I have a client with an open combustion gas furnace in their attic. They recently had the attic spray foamed at the roof and end walls (enclosing the attic within the thermal boundary). 

The house is an original two-story with basement (to be finished) and attic, and a crawl under a 2-story addition with a short attic. The house is located in northern VA (mixed climate). The house has two heating\cooling zones, with AHUs in the basement and attic. Both furnaces are gas-fired and open combustion. The house is cooled with two AC condenser units. 

I tested the furnace in the attic, and draft was acceptable, but CO concentrations were a bit high (32 ppm).

The HOs are considering a few energy improvements, but replacing the furnace in the attic is not one of them. They will be replacing the furnace in the basement with a high-efficiency unit. The furnace in the attic is only 5 years old and it is an 80 AFUE unit. The AC condensers (2 of them) will have to be replaced within the next 3-5 years.

Are there any big issues I am not seeing with an open combustion furnace in a tight attic (that is part of the conditioned space)? I am looking for a good justification for replacing the furnace in the attic and\or upgrading to a heat pump. 

I guess my big question is whether it makes any sense to go with two heat pumps, and say good bye to the gas furnaces and AC condensers? I keep hearing so much fanfare about heat pumps in mixed climates, but I wonder if this is driven by manufacturers and salesmen or by real-world results.

Thanks in advance!

Patrick

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Now that the attic is sealed, where is the combustion air coming from?
From the conditioned space below the attic.

With that furnace in a sealed attic, you need to run worst case CAZ and make sure there's not a problem

Heat pumps are great in a moderate heating climate, assuming the owner can adjust to the lower supply air temps after previously living with gas heat. Of  course you need to do the math and figure cost per BTU delivered from gas vs. electric. If the AC condensers are going to need replacing anyway, I would seriously consider making the switch when they go.

I ran the combustion safety tests and there was sufficient draft, but CO levels were a bit high. 

Thanks for the comment on the mechanicals.

I would explain to the customer that you have an ambient CO of 32, and the red line for a hazardous situation is 35. Safety should always be a top priority, and ask them how comfortable they are with being so close to the red line. This for me would be a no-brainer. I would recommend replacing the unit or at least having it looked at to see if there is a repair that will bring the ambient CO down, and then test again. 

I completely agree.

I highlighted the CO problem first and foremost.

The issue is that their HVAC guy just said it needed "adjustments." They understand it's important to rectify the issue, but I can only hope they will act ASAP so I can re-test and give them the thumbs up or down. Would you bring it to the point of "scaring" them with some embellished info?

I guess I just need some feedback on how much pressure I should put on the HOs with a CO level of 32. Thanks in advance.

I would never try to scare a customer into or out of anything. I think our purpose as (strictly) auditors is to provide the building science perspective and your best advice based on the current standards in our industry. We are the ones who should have the level heads to counter the more robust claims of contractors with agendas.

I would simply tell them that "the industry has settled upon an ambient CO level of 35 ppm as the level where danger is present. The CO level in your house is 32 ppm. My highest priority recommendation is to reduce the CO level substantially and swiftly."

That indicates a level of concern very close to alarm-which is where the CO level stands. 

Regarding the venting, the question is: how tight is the house? There are several issues with this 80% furnace being installed like this. In addition to the question of sufficient combustion air you also run into the issue of proper venting. When the furnace fires it removes air from the house through the normal venting process. It cannot pull the house into a vacuum, so it will not vent properly and they will likely have furnace safeties tripping. This problem will be exacerbated with long run times, additional combustion appliances and any exhaust fans (kitchen or bath). Furthermore it's unlikely that sufficient combustion air is available. You can refer to the manufacturer's installation instructions or the National Fuel Gas Code - it's very likely that this installation is in violation of codes. This is an excerpt from a manufacturer's instructions:

"FURNACE LOCATED IN AN UNCONFINED SPACE

Using indoor air for combustion. An unconfined space must have at least 50 cubic feet for each 1,000 BTUH of the total input for all appliances in the space.

If the open space containing the furnace is in a building with tight construction (contemporary construction), outside air may still be required for the furnace to operate and vent properly."

 

Another question is: where is the CO coming from that you are reading? Was this in the flue, the supply air, or just ambient in the space? The location of the reading would likely strengthen your case. CO readings in the ambient air in the space around the furnace would suggest flue "spillage". (Assuming there are no other possible sources.)

Are there any water heaters or other natural draft appliances in the space?

There are additional issues, but to me these are the major ones.

 

Regarding heat pumps: Heat pumps are a great option, and they have been proven for years. However, if the HO has never lived with a heat pump they should have the differences explained to them (especially the difference in discharge air temps).

I have heard of CO and combustion air issues when going to a "hot roof".  I think if your CO readings are in ambient, you may want to start looking at the duct leakage as the "problem".  Let's say the draft of the furnace is -.03"w.c. or so, if your return ducts are pulling a negative in the now tight space, and the return duct operates at about -.10" to -.30", you can see how combustion products will make a u-turn.

All too often I see HVAC contractors seal the supply ducts to no end, but miss the returns, causing negative pressures in Combustion Appliance Zones that exceed the natural draft appliances!  This is because they don't test duct leakage - even though now a code in MA and RI!

Thanks for the insights. I definitely have to do some more analysis on this one. 

What is your suggestion for a heat pump to replace the furnace, and how to address the existing AHU and ducts?

I don't know about Virginia, but here in PA-the latest NG capitol of the world, and with electricity being a potentially less stable commodity over time-some love, some hate Coal-Nuclear-Wind-Solar, etc and therefore future policies may favor or punish, that is subsidize or tax & regulate; one over the other- I would lean toward recommending a new gas furnace. NG isn't immune to this, but here in PA, it's going to be abundant for a long time, and that means lower prices. Besides, I am not a fan of the lower temp air that you feel with heat pumps. A furnace is just more comfortable than a heat pump. 

Upstairs furnaces don't typically run much around here. If downstairs is warm, just the heat rising will keep upstairs warm for most of the year.

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