0 comments

Do You Need a Bigger Transformer? When a 40VA or 75VA Unit Makes More Sense

Tony breaks down transformer sizing, VA ratings, zoning loads, thermostat power, humidifiers, accessories, and exactly when “bigger” saves your system.

Most people think a transformer is just a transformer — like any generic power brick you plug in behind your TV. They grab the first 120V-to-24V unit they see, toss it in the furnace, wire it up, and hope it works.

Sometimes it does.

But more often?

You end up with:

blown fuses
burnt transformers
buzzing contactors
thermostats rebooting
zoning dampers stuck
humidity systems failing
relays chattering
weak 24V output
overheating furnace boards

And the homeowner says:

“Why does this transformer keep burning up?”

Tony’s answer?

Because you’re using the wrong VA rating.

Today I’m going to walk you through:

what VA means,
why size matters,
what happens when a transformer is too small,
when to upgrade to 40VA, 50VA, or 75VA,
how to know your system is overloaded,
how to prevent another burnout,
and how to size your transformer the way a pro does.

Let’s get into it — Tony style.

What “VA” Actually Means in a Transformer (And Why Homeowners Ignore It)

When you see “40VA” or “75VA,” that’s not some random number.

VA means volt-amperes — basically, how much power the transformer can safely supply to your 24V control circuit.

More VA = more load capacity

Less VA = more likely to overheat and fry

Think of VA as the transformer’s muscle.
If you overload it, it gets hot.
If you overload it too long, it dies.

If you ignore VA ratings, you’re gambling with your system.

And most homeowners lose that gamble.

Why HVAC Systems Need the Right VA Rating: It’s All About Load

Your 24-volt circuit powers:

thermostat
control board
contactor outside
relays inside
zoning dampers
humidifier solenoids
safety switches
accessories
WiFi thermostats
air cleaners
UV systems

Each one of these components adds load.

The transformer can only supply so much current before overheating.

Here’s the real HVAC math:

More accessories = more VA required

More dampers = more VA required

Smart thermostats = more VA required

Older systems = more VA required

Larger homes = more VA required

A 40VA transformer is rarely enough for modern systems.

Let’s break it down.

What Happens When Your Transformer Is Too Small (The Symptoms Are Obvious)

A small transformer doesn’t usually die instantly.
It dies slowly and painfully — and your system shows the signs.

✔ Thermostat fails to power on

✔ Thermostat reboots randomly

✔ Contactor buzzes instead of clicking

✔ Zoning dampers stick

✔ Humidifier doesn’t run

✔ Furnace short-cycles

✔ Board LED flickers

✔ Transformer hums or vibrates

✔ Fuse blows repeatedly

✔ Transformer gets hot to the touch

✔ Control signals become unreliable

Most people blame the thermostat.
Or the furnace board.
Or the outdoor unit.

But the real problem?

The transformer is being overloaded.

Here’s the technical reasoning for these behaviors:
[Effects of Transformer Overload on Low Voltage Control Circuits]

When a 40VA Transformer Is Enough (Tony’s Honest List)

Let’s start with where a 40VA transformer actually makes sense.

A standard 40VA transformer is fine ONLY if all of these are true:

✔ Single-stage furnace
✔ Single-stage AC or heat pump
✔ No zoning
✔ No humidifier
✔ No UV systems
✔ No air cleaners
✔ No smart thermostat (sometimes, but iffy)
✔ No external relays
✔ Simple wiring layout
✔ Short low-voltage runs

If you have a simple, traditional HVAC system with no add-ons, 40VA can work reliably.

But the moment you add ANYTHING?
The 40VA rating becomes the weak link.

Let’s get into the real world now.

When You Absolutely Need a 50VA Transformer

A 50VA transformer is the minimum Tony will use on systems that include:

✔ A heat pump (extra load)
✔ A smart thermostat
✔ Long thermostat wire runs
✔ An outdoor unit with alternate defrost controls
✔ A furnace board that draws heavier current
✔ Multi-speed or ECM integration
✔ Additional relays for staging

Even without zoning or humidifiers, heat pumps regularly need a stronger transformer.

Here’s the reasoning behind this upgrade:
[24V Load Requirements for Heat Pump and Multi-Stage Systems]

When You Need a 75VA Transformer — The Situations Homeowners Underestimate

Here is where the average homeowner and rookie tech get burned:

If your system includes ANY of the following, you need a 75VA transformer:

✔ Zoning system (2 zones minimum)
✔ Zoning dampers (especially motorized)
✔ Smart thermostat + accessories
✔ Humidifier connected to R/C
✔ UV light tied into low-voltage
✔ Air cleaner tied into low-voltage
✔ Additional relays controlling fans or stages
✔ Long low-voltage wiring throughout a big home
✔ Heat pump + electric backup heat
✔ Multi-zone heat pump setup
✔ Commercial air handler
✔ Multiple contactors (like dual-fuel or dual-compressor setups)

Zoning is the biggest transformer killer in the United States.

Every damper motor can pull:

2–5 VA at rest
8–15 VA on activation

Multiply that by 3–6 dampers and the math becomes ugly fast.

That’s why Tony never installs zoning without adding a 75VA transformer.

Here’s the electrical logic:
[HVAC Zoning Systems and VA Consumption Calculations]

Why Smart Thermostats Often Require Bigger Transformers

Smart thermostats like:

Nest
Ecobee
Honeywell T9/T10
Emerson Sensi

…draw significantly more power than old mechanical stats.

Smart thermostats need excess VA to operate:

WiFi
Screens
Internal relays
Processor chips
Sensors
Power-stealing features

If your system barely had enough VA before adding a smart thermostat?

It definitely doesn’t now.

Signs your smart thermostat is starving for power:

screen flickers
reboots
drops WiFi
doesn’t activate cooling or heating
random shutdowns

The solution?

Upgrade the transformer to at least 50VA or 75VA.

Why Heat Pumps Need Bigger Transformers Than Furnaces

Heat pumps use:

contactor coil
reversing valve coil
defrost board
auxiliary heat relays
outdoor fan relays
crankcase heater relay (in some systems)
thermostat terminals O/B, Y, R, C all active

That’s a LOT of 24V load.

A 40VA transformer might power a furnace fine…
but a heat pump with all accessories?

Not a chance.

Tony won’t install anything under 50VA on a heat pump system — ever.

And when zoning is added?

75VA minimum.

How to Calculate Your System’s VA Needs (Tony’s Simple Method)

Forget complicated formulas. Tony uses a simple pro rule:

Write down everything powered by 24V and add up its VA.

Here’s the average VA consumption:

Thermostat: 3–5 VA
Control board: 5–10 VA
Contactor coil: 3–5 VA
Reversing valve: 3–5 VA
Zoning damper: 4–8 VA each
Zoning panel: 10–20 VA
Humidifier solenoid: 12–18 VA
UV light relay: 3–5 VA
Air cleaner relay: 3–5 VA
Extra relays: 2–5 VA

Now total it up.

If it exceeds:

30 VA → 40VA transformer is cutting it close
40 VA → 50VA transformer recommended
55+ VA → 75VA transformer required

Here is the VA-load concept Tony uses:
[VA Loading for HVAC Transformers: Component-by-Component Breakdown]

What Happens When You Oversize a Transformer

Good news here:

Oversizing within reason is safe.

Installing a 75VA transformer on a system that really only needs 40VA?

That’s fine.

It will run cooler, last longer, and provide stable voltage.

The danger is UNDERSIZING — not oversizing.

What Happens If You Install a Transformer That’s “Too Big”?

The only risk is if you install a transformer with too high a VA rating WITHOUT proper fuse protection.

If a short occurs:

The bigger transformer can push more current
The fuse may not blow fast enough
Your control board can fry

This is why Tony ALWAYS adds proper fusing on upgraded transformers.

How Tony Decides Transformer Size in the Field

Here’s Tony’s real-world rulebook — the stuff that matters more than manuals:

✔ If zoning → 75VA

✔ If humidifier → 50–75VA

✔ If heat pump → 50VA minimum

✔ If smart thermostat → upgrade transformer

✔ If multiple accessories → upgrade

✔ If control board LED flickers → upgrade

✔ If transformer buzzes → upgrade

✔ If thermostat flickers → upgrade

✔ If contactor chatters → upgrade

✔ If fuse blows repeatedly → find short THEN upgrade

Never skip the “find short” part — or you’ll cook your new transformer.

Signs Your System Needs a Bigger Transformer RIGHT NOW

If you see ANY of these symptoms, you’re underpowered:

✔ Thermostat randomly resets
✔ Contactors buzz on cooling call
✔ Dampers don’t open fully
✔ Zone board shuts off
✔ Humidifier inconsistent
✔ Furnace board flickers
✔ WiFi thermostats lose power
✔ You’ve replaced more than one transformer
✔ The transformer feels hot when touched
✔ Furnace fuse blows randomly

All of these point to VA starvation.

When to Use Multiple Transformers Instead of One Large One

Sometimes adding one big transformer isn’t the right move.

Tony uses separate transformers when:

✔ A zoning system is installed

✔ A humidifier needs constant power

✔ Smart thermostats need isolated power

✔ Multiple relay kits are used

✔ Outdoor and indoor loads must be isolated

✔ Commercial spaces have long wiring runs

Multiple transformers reduce load and isolate circuits — preventing repeat failures.

Tony’s Final Verdict

When it comes to HVAC transformers, size absolutely matters.

Here’s the truth:

✔ Most modern homes need more than 40VA
✔ Heat pumps need 50VA minimum
✔ Zoning systems demand 75VA
✔ Humidifiers overload standard transformers
✔ Smart thermostats eat low-voltage power
✔ Underpowered systems behave unpredictably
✔ Oversizing is fine if properly fused
✔ Undersizing kills transformers and fuses

A transformer isn’t “just a transformer.”

It’s the heartbeat of your HVAC control system.

Pick the right VA rating and your system runs smooth for years.

Pick the wrong one and you’ll be replacing transformers, blowing fuses, and cursing your furnace all winter.

Tony chooses the right size every time — now you can too.

Tony's method to test a transformer with a multimeter is discussed in the next blog.

Tags: Tony’s toolbox talk