Do you have a carbon monoxide and natural gas or propane detector in the house?

Do you have a high water table with an adequite vapour barrier between the foundation slab and the ground? Are your bathroom ventilation stacks functioning properly? How's the ventilation in the attic? Are you storing paint or any VOC chemicals in the house? How about pool or hot tub chemicals? Is your house completely detached or semi-detached?

IMPO

SR

@faq4cw House built to code in 2001. Yes to the detectors. Yes to high water table but we raised our house 4’ above grade. Stacks functioning. Attic ventilation okay but could be better. No to the chemicals, pool, etc. Attached garage.

This is not a lingering smell. It comes on fast and strong. Then lingers. Not constant same air quality.

" the air doesn't seem as hot as the 21 y.o. natural gas furnace we had. "

fsq, isn't this to be expected?

Hi, ykindschi,

Your geothermal heat pump shouldn't contribute to an indoor air quality problem any more than an air source heat pump or even a gas furnace would.

Most homes operate at a slight negative pressure due to a combination of "stack effect" and a HVAC system that is starved for air flow on the return side. Stack effect occurs when warm indoor air rises to the upper levels of a home, pulling in outside air on the lower levels, like a chimney, of sorts. The driving force for stack effect is greatest in winter when the temperature differential is greatest. You've experienced Wisconsin weather for the past 22 years, so stack effect by itself is not likely the cause of your problem.

Given the change to your HVAC system, I think there's a good chance that the blower on your new air handler is a higher capacity than the old one. If no modifications were made to the duct work system, a higher capacity blower would result in a system that's even more starved for air than the old one. That increases the potential for back drafting--a condition where outside air is drawn into the home by backward flow through fireplace chimneys, hot water heater exhausts, range vent hoods, etc. bringing soot and contaminants into the home. Lastly, let's not forget the addition of the RenewAire heat recovery ventilator. While these are typically set up to exhaust air at the same rate as they introduce fresh air, the intake could be blocked by snow on the roof or a ductwork issue or a blower problem resulting in more exhaust air flow than make up flow and contributing to back drafting.

@ Charles Ross Homes - you have responded to my question posted long ago about thermal bridge marks on the ceiling (https://www.houzz.com/discussions/6299996/ceiling-drywall-ghosting-dark-markings). I have to say everything in your answer to the Geo question doesn't fit. I don't have chimney, range vent hood, etc..., only option would be hot water heater and that is a very long shot. The exhaust and intake on the RenewAire is at ground level. This smell is not all the time. It might be every 3-5 days and it is a chemical smell out of nowhere.

@ Elmer J Fudd, it is an observation. My first go at Geo, now I know the difference.

A few observations and perhaps some further questions.

Elmer:

" the air doesn't seem as hot as the 21 y.o. natural gas furnace we had. "

fsq, isn't this to be expected?

Yes, this is to be expected. However, OP says, “House built to code in 2001”. Therefore I would have the expectation that the ductwork was adequately sized for air-conditioning and ‘should be’ OK for the lower temperatures of heat pump heating. Smaller ductwork of older houses designed in the era pre-dating air-conditioning would present more of a problem and would likely require modifications.

I also suspect that Charles is on to something regarding negative pressure and should not be lightly dismissed, particularly since the DHW tank is gas heated.

The OP also says, “It typically happens about 6:30 - 7:30 p.m.” So that’s around dinner time when the use of hot water and any gas is likely to be increased. So that’s something to ponder.

I don’t think the geothermal system has a direct effect. Although the HRV might.

Another question for the OP, have you had an exterminator visit your home, anyone spaying any chemicals or foam insulation, dead, trapped animals, strange hobbies? Might a renovator left somethings in the walls etc.?

Are you on city water or well water? Do you have a septic tank?

IMPO

SR

Hi, Ykindschi,

You noted that you have a gas fireplace--which can be another potential source of back drafting. Essentially, any connection between the interior of your home and the exterior--including bath vent fans (the backflow dampers don't seal well) and even gaps around windows and doors that allow for air exchange are potential routes for air to enter your home. You can also pull air from your crawl space into your home either via leaks in the return duct work or through penetrations in the 1st floor. Check also for critters in the crawl space. They can be a source of sporadic "odiferous events."

You noted an increase in air flow with the new system which suggests a larger capacity blower. If no modifications were made to the duct work system, my bet is that the system is more air starved than before. A higher capacity blower will increase the amount of air being pulled though any leaks in the return ductwork. Fortunately, that's something that can be easily measured. If you set up pressure monitoring, you'll be able to check how it varies over time, and hopefully correlate it with either atmospheric conditions or a set of HVAC operating conditions.

Another potential route for back drafting is the plumbing vent system. If you have a bathroom that's not used frequently, the traps may have dried out enough to allow sewer gases to be pulled into the home. I've seen this occur in homes where the guest bathrooms were not used for long periods of time. Traps will dry out faster in winter than in more humid periods.

@faq4cw

We are just 2 people who have been eating pretty small dinners and I have dish drawer style dishwasher, so not crazy hot water use at that hour. We have a 2” water line that brings water from an iron curtain and high filtration water system at our farm a quarter mile away. (No livestock so not the source). Our septic tanks are about 60’ and the mound is about 200’ away.

@Charles Ross Homes - we have a full basement. Pretty normal in WI. I’m familiar with dry traps and it’s not that.

It’s not a dead animal smell. We’ve had our fair share of dead critters. Temps outside are freezing, so nothing is rotting outside. It’s not sewer gases. It is chemical in nature and shoots our carbon dioxide and VOC levels up in a matter of minutes. Makes me light headed, sick in stomach, burning throat issues, feels like I am being asphyxiated.

Two HVAC companies said our ductwork was adequate for geothermal in our option assessments.

I should have never mentioned the temperature of the heated air. The differences of the two are just an observation, but have nothing to do with the random smell and poor air quality. It’s been explained to me and is a whole other hindsight issue.

Our house is not air tight. Lots of glass. I felt like we had plenty of air so the RenewAire seemed like a dumb idea to me, but they are now code in Minnesota for all new builds, and our temperatures and weather are similar. Thought it could help and not hurt.

Keep thinking. 🤔

@ Charles Ross Homes, I have asked countless times about the natural gas fireplace. It is a sealed firebox and I have been assured absolutely nothing from that can come in the house. Our monoxide detector is not going off during this event.

Might it be possible to call your local fire department to pay you a visit and have 'sniff' when this happens? They often have specialized equipment to detect what's in the air.

IMPO

SR

It's certainly a handicap trying to diagnose any issue without being able to visit the site-- let alone one that occurs sporadically. I'll second the suggestion to reach out to your local fire dept. for detection equipment. I think you'll also need to reach out to someone local with a building science background---perhaps a HERS rater if there's one in your area.

@ykindschi,

I'm still scratching my head on this one. It really reads like a back draft situation. A non-detectable level of CO wouldn't eliminate it as a possibility. You removed a gas-fired furnace. Did that furnace share a common vent/chimney with the water heater? If not, was the exhaust vent for the furnace completely removed or was it blanked off?

@ Charles Ross Homes - the vent of the gas furnace was not shared and is capped off. The gas water heater vent has been checked a number of times and is not leaking. It has been a week since fumes raised the levels like that and now the RenewAire runs 30 min per hour. I’m just waiting for it to happen again to try and find the source.

@ykindschi,

It may be difficult to identify the source of the pollutants entering your home, but you can do a couple of things to address the negative pressure inside your home which is drawing them in. In the end, if the problem goes away, the specific pollutant source is academic. Start by measuring the differential (indoor vs. outdoor) pressure. You can do that using a manometer and some tubing you can purchase for less than $100. It will enable you measure the contribution of various exhaust equipment (e.g., bath vent fans, clothes dryer, fireplace, hot water heater, etc.) as well as each change you make. Two changes I'd suggest are reducing the blower speed and sealing the ductwork. Both should improve the operating efficiency of the system and mitigate the potential for drawing pollutants into your home.