Lies, Damned Lies, and Manometer Readings | Hacker News Reader

Lies, Damned Lies, and Manometer Readings

asteriskmag.com

100 pointssavanaly30 comments

Few people understand HVAC. Although if you have anything to do with running a data center, you have to learn the basics.

I was once in a computer room at Stanford which had two large industrial AC/heat units. I was there to copy old backup tapes (alumni were saving some history) and had a lot of free time while the tapes spun. I noticed that one A/C unit was on heat, and the other was on cool. So they were wasting a vast amount of energy.

You're supposed to wire those units together so they cooperate. Their controller can do that. Nobody knew that.

University HVAC systems are weird. We had a steam valve stick open in our building one summer. Brought the internal temperature up to 95. Apparently the chillers bring them temp down to 55F to condense the humidity, then they reheat to 74F. I would not have thought that steam was required for proper AC

Steam is highly economical as a heat transfer medium if you already have a boiler with plenty of capacity. It's versatile and packs an incredible punch - you need very little heat exchanger area if you're condensing steam on one side.

Steam isn’t required, electric reheats can manage humidity just fine.

It was the University of Minnesota. They had a big steam plant for heat in the winter

Cool! I’ve been in the St Paul campus steam tunnels :) The U of M also has district cooling on both campuses and an east bank steam plant.

When you have district heating, it makes sense to use steam for the reheats instead of electrical resistive heaters. There are also steam powered chillers, for places that don’t have district chilled water.

You mean the licensed, bonded, 4-year apprenticed installed and twice inspected system was set up inefficiently? Mild shock!

Before the pandemic, I had to replace my outside AC unit. I got three different quotes, each of which suggesting three different required changes, from electrical to new duct work.

I asked the first quoter about all the other work #2 and #3 suggested and he scoffed at it and said he did not want to keep his guys working in my attic one second longer than necessary in the August heat and that the electrical work was completely unnecessary.

Similar to dentistry, they're doing something useful but it's hard to know how much of it is actually valuable

It took some digging, but I managed to find the specifications for the temperature limit switch in my High Efficiency heating system... for $7.62 I was able to buy a replacement from Grainger, and get it installed.

I strongly suspect if I had called for service, I'd be told I needed a new furnace, even though this one is less than a decade old.

Where was this limit switch?

If it was on the flue, then it's a symptom of a drafting problem when they fail. Basically the thing should rarely ever click, but if the flu isn't drafting well, then the snap disk will get triggered frequently. You'll never notice anything wrong because the heat still works fine.

Then the thing fails and you fix it, but you only fixed a symptom and not the problem.

Even so, I still think it's better to get in there and figure stuff like that out than to avoid it. "just let the professional do it and do whatever they suggest" only works if you have some way of finding a guy who is both knowledgeable and honest. Yeah there are obviously risks like being unaware of invisible unsuspected things like above, but life is risk regardless.

Check your filters. Some residential units have a mesh between the squirrel Cade fan and the fire box. Mine got partially block by an errant piece if insulation that was restricting air flow and causing the unit to short cycle. I too thought it was a bad switch. The HVAC dude said my condenser coil was clogged and I needed a new unit. One disassembly and cleaning later and it runs like new. When taking things apart take pictures showing where plugs wires and screws go.

If it’s the high limit temp switch, that can be triggered by extended furnace runtimes when the temperature is so cold outside that the furnace must run nonstop to keep up with thermal losses (polar vortex and whatnot). This will potentially manifest in smart thermostats as frequent reboots, as the limit switch cuts off power to the furnace (which cuts power to the thermostat) to prevent it from bursting into flames. I can’t speak to Nest thermostats, but ecobee will send you an alert.

This is close, and I'd grant that limit trips are associated with longer run times. But in principle continuous furnace run time shouldn't cause a high limit trip. The limit is mainly to protect the heat exchanger from damage. The primary cause is usually low/reduced airflow, although bad sensors are also common.

A high limit trip will generally cycle the furnace off for a time period. Somewhere between 3-5 off cycles will result in lockout and the furnace will stay off. The control for this is more likely to reside in the furnace board vs an aftermarket ui.

I took great pains to setup my boiler to run nearly continuously by using its outdoor reset controls and an outside temp sensor (a simple 10K NTC) to adjust the flow temperature to match/ever so slightly exceed what’s needed to match the building heat loss.

This results in higher efficiency and better steady state comfort as the heat is on for always 18+ hours and often 22+ hours per day. It’s hard to say with confidence on the precision, but it seems like this is saving 8-10% in gas (in therms) vs last year when I had a constant flow temp (that I’d adjust manually every so often to try to match but where I’d have to leave it “too high” or risk a cold house on the coldest days).

The exhaust temps in the stack reach steady-state far before even a 30 minute run would be finished.

The high temperature limit switch should never be tripping, even when the furnace runs continuously.

This indicates you have a problem with temperature rise across the heat exchanger, probably from inadequate airflow. Your ducts may be sized too small/furnace too large, fan speed set too low, plugged filter, etc. You may be able to mitigate the problem by increasing fan speed, limiting heat by using stage 1 only, putting in less restrictive filter, add additional cold air return, etc.

Usually a failed limit switch means cycling and a bigger problem with airflow. However I have seen a high temperature limit switch fail without cycling - fell apart with age (40 year old furnace).

It was the high temp limit switch, and it was tripping at about 110 degrees instead of 180. It kept short cycling, and the house got down to 58F. Brrrrr

It took an extended run to get the house back up to temperature, but it had no problem with a correctly functioning switch.

If you call an HVAC firm in Tucson, often they simply use the call to try to sell you a brand new unit, making disparaging comments about the condition of the current unit, and outright lying about its fixability.

I feel like everyone is guilty of something like that. Nobody wants to understand how the existing system works, they just want to rewrite it from scratch ;)

I would argue that hvac is uniquely bad

This is a weird rule of life: if something can be done by an ordinary person, but you hire someone else to do it, be prepared for a huge bill and/or chicanery.

Obviously not all HVAC is in the realm of a normal person's abilities, but much of it, and almost all problem diagnosis, only requires reading the diagram inside the furnace, using a multimeter, and a screwdriver or wrench.

It's all very trouble-shootable. If you have those basic skills you should be able to identify what exactly is causing the issue, and the tech won't be able to lead you down the garden path.

I designed and installed a heat-recovery ventilation system in an Alaska house (new construction). I was aiming for five-star or five-star-plus rating but there was enough leakage that it only got four-star-plus. Still nothing to scoff at compared to the standard contiguous U.S. houses, but the amount of sealing at joints, electrical/plumbing penetrations, HVAC, and vapor barriers was pretty intense. Even the seal under the front door can be enough to lose a rating point if it's bad.

Balancing the HRV was done by some professionals once the install was done, and only required a manometer to adjust some wall vents and a baffle in the exhaust duct, which wasn't quite the same as heating/cooling calculations would be, since heating was hydronic and AC isn't really a thing in AK.

Contractors typically/frequently install AC condensers and furnaces that are too big for the house. These cost more and the contractors don't have to worry about call-backs for insufficient cooling or heat. Unfortunately they are much less efficient because they run for such short cycles.

Would variable speed drives help with this issue at all?

VSDs, in general, are MUCH MORE efficient than typical 3-phase heat pump.

How about moving everyone to heat pumps?

A heat pump can benefit from a VFD, it has a compressor pump.

So build a walk-in freezer, and run them for that?

I’d love suggestions for measurement tools to quantitate my home AC units…

I'm the author of the article. The dual manometer suggestion below is excellent. I use external static pressure to calculate airflow using manufacturer tables quite frequently. It's fast and only slightly less accurate than using an expensive flow grid. I also use a manometer to measure pressure drops across filters and AC coils quite a bit.

I'd add a tool for measuring temperature, ideally a digital psychrometer, but even something that just measures dry bulb is useful for heating and bad AC failure.

Finally a multimeter, ideally with clamp on amp readings.

https://www.youtube.com/watch?v=eFmDmk1Siqc

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