Exhaust Gas Heat Exchanger

exhaust gas heat exchanger

exhaust gas heat exchanger
Guard your home from fire
The news that last Friday’s devastating fire on Joyce Avenue was caused by a child playing with a lighter should serve as a reminder for people to practise fire prevention in their homes, officials said yesterday.
Heater works when it wants to, why?

Central gas heat. Sequence is: (1) t-stat asks for heat, (2) exhaust blower kicks in and makes exhaust pressure switch (3) glow plugs fire (4) gas flow starts (5) flames, heat exchanger heats up (6) fan limit kicks in and heats house.

This works fine … sometimes. It’s easy to find a broken component, harder to find a temperamental one. Where would you start?
OK. Following the circuit leads me to something called a “sequencer”. I have a wiring diagram with the furnace book.

This has no mechanical elements, so I pulled it out, checked it for cracks and burn marks, sandpapered the contacts and made sure none rattled and put it back together.

It is working now, we will see for how long.

Could be the burner ‘snorkles’ are dirty, or the Rollout switches (Also called ‘Gas Spill Switch), are going bad. May be just the rollout switch on the far end, away from where the gas comes into the manifold pipe. If this rollout switch doesn’t sense the proper heat, it cuts the gas valve off. It isn’t going to sense the proper heat value, because one or more burners are dirty. (Most of the time, the burner tubes are dirty from not changing the filter regularly, or at an interval of overly dusty conditions) (Or it, or one or more of the other rolloff switches are bad. Read on)

Let’s see if I can explain this better. Let’s use an example. Homeowner has a furnace that when the thermostat calls for heat, the burners flame on temporarily, then turn off. There are 6 burners in this unit coming off of the manifold pipe. Where the gas comes in from the gas control valve to the manifold pipe, there are two burners that flame on. The other four do not. The rollout switch at the far end of the manifold pipe, does not receive the necessary heat value, to keep the gas control valve operating. The gas control valve shuts off. This is because the last four burners are dirty. In the manifold pipe are brass jets. There is one for each burner. The end of the burner that goes around the jet, has a round shape. It’s shaped like a donut with a large hole in the middle, and small holes going around the circumference. There are also slits in the large round shape. If these slits get dirty, and clogged, they cannot make the swirl pattern for the gas. They have to make this pattern to ignite the next burner on down, and so on.
If the last burner at the end of the manifold pipe doesn’t flame on, the rollout switch at the end near it, senses no heat, and shuts the gas valve off.

It all boils down to routine maintenance. Plus those rollout switches may be going bad. I have found that the A-coil for units that have AC and Heat, will get clogged up. They usually get clogged because the owner doesn’t change the filter regularly. This causes condensation to form, and rolls down inside the unit. The rollout switches are one device that receives the rusty water. This causes them to go bad after time.

The blower fan, is not only there to force warm air in the ducts leading to the registers, it also keeps the flames from the burner from ‘rolling out’ of the heat exchanger. Hence this is why there are Rollout switches. If the flame rolls out, the rollout switches shut the gas control valve off.

There is one more area, to look into that gets dirty, and is easily overlooked. See the small induction blower? See the large shiny object, that has a hose that leads over to it? This object looks like two pie pans put together. Inside is a rubber diaphragm. This is a Pressure switch. You can take the rubber tube off of the induction blower, and GENTLY suck on the tube to see if you hear it click. DO NOT BLOW in the tube, or suck too hard. (NO, no funny remarks are needed here) The rubber diaphragm inside is VERY thin! If you blow on it, it WILL break! If you use too much suction, it CAN break! A lot of times dust gets in this tube, and prevents it from operating the Pressure switch.

Best thing to do? have a trained professional check your furnace out. Around here it’s a $60 service call. They come out and check the system. If need be, they will clean the burners, and the A-coil fins. This is usually included in the service call. I imagine the rate is pretty low here, compared to other areas.

Help the technician out.(May end up being cheaper for you in the long run. Read on)
Have you ever worked on a car with a carburetor, and had to set the choke? You have to wait until the engine is cold, and then do an adjustment. This may lead to running the car, and waiting until the next day, to make sure the choke has been set properly, because the car will be cold again.
Same thing goes for a furnace. The computerized control box has an error code, it will show when something is wrong. When heat is called for the unit tries so many times, then the control box locks out. It will keep flashing the same error code over and over. If electricity is cutoff from the furnace, the code will be lost. If you view the bottom panel of the furnace, you will see a sight glass. Looking through the sight glass, you will see flashes if your furnace isn’t working correctly. There is a chart on the outside panel, or inside the furnace, that has the codes inside. IF you remove the bottom panel, you will let a safety switch turn the furnace power off. This will DELETE the code! This is a plunger switch that the panel holds in. Take the panel off, and the plunger sticks out, turning the power off. Watch the number of times it flashes at one time. It will flash so many times then stop. Then it will resume again. The tech needs this info. It may only do this at sporadic intervals, and not when a $$$$$$ tech is there! Gather this info!

23 Subaru project – Part 1 Building a new exhaust system Heat Exchanger GEET