r/askHVAC

Hi everyone,

We had a carrier/bryant furnace installed in 2011. It's been good overall since then (writing in April, 2026), but we had to get some motor part repaired in October, 2025 that cost $1,500. At the time the guy said the furnace looked good otherwise.

Fast forward to last night: we got a malfunction code on the thermostat that said there was an issue with the outside temperature monitor. We do not have a humidifier, so that function only tells us outdoor temp and humidity. Furnace seemed to still be doing okay. Middle of the night last night and the furnace was not working and there was another error code. I shut off furnace and restarted it and seemed to be okay. In morning error code was on and furnace was not working.

Just had a couple service guys out. They said the code was for the 'pressure switches' and said when they restarted system and hit pressure switch lightly with a flashlight it started up again. They looked at some diagnostic stuff and said that the 'heat exchangers' in these furnaces are problematic. He showed me images that he said showed little yellow 'boogers' accumulating in a way in part of the system that indicates there is a developing issue with the heat exhanger. He said that I could pay a bit over $500 for the pressure switches to be replaced, but that it is a temporary fix and that the heat exchanger will eventually need replacement too. He said that costs $3800 but could be as low as $2500 if it's under warranty because we had the furnace installed.

We are having someone come out in a bit to give us some quotes on a new system, but I want to see what you all think about this story. I'm hesitant to put more money into an old system that has indications of an issue that will be too costly to prepare down the road, but I'm also super bummed about having to get a new furnace when we just put $1500 into it a few months ago and were told it looked great.

The company said they install Lennox furnaces and that they are good. We are in Minneapolis.

thanks for any thoughts/advice/experiences with this.

I’m by no means a professional when it comes to HVAC, but I’m going to do my best to explain the situation and keep it brief.

I’m buying a condo in a high rise that was built in the 70s. All utilities are included in the HOA including heat and AC. We purchased the property with no inspection, as our understanding was there was really nothing to inspect aside from appliances and toilets.

We discovered today tenants are actually responsible for their units HVAC, but the heat and AC getting to the units is controlled by a boiler that the building operates and maintains. Essentially the unit just has a blower that turns on/off by the thermostat based on the unit temp.

The units HVAC and thermostat are original from the 70s. Finding out I am responsible for them after reading the HOA docs gives me pause, considering I have no idea how much it’s cost to replace or the current condition.

If anyone has experience on how much this would roughly cost to replace, I’d greatly appreciate if you could share the knowledge. I have 24 more hours to back out after reviewing the HOA and am trying to decide best action.

Extra caveat, I’m buying from an estate. The owner died, and the HOA won’t share any specifi details about the repairs with me and refer me to the owner. Who’s dead.

Disclosure up front: I'm building a tool in this space (Vent-off), so I have skin in the game. But this post isn't a launch — it's an observation from the trenches that I wish someone had told me 6 months ago, and I'd love to hear if other people building or using construction tech are seeing the same thing.

Background: I'm a product development engineer at an HVAC manufacturer. I spent the last few months teaching myself React and Python and building an AI tool that extracts equipment schedules from mechanical bid sets. Going in, I assumed the hard problem was extraction — getting the model to reliably read a fan schedule on page 47 of a 200-page PDF and pull out the tag, CFM, HP, voltage, manufacturer. That's the demo everyone shows.

It turns out extraction is the easy part. Modern multimodal models (Claude, GPT-4, Gemini) handle clean schedule pages reasonably well once you give them the right context window and a structured output schema. The accuracy isn't perfect, but it's good enough that human review catches the misses.

The hard problem is scope separation. A mechanical bid set isn't just mechanical. It contains equipment that belongs to electrical (disconnects, VFDs, controls power), to plumbing (water heaters, drain pumps), to fire protection (smoke dampers, fire/smoke combo dampers), to controls/BAS (sensors, actuators, valves), and to the GC (concrete pads, structural support, roof curbs). If your tool naively extracts every "equipment-like" tag it finds, you hand the estimator a list that includes thousands of dollars of work that isn't theirs to price — and now you've made their job worse, not better, because they have to manually filter your output before they can use it.

The interesting design problem is teaching the model to understand trade scope — not just "what is this object" but "whose responsibility is it on this project, given the spec section it appears in, the drawing discipline, and the project delivery method." That's not a vision problem. That's a domain knowledge problem, and the models don't have it out of the box. You have to encode it.

A few things I've learned trying to solve it:

1. CSI division headers in the spec book are the strongest signal, stronger than visual cues on the drawings. If equipment is called out under Division 26, it doesn't matter that it appears on a mechanical sheet — it's electrical scope.
2. Plan/schedule reconciliation matters more than I expected. A unit shown on a floor plan but missing from the equipment schedule is usually a red flag (the schedule is the source of truth), and vice versa. Models don't naturally cross-check these unless you force them to.
3. The "basis of design" vs "or equal" language is its own rabbit hole. Whether a contractor can substitute affects pricing strategy, and it's almost always buried in spec language the model has to read carefully.
4. A council-of-models approach beats a single model. I'm running Claude, GPT-4, and Gemini in parallel and using a synthesis layer to reconcile disagreements. When two of three models agree on an extraction, confidence is high. When they disagree, that's usually where the interesting edge cases live, and it's where I route to human review.

Where I'm genuinely stuck and would love opinions:

• How do you all think about the trade-off between automation depth and auditability? Estimators don't trust black boxes. Every line item my tool produces has to trace back to a page reference and a snippet of source text, or it's useless. But that adds UI complexity and slows things down.
• Has anyone here built or seen a takeoff workflow that handles multi-discipline scope separation well? I haven't found one I'd consider production-grade for mechanical sub use, but I'd love to be wrong.
• What's your read on where construction AI tools actually get adopted vs. demoed? My hypothesis is that adoption follows pain that's measured in hours per week, not impressive demos. Curious if that matches what others are seeing.

If anyone wants to see the tool I'm building, it's at https://www.ventoff.ai/ — but the post isn't really about that. I'm more interested in whether the scope separation problem resonates with other people working in this space, and what you've learned about it.

— VJ
HVAC Engineer & Founder, Vent-off

Hello ST Members — I'm an MBA student researching where HVAC software falls short for technicians. I'm offering a $50 Starbucks gift card for a 30 minute phone call to hear about your experience. Must be a GM or Owner. DM me and I'll send a calendar link. Thank you!

Been doing some research on home service businesses and I keep seeing the same pattern, a lead comes in at 2pm, owner's on a job, calls back at 5pm, lead's already booked someone else.

Talked to a few contractors and they said speed-to-lead is brutal to manage without a dedicated person.

Curious what you guys actually deal with day-to-day:

— Do you have someone calling leads instantly, or is it you?

— How many leads do you estimate you lose per week just from slow follow-up?

— Have you tried any tools (CRM, auto-dialers, anything) that actually helped?

Not selling anything. Building something in this space and want to make sure it solves a real problem before going further.

Would love to hear from people actually running these businesses.