I am considering designing and installing a fully redundant HVAC system for my residence. I cannot find anything on this subject, because all searches lead me to zoning (please understand: this has nothing to do with zones, I am talking about full redundancy).
I currently have a 60k BTU furnace and a 2-ton AC conditioning the two finished floors of my 1500 sq. ft. brick house (c. 1900). The furnace was installed originally just heating the lower floor, but I figured out a great way to run returns and the other registers, then added AC per my own Manual J calculations. This single zone solution actually works out very well in my house with no real trouble spots. Blower/furnace is in unfinished basement, as is the duct trunk. Registers going to upstairs are for individual rooms and are hidden in walls and built-ins. Return is in first floor and panned upstairs through a false wall/part of a closet.
I like the idea of placing a similar furnace/coil beside the other, “T-ing” the return ductwork into this one (i.e., the return would come down between the two units), and feeding the outflow ducting just as the existing unit does. I’d probably like to do a heat pump this time for the outdoor unit. I could run a separate thermostat. Conceivably, I’d need to use dampers to seal off the unit I’m not using (both return and register) each time I switch to a different unit.
I know this is overkill, and I don’t intend to debate whether or not I should do it or why I want to do it, but I’ve always wanted to do something like this just so if something happens to the heat or AC when I’m gone, the problem can be deferred until I get back (thus, not being a burden to my family). And I’m recently interested in this again when I ran into the old, “your Trane furnace is having issues due to a manufacturer defect and it’s under warranty, but no HVAC contractor would want to do anything less than make you buy a whole new system” trick. So the idea is if I put in a brand that I can buy myself for the new one (e.g., Goodman or similar), I can run the old one until it’s dead and then change it out at my leisure. This is all about being in control for me, and I don’t mind spending money to be in control, especially because I can buy two new Goodman units for $2k, but going Contractor will cost me no less than $5k for one unit…and then I have to deal with their BS. Also, I’m just a nerd who likes doing stuff like this.
Anyway, has anyone seen or done anything like this? I can’t imagine it wouldn’t work fine, but there’s always stuff I don’t know until someone tells me. What is the best way to do the dampers and that kind of stuff? Any speculation or discussion outside of just telling me my requirement is stupid is highly appreciated.
Replies
You can install a second unit in parallel.
If you hook to the existing duct work, it should work, as long as the ducting can handle the combined fan output from both systems. if only one is running, it also would be fine, but the system not in use would be fed backwards, and heated up in the process.
You can install a backdraft damper to prevent this backwards flow of air. You could do so only on the second system, or in both systems. There will be some minimum flow pressure to operate the dampers, and some pressure loss.
power activate dampers can also be installed (dampers made by EWC or others), but the control could be a little tricky. you want to integrate damper operation with the call for heat and cooling, and have the configuration set before the furnace starts pumping.
EWC makes controller boards, but they are focused on zoned systems. You could probably apply one of them designed for a two stage furnace, and control the dampers with two thermostats. The duct sizing would still need to be enough to handle both systems operating. operation where the primary is not working would be delayed since the second stage usually is controlled by a time delay or temperature differential.
You also could use manual dampers to enable a manual change-over.
(your family or a contractor tech visit could follow a checklist to change the system over) (operate damper a to position 1 operate damper b to position 2, operate switch A to off, switch B to on.)
Thanks, UncleMike42. (We always say everyone has an Uncle Mike).
I was just thinking about doing a manual damper system for now, with a laminated "how-to" checklist and color-coded labels. I was thinking of just sliding in a thicker piece of sheet metal - perhaps with rubber edging - into a slot in the ductwork meant to be the damper slot. I would need to gain some sheet metal skills or figure out where to buy specialty duct sections.
However, it would be much more impressive to do as you said, and connect it to the call for heat and low voltage wiring. I could see that being the second stage of the project, what I would consider the more fun stage, since the system would already be working and this would be adding functionality.
I also envisioned just having one system on at a time since my current system is properly sized, but I love the idea of a lead-lag system (like they have in Communications shelters and Telecommunications Rooms) where both units would be multi-stage and either could assume the entire load if necessary. I might even work toward this, because I will be considering higher functionality units for the future (everything works great now, but I'd like to have a heat pump, and the new infinitely adjustable models I've heard about from Mitsubishi could really be a beautiful thing...)
I did not know about EWC, so I will look into that; thank you.
Also, I'm concerned about my headroom for the second unit if I want to keep it as an upflow model and still get a coil on there. Know of any manufacturers that make a squat unit?
https://www.trane.com/residential/en/products/gas-furnaces/xc95m/
furnace is 40 inches high,
https://www.trane.com/residential/en/products/comfort-coil/
you can put a 17.5 inch high coil on top.
combination would be 57.5 inches high.
https://www.goodmanmfg.com/products/gas-furnaces/90-afue-gas-furnaces/96-afue-gmvc96#technical-specifications
34.5 inches high
https://www.goodmanmfg.com/products/air-handlers-and-coils/coils/capf-series
18 inches high,
so total would be 52.5 inches
You can get a gas furnace with coil on top under 5 ft tall, how short are you needing?\
a couple minutes with google will get you some info.
I looked at goodman and Trane.
My current system with coil stands at 57", which I think is the normal size. My duct work is at 65", but the new one would need a duct transition piece that wouldn't quite fit, at least without bottlenecking it. Certainly something can be done, but I'll have to ponder that solution for a while. I mean, I could move the new system up further than is ideal to make this work, but then it causes some other ductwork transition problems. Yeah, I have some real research time I need to put into this (and have you noticed the Internet is not what it once was?...everything is an ad now...Man took a world-changing invention and ruined it - along with our chance of really improving - with greed).
Your idea of a fully redundant HVAC system sounds pretty unique, and it's great that you're enthusiastic about taking control of your heating and cooling solutions.
To address the dampers and other technical aspects, you might want to consult with an HVAC professional or an engineer experienced in residential systems. They could provide valuable insights into the best way to implement your redundancy plan and ensure it works seamlessly. Sometimes, practical advice from experts can save you time and headaches.
Your dedication to self-sufficiency and your willingness to explore unconventional solutions are commendable. And on my end, as I'm renovating my house, I'm keeping an eye out for those Cheap Radiators to keep things cozy.