical difference nobody writes about

Here is the one that actually affects your installation, and it comes straight out of the installer documentation rather than the marketing.

An HRV produces condensate. An ERV frequently does not.

When warm, humid exhaust air passes through a heat exchanger and cools, moisture condenses out of it. That water has to be drained. HRV units therefore have condensate drain connections that need plumbing to somewhere sensible.

With an enthalpy core, the humidity in the extracted air is partly transferred into the fresh supply air instead of condensing out. In that case there is no condensate to drain, and the drain connections are sealed off with caps — the unit is not airtight if they are left open.

For a retrofit, this matters more than it sounds. Finding a drain route from wherever the unit is mounted is one of the fiddlier parts of the job. Not needing one is a genuine simplification.

And if you do need a drain, do not use a normal trap

This detail is specific to warm climates and it is easy to get wrong.

Standard plumbing practice for a condensate drain is a U-bend trap holding a plug of water that blocks air movement. Manufacturers of these units specifically warn against fitting one, for a reason that will be familiar to anyone who has smelled a rarely used floor drain: on warm days the water evaporates out of the trap.

Once it has, the seal is gone. The drain line is now an open air path into the unit, which defeats the balanced airflow the whole system depends on.

The specified solution is a dry siphon — a mechanical trap that seals without relying on standing water. In a climate where attics and utility spaces routinely get hot, that is not an optional refinement.

Sizing against what California actually requires

California requires whole-dwelling mechanical ventilation on low-rise homes, calculated as:

1 CFM per 100 square feet of floor area, plus 7.5 CFM per occupant — where occupants are counted as the number of bedrooms plus one.

Work that through for a fairly typical property. A 2,000 square foot, three-bedroom house:

• Floor area: 2,000 ÷ 100 = 20 CFM
• Occupants: 3 bedrooms + 1 = 4, × 7.5 = 30 CFM
• Total: 50 CFM

Now compare that against what residential balanced units actually deliver. Even modest models top out several times higher — in the region of 200 CFM and upward — while minimum continuous rates sit around 45 CFM.

Two things follow. First, capacity is rarely the constraint; a small unit comfortably covers a normal house. Second, and more usefully, the unit will spend most of its life running near the bottom of its range, which is where it is quietest and most efficient. Selecting a much larger unit than the calculation calls for buys you nothing and costs you both money and space.

Filtration: the part that matters most locally

This is where balanced ventilation earns its place in Los Angeles specifically, and it is worth understanding how the two filters differ.

A balanced unit has two filters, and they are not the same:

• Outdoor air side: a fine filter, typically around MERV 13. This is the air entering your house, and this filter is the reason a balanced system helps rather than hurts on a smoke day.
• Extract air side: a coarser filter, around MERV 8. Its job is protecting the heat exchanger from indoor dust, not cleaning air you are about to throw away.

Contrast that with the cheap way of meeting the ventilation requirement: a continuously running exhaust fan. It pushes air out, slightly depressurises the house, and replacement air is pulled in through every gap in the building envelope — around windows, past the attic hatch, through the floor. Completely unfiltered.

On an ordinary day the difference is modest. During a smoke event it is the difference between introducing filtered outdoor air deliberately and sucking unfiltered smoke in through the cracks. Both approaches satisfy the code.

What we would actually recommend

For a typical Los Angeles home:

1. Start with an ERV unless something about the house argues otherwise.
2. Size from the code calculation, not from the largest unit that fits.
3. Confirm the filtration on the outdoor air side is fine enough to matter — this is the point of the exercise here.
4. Check the drain requirement before choosing a location. It may not need one, and if it does, specify a dry siphon rather than a conventional trap.
5. Do not switch it off. It is designed to run continuously, and code requires the switch to be labelled precisely because people turn them off.

The short version

Most HRV versus ERV advice online is answering a cold-climate question. Here, an ERV is usually the better fit, it often removes the drain requirement entirely, and if a drain is needed it should not be a conventional water trap.

But the reason to fit a balanced system at all in this city is less about heat recovery than about where the incoming air comes from and what it passes through on the way in.