I'm about to replace the furnace in our house. The existing furnace uses a 6" stack through the roof. I know most new furnace installations just use PVC and poke out of the wall at about ground level. Question is, can I reuse the existing stack since it's already there? Or are we better off sealing it up and forgetting about it. I'd rather not go through the wall if possible just because we have a deck right on the other side of the wall where the furnace is located.
Using existing exhaust stack for new furnace
exhaust-ventfurnace
Related Solutions
According to the National Electrical Code, a gas furnace (or any other central heating equipment) must be on its own circuit. So connecting a water heater (or anything else), would be a code violation.
National Electrical Code 2014
Chapter 4 Equipment for General Use
Article 422 Appliances
422.12 Central Heating Equipment. Central heating equipment other than fixed electric space-heating equipment shall be supplied by an individual branch circuit.
Exception No. 1: Auxiliary equipment, such as a pump, valve, humidifier, or electrostatic air cleaner directly associated with the heating equipment, shall be permitted to be connected to the same branch circuit.
Exception No. 2: Permanently connected air-conditioning equipment shall be permitted to be connected to the same branch circuit.
Chapter 1 General
Article 100 Definitions
Branch Circuit, Individual. A branch circuit that supplies only one utilization equipment.
This is a complex problem, and it's going to be difficult to provide an accurate answer without inspecting the equipment. So this is going to be more like a few comments, than an actual answer.
Water heater
I'll start with the water heater.
Old units aren't efficient
If the unit is older, it's likely not performing at peek efficiency, which is wasting energy. Updating the unit could save you money in the long run.
Tank heaters waste energy
No matter how much hot water you use, the heater is turning on from time to time to heat the water in the tank. The only way to eliminate this waste, is to install an on-demand unit.
Fuel source cost
Depending on the costs of different fuels in your area, switching fuel source could save money. Installing a gas fired on-demand water heater, could save you money long term.
Fireplace
If the fireplace doesn't have a fan, that is blowing heat into the home. You're quite possibly drawing more cold air into the home, than the amount of air the fireplace is heating. By using the fireplace, you could be blowing money out the chimney.
Also, the cost of wood is typically quite high, compared to other fuel sources.
Heat pump
Heat pumps use a lot of electricity, which in many parts of the country is an expensive fuel source. In temperate climates, heat pumps make sense because they're not typically used much. In climates where the temperatures often drop below freezing, heat pumps are virtually unheard of. This is because heat pumps aren't so good at heating homes, and often have to rely on supplementary electric heating elements.
Use the furnace on cold days
Since you have a gas furnace in the home, you can use that when the heat pump can't keep up. Depending on the local gas price, this could save you some money. In most dual fuel systems (like yours), if the heat pump can't keep up it shuts down and the furnace runs instead.
From your description, it sounds like the furnace and heat pump are running at the same time. This is not a good idea, and can actually cause damage to the system.
You'll want to configure the system (or have it configured) so that when the thermostat switches to auxiliary heat, the heat pump shuts down and the furnace kicks in.
Without actually being at your home, it's difficult to diagnose the problem. You might want to have an HVAC technician come out and evaluate the system, to help determine if you're wasting money.
Best Answer
The problem with reusing non-HE (High Efficiency) exhaust for a HE appliance (furnace or water heater) is that HE exhaust gases are much more acidic and humid than non-HE exhaust.
This means that for typical galvanized steel exhaust, a HE appliance will cause corrosion in short order, eventually perforating the stack, causing leakage of exhaust into the house.
The use of PVC in HE appliances is simply cost - PVC pipes are cheap, and the temperature of HE exhaust is low enough to allow for PVC to be used without melting (although there is some controversy around that, especially if the PVC pipes pass behind finished or through otherwise confined spaces). The other alternative is stainless steel, which is much more expensive.
If your existing exhaust stack is stainless steel, you should be able to reuse it for a HE furnace. If it's the more typical galvanized steel, your best bet would be to abandon it and run a new PVC exhaust stack. Going through the wall is typically the shortest path (and is allowed for HE exhaust), but depending on the installation you can also run the exhaust through the roof, perhaps even re-using the existing exhaust path.
When I installed a HE furnace (which replaced a non-HE one), the contractor ran about 12 feet of PVC pipe to move the exhaust on the exterior wall away from the power and gas meters, so in your case you could probably ask them to place the exhaust away from your deck. It all depends on the nature of the installation (finished/un-finished area, location of furnace in relation to the walls/roof, etc.).