Heat/a/c cusing a vacuum

Glenn Harrison_2

Does your customer have an attic fan? Also, here are they combustion air vents ducted to. My thought is, 1, the combustion air vents are ducted to the attic, and the attic fan is sucking air out of the C.A.V's.

2. The attic fan is sucking the whole house into a negative pressure.

These are assuming they have an attic fan.

One other thought, is the return ducted into the attic at all? If so, and assumng the C.A.V's are ducted into the attic, the return could be suckking air out of the attic, and therefore out of the Combustion Air Vents.

I'm making a lot of assumptions. Hope one of them is helpfull.

STEVE PAUL_3

sucking water heater flue

We are having a problem with a water heater install we did about a week ago. We replaced an older 40 gal. 40,000 btu water heater with the same parameters except the new one has the flamable vapor flash back system. The problem is that when the fan in the furnace in the same room as the water heater is running, (heat, A/C or fan only)we get a back draft down the water heater flue. Enough to set off the CO detector when the water heater is firing. The room also has 2- combustion air grills for make-up air that are sucking as well. If we open the front door, the problem stops. The owner says it never happened with the old water heater. We don't know if this is true or not. We suspect that the 15"X15", grill on the return plenum is not big enough for the 24"X16" return box.We are thinking of putting a tight fitting wheather stripping on the bottom of the furnace rooms solid door. Do you think this will help or will it just suck from another location?

Techman

System Sucking

Hi Steve.I beleave the return from the HVAC unit is NOT allowed to be in the same area as the flue from any device, to prevent this problem from happening .

STEVE PAUL_3

The return grill is outside the furnace room. We think that we should put the wheather stripping on the door to seal the furnace room against the return negative pressure.

bill_71

i take it everything is in the basement and the "furnace room" is the part of the finished basement that is walled off so nobody has to look at the unsightly but ever so important shtuff? how long ago was the basement finished off? drop cielings or sheetrocked?? it is entirely possible that there isn't enough combustion air for these peieces of equipment in the basement as a whole, and the return in the basement is not helping things at all. the homeowners may not have had this problem until they finished their basement 2-3 year ago and then the problem started but wasn't apparent until they bought a co detector around the same time as you put in a water heater???? sound about right or am i way off base here???

STEVE PAUL_3

This job is a single story house on a slab. The water heater and furnace are in a seperate room with dedicated combustion air grills. The return register is in the hallway wall, connected direcltly to the furnace through the wall return.We think that the blower is pulling air from the flue piping. We are going to weather strip the door toor to seal the room from air flow into the house. I am thinking the house is too tight for the system.

Techman

Steve Paul

Morning!The return duct is sized for a 3ton unit,what do you have for AC BTU's?

JackEnnisMartin

Wate heater problem

Pleae do this on Monday morniug for my peace of mind and your own. CLose off the return air grill in the furnace room. This is a very bad installation gafe by the original installers and it has to be closed off. Your customer could be getting a lot of Co in his/ her building air. The solution to your problem is sit down with the gas code book and size out the combustion grill size for a power vented system. The grills are sized for a natural draft gas unit. The advent of powered burners for residential use has caused many headaches and the people who are supposed to send out amendments to the gas code are not keeping up. You will be amazed at the difference in the size of the grills required.In any event the return in the furnace room is a wrong wrong wrong -- change it.
Jack Ennis Martin

Steve Paul_4

CLARIFICATION

I will explain the situation yet again.The water heater and the furnace are in the furnace room. There IS NO return register in the furnace room. There are no heating or A/C registers in the furnace room. There ARE 2 combustion air registers in the furnace room that are connected to the outside air. When the furnace fan is running, for heat or A/C with or without any furnace fire, air is pulled from the water heater flue to the extent that the water heater back drafts. It can not overcome the backdraft. It has tripped the CO detector at least once that we are aware of. The 2 main return registers are in the hall outside of the furnace room. They are high and low, using the wall cavity as a plenum to connect through the wall directly to the return box on the furnace. The only work we did in this house was to swap out a 40,000 BTU water heater with a 3" flue with the same. We think that the reurn is pulling air from under the door from the furnace room. We have no intention of making any modifications to the house structure unless we are paid additional money. We are certain this is a pre-existing condition. We just want to be able to show the home owner that we are in no way at fault.