For 2nd-year Thermodynamics and Heat Transfer course at McMaster University in Hamilton, Ontario, Canada.

1. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
For AUTOTECH 2TS3
Dr Timber Yuen
Design, Construction and
Testing of a Heat
Exchanger to Recuperate
Heat Normally Lost from
Exhaust Fumes of a Gas
Hot Water Heater

2. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
-Gas water heaters burn methane (CH4) which
gives off CO and CO2
-These gases must be ventilated to outdoors to
protect health
-Tremendous amounts of heat are lost due to the
hot fumes being ventilated
-We wanted to build a device that uses the heat
from these fumes to preheat cold water to save
gas during the water heating process.

3. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
-Normal Hot Water Heater Configuration -Our planned configuration

4. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
-Cold water would normally come in at 11°C
and be heated to about 55°C
-This heat exchanger would “preheat” the
cold water to a slightly warmer temperature
(ie, 20°C) and reduce the amount of energy
required to get to 55°C
-The blower is only active when the tank is
filling

5. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
Design
-Helical pipes wound within air pipe
-In reality, pipes would go in and out sealed
holes in outer casing
-To simplify construction, pipes protruded
from open end of exhaust pipe.

6. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
Design
-We needed a source of hot air that would
simulate the exhaust of a gas water heater
-We approximated the temperature and air
flow rate of a space heater to be that of a
water heater exhaust unit (may have been
inadequate)
-Using an approximate input air speed of
4 m/s, a fluid simulation was done to look
at the speed of air as it passes the coils

7. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
Construction
-Copper was used for water tubes due to high
thermal conductivity
-Copper tubes were wound around a smaller pipe
to obtain the helical shape
-4 tubes were used (two purchased and cut in half)
-Difficulty with kinking, used ice to hold structure
of tube
-We bent two of the coils in the wrong direction –
had to re-coil

8. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
Construction
-A space heater was used to simulate hot exhaust
-Duct fittings were used to route the air into the abs
pipe
-Spray foam was used to insulate steel ducts from
convective air

9. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
-Copper Manifold used to split flow to 4 coils
-Flare fittings to vinyl hose
-Only potential source of leaks
Construction

10. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015 Testing
- We wanted to know what effect the flow rate
has on the rate of heat transfer
- We expected an optimal rate of fluid flow
that balances the convection coefficient h and
the time t each element of water spends in the
elevated temperature environment
-The optimal rate of fluid flow will then be
compared to what is expected from an average
water heater

11. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
-Divide fluid flow range to four different rates
-At each rate, measure flow rate using graduated
cylinder and timer
-Measure output temperature at 30 seconds and 2
minutes
-Turn space heater on fan only to clear out any heat
between tests, cool down heater
-Repeat for every flow rate
Procedure

12. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015 Testing
-This experiment had considerable danger
-Space heater has exposed electrical components,
any contact with water could transmit voltage to
casing and heat exchanger components
-A GFCI outlet was used to ensure shutoff if water
contacted any electrical parts
-Problems with temperature limit on space heater
-Reduced power to 1300W, reduced cycle time,
removed cap of air pipe

13. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015 Testing

14. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
Results
10
11
12
13
14
15
16
17
18
19
0.007413 0.012893 0.0256 0.054167
Temperature(°C)
Flow Rate(kg/s)
Outlet Temperature vs. Flow Rate
Outlet Temperature at 30
Seconds
Outlet Temperature at 2
minutes
Inlet Temperature
= 11°C
Flow Rate
(kg/s)
Outlet
Temperature at
30 Seconds
Outlet
Temperature at
2 minutes
Heat Transfer
Rate at 30
Seconds
Heat Transfer
Rate at 2
Minutes
0.007413 14 18 92.95902 216.90438
0.012893 14 16 161.67822 269.4637
0.0256 12 13 107.008 214.016
0.054167 11 11 0 0

15. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
Results
0
50
100
150
200
250
300
0.007413 0.012893 0.0256 0.054167
HeatTransfer(W)
Flow Rate (kg/s)
Heat Transfer Rate vs. Flow Rate
Heat Transfer Rate at 30 Seconds
Heat Transfe Rate at 2 Minutes

16. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
0%
5%
10%
15%
20%
25%
0 0.02 0.04 0.06
Efficiency
Flow Rate (kg/s)
Electrical Power to Water Heating Power Efficiency
Efficiency @2min (1300W)
Efficiency @ 30s (1300W)
Results

17. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
- We observed a maximum heat transfer rate at
0.013 kg/s
-Expected flow rate to supply a hot water
heater is 0.134 kg/s (2012 Ontario Building
Code)
-At that rate, our heat exchanger would be
virtually ineffective
-We would need to increase the number of
coils, or increase tube diameter, or a
combination
-Designing an experiment to find an optimum
configuration of tubes would be difficult
Analysis
National Research Council of Canada, 2009

18. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
- National Research Council of Canada did a study that finds the average family of four uses 167 L/day of hot water
- We wanted to calculate how much energy we can save heating 167 litres of water:
- We used 14¢ as the cost for 1kWh
-
167𝐿∗1𝑘𝑔/𝐿
0.013𝑘𝑔/𝑠
= 12,846 s
- Using our best energy transfer rate 269.46W:
- 12,846s * 269.49W = 3461kJ
-
3461𝑘𝐽
3600𝑘𝐽/𝑘𝑊ℎ
∗
14¢
1𝑘𝑊ℎ
= 13.44¢/day = $49/year in savings
- This value is highly theoretical because it uses the heat transfer rate when the flow is very small
Analysis

19. Domestic Water Pre-Heater using
Gas Water Heater Exhaust Heat
Jack Gillies & Joshua Molina
March 31, 2015
Conclusion
-The heat exchanger would have to be redesigned to accommodate higher flow rates
without sacrificing surface area
-Although we don’t know the actual savings, we know that a device like this has potential
to save considerable amounts of energy
-Devices such as this would not be difficult to retro-fit into existing residential plumbing
-Other heat recovery systems are becoming popular ie, drain heat recovery
-The testing showed that there is in fact an optimum flow rate for heat exchangers, as our
hypothesis predicted
Questions?