School me: Combi boiler/tankless water heater (gas)
 

Looking to learn more about combi boiler and tankless water heater (gas)

We currently have a furnance, AC coil, 2ton AC, hot water tank all with forced air. Older home.

Some have told me i can use a combi boiler to heat the home (no furnance?) and heat the water (no hot water tank).

i'm pretty lost on how all of this is supposed to play together.

Take me to school.

Is your forced air furnace direct gas fired, or is does it have a hydronic (hot water) coil?

If it's the former (direct gas fired) then there's no reason you would need a combi boiler.

I have a navien combo unit in my garage for in floor radiant heat and hot water for the shower. It’s about 2000sq ft and maybe $200 to keep it very toasty in the winter in the okanagan. One thing I didn’t realize is it only works one at a time so if you have a shower then it shuts off the in floor heat which isn’t a big deal because the slab stays warm for a long time.

I loved my tankless unit but the only thing i wasn't happy about is if there was a power outage it wouldn't work. I know its rare but where I lived I was out of power for 3 days a number of years ago and wow it sucked having zero hot water.

You would use the combi boiler to replace the hot water tank and the furnace.

The furnace will be replaced with a hydronic air handler that will also integrate your 2 ton AC for cooling - basically it's like the car's climate, you have a hot side and a cold side, and the combi boiler will supply the hot side and the AC will supply the cold side and the air handler has a fan to blow air over the radiator coils to heat or cool them as necessary.

I have a Navien unit put in last year and it's been great - size it appropriately and it will handle both DHW (domestic hot water) and space heating demand simultaneously. If the DHW draw is more then it will prioritize that over space heating (i.e. 2 people showering at once and running dishwasher so on). For me it was great as it's a lot smaller space-wise than a tank heater and a furnace, and my mechanical room is tiny in my townhouse. Also the unit has been very efficient in terms of running costs which is nice. I am looking to integrate either an AC or a heat pump in the future for cooling, once it's been approved by our strata. I had to replace my furnace and hot water tank simultaneously so decided to do the combi/AHU combo instead of separate hot water/furnace.

However like murd0c said, if the power goes out so does your hot water/space heating. Though that's probably true of any of the modern efficient units that use eletronic ignition/computer controls etc.

To be fair, a hot water tank probably wouldn't have held its heat for 3 days

This would be a terrible solution IMO.
Best option would be to replace both furnace and 2ton AC with new Heat pump and separate Gas fired hot water tank or gas fired instant water heater. Can also take advantage of rebates this way.

Other than newer or high efficiency tanks with electronic firing controls, most standard gas fired tanks don't need external power and would continue to work in a power outage.

direct gas fired.

Well, the dude asked about the combi boilers (which are basically bigger, double-sided tankless water heaters) so that's the general route if that's how he wants to do it. If he can get a heat pump instead then that's even better but the rebate process requires you to jump a bit of hoops. If his AC system is relatively new then at least with the AHU he can still make use of that investment. The heat pump will still require an alternate source of heat anyways so this won't hurt.

My strata did not allow for heat pumps or central AC when I was doing my upgrade/renovation, we are voting to allow it this year grrr.

I'm not looking to go heatpump. Hydro is a lot expensive than gas.

How much did this setup cost? curious.

~$11k installed. That included some ducting work and a Navien NCB-240/130H/iFlow iFLH – 16000W​.

Yeah I'm in HVAC for a living, so I know what combi boilers are.
It makes no sense to install for a hydronic furnace, it'll cost more to install, there's efficiency losses and increased complexity that makes no sense when high efficiency direct fired furnaces exist.

And Cold Climate heat pumps don't need backup heat. I have one with no backup heat. The rebate process is easy, and I got 11k back. I've posted it on here in a couple other threads, but I only paid ~$4k out of pocket for my install.

Maybe do some research on that, that's old school false thinking. The energy costs a good efficiency HP would use is on par with gas these days. My hydro costs for heat so far this year has been all of $200.

And don't be surprised if taxation on gas goes up as the government tries to shift us away from carbon fuel.

With modern high efficiency combi boilers now, you can run many different configurations including, in your case, an air handler to replace you gas furnace and have the combi boiler provide hot water to heat the coil. This wouldn't make any sense as the up front cost would be more than the amount you can possibly save because it's higher efficiency (unless you're using an OLD furnace from the 1960/70's that are 45% efficient)

If you have the space at your place, keep space heating and domestic hot water production separate. If something fails on a combi boiler, chances are you will lose both hot water and space heating at the same time.

Another thing with high efficiency boiler/combi boiler - put away a couple hundred bucks every year. Even when you have your equipment serviced regularly, eventually something will fail and it will cost a bit of money to diagnose and repair.

Go buy yourself a UPS power supply. A tankless water heater has low amperage draw even during start-up/high fire.

For my case, the furnace was indeed from the 70's, and the hot water tank from the mid-90s. So they needed replacing either way. And a 8x4 mechanical space.

8'x4' mechanical room is large by today's standards.
This was in a nook/work space that was no wider than 44" and a depth of less than 15". A cover panel and desk was built in front of this at finishing.

https://i.imgur.com/0PbyeWg.jpg

Rinnai is awesome. I've used one for 6 years with no issues whatsoever. Also love burners as you can set the output temp to your liking so you can just crank the faucet to the hottest position and be perfect temp everytime.

It's the access though - it's through a doorway that is ~24 inches wide down a stairway that was a pain in the ass to get the old furnace out because it makes a 270 (who the fuck builds it that way).

Also I remeasured, it's 7x3. At any rate the AHU/combi package leaves so much more room to get in and check things/turn off valves etc as needed.