Advice on backup system needed

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Jon_E

Burning Hunk
Feb 24, 2014
135
SW VT
I am looking for advice from similar-minded people, you guys on this forum, on an effective and appropriate backup system for domestic hot water production.

Some background information first. I built a timber-framed home, completed and moved in in December 2007. It has around 4300 square feet of finished space, including a basement with radiant in-slab tubing, and two floors above with staple-up tubing. I have three bathrooms with showers, plus two half baths and the usual complement of appliances that use hot water. Currently I use a Central Boiler CL5648 classic outdoor wood furnace that was bought new when I started the house. It is a Dual-Fuel model with a Wayne gas burner installed. This is the only heating system I have. There are no other wood stoves, fireplaces, backup systems, anything at all that would allow me to produce sufficient heat or hot water if the OWB failed for any reason. For that reason, the CB unit has been running continuously since approximately October of 2006. I've had it offline for short periods (4-12 hours) for maintenance or repairs occasionally.

I am now in the process of building a detached 900 sf garage, which will also have radiant in-slab tubing heated by the same OWB. The garage only needs to remain above 62 degrees, and will have no separate hot water demands. I am planning (but not 100% definite) to put a large solar array on the roof of the garage which will be a grid-tied PV system. I also have an above-ground pool that I am working on a heat exchanger system for boosting the pool water temperature just a little bit with the OWB.

What I am looking for, is advice on some kind of backup or secondary heating system that will cover domestic hot water in July and August, and any other time a backup is needed (i.e. winter vacations) Not more than three months out of the year. I want to be able to take my wood boiler offline for greater periods of time than just one day, for extended maintenance, cleaning, or simply to take a break. It is somewhat annoying to feed a fire when it's 90 degrees outside.

I am looking for something very small, that can be installed on a wall in the basement, and can tie into my existing system. I'd like to be able to flip a switch and have the unit start producing hot water that would then supply my existing 40-gallon passive hot water tank. Either gas or electric would work, and I am not leaning specifically one way or the other. Electric I think would be "easier" but I worry a bit about the cost - although a solar array would certainly offset the electrical costs.

What success have you guys had with the available technology? I will answer any questions about the sizing or layout - I'm not really looking for professional advice but more of an educated opinion on what works for you and why. Looking for pros and cons, what you're happy with, brands and models if you want, etc. Thanks.

EDIT: I probably should add that I am really unhappy with the efficiency of the gas portion of my OWB, and it doesn't really allow me to totally shut the system down. If I develop a leak, for example, and have to re-weld my firebox - I have no heat or hot water at all.
 
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I put in a cheap 40 gal electric hot water heater from Home Depot instead of running an oil boiler to an indirect tank in the summer because it was cheaper to run. I piped it in parallel to the indirect so I could just flip a switch and align some valves. (Note: there are legionella concerns with standing water, so....). It doesn't hang on the wall but it's cheap and effective. I also bought a heat pump water heater add-on for it, but it is a little noisy compared with the TOTALLY silent electric water heater. A friend of mine has a pool and and OWB but he also has a solar pool heater that he built on a low rack-I don't think it cost that much. Not sure how much he uses the OWB for the pool.
 
+1 on an ordinary electric water heater.

I've got an 80 gallon one that I heat with wood via flat plate hx. I heated it all last year doing that, with one burn a week. But didn't the year before, and might not this summer, depending what wood I have laying around. It only runs us around $25/mo @ $0.18/kwh to just use electricity, family of 5. No worries about switching stuff off & on or opening or closing valves - if the tank gets down to 115 (or whatever), the elements come on. Cheap & easy. Burning wood in the summer for household dhw is a bit overrated - as is doing it with solar. I'd also seriously consider a heat pump water heater - some states seem to have very good incentives.
 
I Love the Nyle Geyser heat pump, flip a switch, heats up my 56 gallons no problem, and dehumidifies my basement at the same time. $35 per month to run, does my domestic 12 months a year....
 
I've been considering something similar, and I wonder why I can't just install an electric or propane hot water heater/tank downstream of my storage. When the storage is hot, the downstream tank will essentially just be additional storage, but if storage cools, then the tank will heat the water before it gets to the tap.
 
Thanks for the replies. Been doing some research in the meantime - there sure are a lot of options. Right now I think the solar array that I'm contemplating is irrelevant. The local plumbing supply store has a deal on GE hybrid heat pump tanks - 80 gallon tank is $700-ish out the door. However, I don't really have room for much more than replacing my existing 40-gallon tank with something taller and a little larger in diameter. Ideally the units I've seen with a single coil and backup electric element (designed for solar or geothermal systems) seem to be my best direction. Lochinvar makes them, as do several others. The reality is that I would expect to pay $50-$80 during the course of the year to run an electric element for a couple of months in the summer. I also need to be able to use the system as backup in case of a winter vacation or even a system failure. Close a couple valves and flip a switch and the OWB is offline.

The electric system also means that I can get rid of the gas burner in my OWB, although I will probably just leave it there.
 
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