high efficiency gas furnaces - carsondunlop.com

Copyright 2019 Carson, Dunlop & Associates Ltd.

HIGH EFFICIENCY

GAS FURNACES

Alan Carson

CARSON DUNLOP

March, 2019


Before we start

–Want to make sure we are all

on the same page……


Definition of a Home Inspection

A business with illogically high liability, slim profit

margins and limited economies of scale. An

incredibly diverse, multi-disciplined consulting

service, delivered under difficult in-field

circumstances, before a highly stressed, hostile

audience with differing interests, in an impossibly

short time frame, requiring the production of an

extraordinarily detailed technical report almost

instantly, without benefit of research facilities or

resources.


GOALS

–Rules of the game

–Identify high efficiency gas

furnaces

–Understand how they work

–Understand how they fail

(Focus on upflow furnaces)


OAHI STANDARD - HEATING

Inspect:

• Installed heating equipment.

• Vent systems, flues and chimneys.

• Fuel storage systems

• Distribution systems.


OAHI STANDARD - HEATING

Describe:

• Energy source – natural gas

• Heating method – furnace

• Chimney or venting material – plastic

• Combustion air source – outside/basement

• Exhaust venting method – induced draft

/direct vent


WE DESCRIBE

• Fuel

• Size

• Age

• Efficiency

From data plate and

Technical Reference Guide


WHAT WE REPORT

• Significantly deficient or near end of

life

• Why it’s so & why it matters

• TELL THEM WHAT TO DO!!!!!

– correct or monitor

• Anything we didn’t inspect and why


NEAR END OF LIFE

Need to know -

1. Life expectancy (e.g. 15 to 20 years)

2. Age?

1. Carson Dunlop’s Technical Reference

Guide

2. Manufacturer


IDENTIFICATION

•How do we tell if

it’s a high

efficiency furnace?

•Just look!


HERE ARE A FEW


3.5 WAYS TO IDENTIFY

HIGH EFFICIENCY

1. Efficiency rating

• 90% + AFUE – What do letters mean?

• Check data plate - (Output/Input)

• Conventional – 80% SS: 60% Seasonal

• Mid – 80% SS: 80% Seasonal

• High – 90%+ SS: 90%+ Seasonal


IDENTIFYING HIGH

EFFICIENCY FURNACES

2. “Condensing furnace”

• Exhaust gases cool and condense

• Condensate drain line


IDENTIFYING HIGH

EFFICIENCY FURNACES

3. PVC/ABS vents (150°F/180°F)

– 100 to 150°F exhaust

• S636 PVC or CPVC (200°F)

– orange labels

– Since 2007

• PP (Polypropylene) – the new kid!

(230°F)


ABS

PVC


SIDEBAR:DON’T USE PVC ON MIDS!


IDENTIFYING HIGH

EFFICIENCY FURNACES

3.5. Sealed Combustion/Direct vent

• Use outside air

• Three advantages!

1. No depressurization/back-drafting

2. No waste of warm house air

3. No pollutants into furnace – gas, paint

remover, bleach, cleaning products, water

softener salts, etc.


SEALED COMBUSTION

OR DIRECT VENT


IDENTIFYING HIGH

EFFICIENCY FURNACES

Why 3.5?

• Direct vent - means high

efficiency

BUT….

• High efficiency - does not

necessarily mean direct vent


IDENTIFYING HIGH

EFFICIENCY FURNACES

SUMMARY

1. 90%+ AFUE rating (data plate or model)

2. Condensate collection

3. PVC/ABS/S636/PP venting (low temp

plastic)

4. Sealed combustion (direct vent) – optional


GETTING HIGHLY

EFFICIENT

So, how do we do it?

• Extract more heat from

combustion products

• How?


GETTING HIGHLY

EFFICIENT

• Keep exhaust products in house

longer

• Expose to more house air

• More heat transfer

Longer heat exchanger surface

Multiple heat exchangers


GETTING HIGHLY

EFFICIENT

• But there’s more to it…..

• Magic in condensation

• LATENT HEAT OF

VAPORIZATION


GETTING HIGHLY

EFFICIENT

• Main products of combustion of

natural gas?

– Water

– Carbon dioxide

• The water (steam) is the key


LATENT HEAT OF

VAPORIZATION

• Define BTU!

• Heating 1 lb. of water

• From 32°F to 212°F

– takes 180 BTUs

• From 212°F water to 212°F steam

– takes 970 BTUs!

• Condensing steam releases 970 BTUs!


GETTING HIGHLY

EFFICIENT

•So, if we can get the

combustion gases to

condense……


TWO ADVANTAGES

OF CONDENSING

We can get way more heat!

Plus –

Exhaust is so cool, we don’t need

chimneys!


SIDEWALL VENTING


DISADVANTAGES

OF CONDENSING

But…….

Condensate rusts metal

–E.g. heat exchangers

Can we control the water?


How much water does a

typical furnace produce

in a month?

Prize!


a. 10 gallons

b. 25 gallons

c. 50 gallons

d. 100 gallons

e. 200 gallons


200 gallons!


WE NEED TO COLLECT IT


CONDENSATE IS CORROSIVE

Need special heat exchangers

• Stainless steel

• Plastic coated steel

• Just the secondary HE – Why?


PLASTIC COATED

CLAMSHELL


GETTING HIGHLY

EFFICIENT

• So, we can condense the combustion

gases

• And capture way more heat

• If we collect the water properly

• And we don’t rust the heat exchanger


SEQUENCE OF OPERATION

1. Thermostat call

2. 25-second purge

3. Draft checked

4. HSI warms up

(15/20 sec.)

5. Gas valve opens

6. Prove flame

(2/8 sec.)

7. Blower on 30-60 sec.

8. Firing rate & blower

rate may adjust

9. Thermostat satisfied

10. Burner off

11. 15-second purge

12. Blower off 60-240

sec. later


THE COMPONENTS

• Let’s look at each one


THE COMPONENTS


1. CABINET

Same as any furnace:

• Rust?

• Damage?

• Out of level?

• Combustible clearances –

negligible


2. COMBUSTION AIR

• Also called dilution and ventilation

air

• Inside – need lots of space

• Outside - direct vent – pretty

straightforward~ish

• Note: Many furnaces can get

combustion air from in or out


FIND COMBUSTION AIR SUPPLY!


COMBUSTION AIR SUPPLY!


3. BURNERS

• Typically in-shot/monoport

• Not ribbon

Flame should be -

• Mostly blue

• Stable

• No lift off or floating


MONOPORT BURNERS


ANY PROBLEM HERE?

Photo courtesy of HVAC-Talk.com. and Kenny Hart


3. BURNERS

• 1 stage, 2 stage, modulating

• Not infinitely variable

• Does house air fan speed change as firing

rate changes?

• Does induced draft fan speed change as

firing rate changes?


3. BURNERS

• 1 stage, 2 stage, modulating

• Not infinitely variable

• Does house air fan speed change as

firing rate changes? YES

• Does induced draft fan speed change as

firing rate changes? YES for most


3. BURNERS

Burners at top of furnace – Why?

1. Better heat transfer - Coolest

exhaust gas meets coolest

house air

2. Condensate runs down – away

from burner

Copyright 2016 Carson Dunlop and Associates Ltd.

BURNERS AT TOP

Coolest exhaust

sees coolest

house air

(upflow)

Condensate

collected at

bottom, far from

burners

Cool

House

air

flow

War

m


4. IGNITION SYSTEM

No standing pilots!

Most use one of these

–Hot Surface Ignition (HSI)

• most popular

–Direct Spark Ignition (DSI)

–Intermittent Pilot (IP)


4. IGNITION SYSTEM

Hot Surface Igniters (2,500°F)

–Silicon carbide

–Silicon nitride (newer/more

durable/less energy)


HOT SURFACE IGNITER


4. IGNITION SYSTEM

• Early Hot Surface Igniters

(HSI) were troublesome

• Newer ones are more

reliable


CARRIER HSI


5. HEAT EXCHANGER

• May be 1, 2 or 3 heat exchangers

• Most new furnaces have two

• 1st conventional steel or aluminized

steel

• 2nd plastic coated or stainless

• Sees harsher environment –

condensate


PLASTIC COATED

CLAMSHELL - AGAIN


5. HEAT EXCHANGER

• What is warranty on heat

exchangers?

• Can’t see cracks or clogging

• Small heat exchangers may clog

– Construction dust!

• Carbon/soot at exhaust termination

• May see evidence of leak below


5. HEAT EXCHANGER


SOME DO RUST!Photo courtesy of HVAC-Talk.com. and Kenny Hart


SOME DRAMATICALLY!

Photo courtesy of HVAC-Talk.com. and Kenny Hart


RUSTED, CRACKED

HEAT EXCHANGERPhoto courtesy of HVAC-Talk.com.


CLOSE UPPhoto courtesy of HVAC-Talk.com.


Soot on

the filter

is a clue

http://hvac-talk.com/vbb/index.php


6. CONDENSATE SYSTEM

• Need to condense

exhaust gases to get

efficiency

• Latent heat of vaporization


6. CONDENSATE SYSTEM

• Highest efficiency at startup!

–Don’t need long running

cycles

–Cold heat exchanger makes

for good heat transfer


6. CONDENSATE

• Where do we collect it?

–2 or 3 places

• How does it flow?

–Gravity mostly


6. CONDENSATE

• Can make >1 quart in 30 minutes

• Again, 200 gallons/month


6. CONDENSATE


THE RISKS AROUND

CONDENSATE!


WHAT’S THE (BIG) ISSUE?


7. INDUCED DRAFT FAN

• No dilution air

– Use 50% less air

• Pulls Combustion air in > Burner > Heat

Exchanger

• Pushes exhaust > Vent

• Heat exchanger more restrictive

– Get more heat out of the exhaust


You don’t need a picture of an

induced draft fan, do you? ½ way



•Early systems not so

reliable

•Better now



• Plastic housing > metal

• Listen for bearing noise – squeaking,

grinding



• Listen for splashing/gurgling on start-

up

– Condensate collection problem

– Low spot in venting system


8. AIR PROVING SYSTEM

• Makes sure fan is moving air

• Combustion and exhaust

• Pressure switches

–Fixed

–Differential


8. AIR PROVING SYSTEM

• Not much to check

• If problems, furnace won’t run


WHERE’S AIR PROVING

SWITCH?

Where’s

the air

proving

device?


WHERE’S AIR PROVING

SWITCH?


8. DRAFT PROVING

SYSTEM


9. CONTROL BOARD

–Complex/expensive black box

–Solid state

–Diagnostic LEDs

–3 year warranty typical

–Poor location!!!!

–$700 to replace????????????


CONTROL BOARD


Where

would you

put the

control

board?


10. VENTING SYSTEM

• PVC or ABS typical

• S636 White – 150ºF (PVC typically)

• S636 Grey - 200ºF (CPVC typically)

• Required since 2007

• Roof or sidewall

• 2 to 4 inch diameter typical


S636 VENTING SYSTEM


POLYPROPYLENE VENT

• S636 rated

• 230ºF rating

• 6 ft lengths

• Push-fit connectors (no adhesive)

• Light, thin wall

• Flexible available for vertical only

• Grey, white, black for outdoor pieces


POLYPROPYLENE VENT

Male ends face upstream

Only approved by some furnace manufacturers

Courtesy of Centrotherm


10. VENTING SYSTEM

• May be coaxial/concentric with

combustion air intake


COAXIAL/CONCENTRIC VENTING


10. VENTING SYSTEM

• Vent length

• Lots of variables

• Minimum 15 ft typical – Why?

• Minimum 12 inch offset inside

home – Why?


10. VENTING SYSTEM

• Minimum 15 ft typical – Why?

• Minimum 12 inch offset inside

home – Why?

May get water at termination


10. VENTING SYSTEM

• Question runs > 40 feet

• Question > 3 or 4 elbows

• Elbows add ~5 feet each

• Larger diameters = longer runs

• Vertical can be longer than

horizontal


10. VENTING SYSTEM

• Slope for sidewall systems

• To outside or back to

furnace?

• How much slope?


10. VENTING SYSTEM

• Slope for sidewall systems

• To outside or back to

furnace? Furnace

• How much slope? ¼ in./ft.


10. VENTING SYSTEM


IF VENT SLOPES

DOWN TO OUTSIDECourtesy: hankeyandbrown.com


10. VENTING SYSTEM

• Termination – Roof– 18 inches above roof (12 inches above snow)


10. VENTING SYSTEM

• Termination – Sidewall – see manual!• 12 inches above snow level (Not grade!)

• Not over walkway

• 4 feet from gas regulator

• 6 feet from mechanical air inlet

• 4 feet from other air inlet beside vent

• 12 inches above air inlet below vent

• 12 inches from windows or doors

• Not right under eaves/No inside corner


10. VENTING SYSTEM

• Vent should be insulated in unheated space

(e.g. attic, garage, cantina/cold room)

• Direct vent system – both go thru roof or

wall (same pressure zone)


10. VENTING SYSTEM

• Often close together, side by side

(e.g. 3 in. for roof, 6 in. for wall)

• Exhaust often 12 in. above inlet


10. VENTING SYSTEM

• Wall termination – see manual!

• Varies by manufacturer

• Vent may have to extend out at least 12 in.

• Point straight out


10. VENTING SYSTEM

• Intake should be as short as possible

• Often points down

• Inspection tip – look for carbon/soot at

vent termination

Copyright 2019 Carson, Dunlop & Associates Ltd.Photo courtesy of HVAC-Talk.com. and Kenny Hart

Keep manual handy!


INSIDE CORNER – NOT GOOD


LOW TERMINATION?


LOW, OBSTRUCTED

TERMINATION?


11. HOUSE AIR FAN

• Most are multi/variable speed

• May be ECM/Brushless DC

• More efficient – uses <1/2 the electricity


11. HOUSE AIR FAN

ECM Motors

• Run cooler

• Wide operating range

• Multi-stage furnaces

– operate faster with higher firing rates


11. HOUSE AIR FAN

• ECM adds to furnace cost ~ $500?

• Energy savings

• $200/year often claimed

• May be true if AC

• More if fan always on

• Some studies say buy if fan always on

• Costs are down & warranties are up


IDENTIFYING ECM

Motor

Control module


ECM – LOTS OF WIRES!


JUST READ IT!


11. HOUSE AIR FAN

• Adjust fan speeds to get right temperature rise

• Rise too low? Slow fan down

• Rise too high? Speed fan up


Watch Temperature Rise!

Hi-efficiency replaces conventional/mid

• Ductwork may be restrictive

• Air conditioning coil restricts flow

• May be hard to get adequate airflow

• Excess temperature rise voids warranty

• Modify ductwork?


12. SAFETY DEVICES

1. Limit switch

2. Rollout switch

3. Air proving switch

4. Flame sensor

5. Blower door

interlock


12. SAFETY DEVICES

1. Limit switch

–Shut off on high temp

–Most auto reset

–Near burner or top of heat

exchanger

–Remember Fan/Limit switches?


LIMIT SWITCHES


WHERE’S THE LIMIT SWITCH?


12. SAFETY DEVICES

2. Rollout switch

(Spillage switch, heat switch)

–Detects flame coming out of the

burner instead of into the heat

exchanger

–Shuts down furnace


ROLLOUT

SWITCH

http://www.hvacpartstore.com/product_details.php?products_id=52


WHERE’S ROLLOUT SWITCH?


12. SAFETY DEVICES

3. Air proving switch

(differential pressure switch)

• Makes sure air flows from intake

right through to vent termination


12. SAFETY DEVICES

4. Flame sensor/rod/electrode

• Replaces thermocouple

• Proves flame in 2 – 8 seconds

• Often dirty – white crust

• Corrosive environment

– Kitty litter!

– Paint, laundry detergent


Courtesy of askmehelpdesk.com

FLAME SENSOR


Where are they?

Roll-out Switch Igniter Flame Sensor


IgniterRoll-Out

SwitchFlame

Sensor


12. SAFETY DEVICES

5. Door interlock

–Prevents furnace operating with

cover off

–Handy for us!


DOOR

INTERLOCK


DOOR INTERLOCK!


SIMPLE APPROACH

1. Inspect first

– Rust, leaks and age (size?)

2. Then test operation

– Fire up properly?

– Leaks, splash, gurgle?

– Shut down too soon?

– Temp rise after 15 minutes?

– Airflow at supply/return grilles

3. Suggest annual maintenance program


Annual Maintenance

• But it has a 10-year warranty!!!!

• Prevention better than cure

• Diagnostic visit is not warranted

• Overtime is not warranted

• Warranty void if maintenance is poor

• We always recommend a contract


WE’RE DONE!

Thanks very much!

Alan Carson

CARSON DUNLOP

800-268-7070

www.carsondunlop.com

high efficiency gas furnaces - carsondunlop.com

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