Combi + Hydronic Forced Air, vs. Furnace + Tankless?

ThomasZ

We recently moved into a 3 BR, 2 bath split-level house with no basement. 1900 sq. ft. We currently have a 30+ forced-air furnace, and a ~20 year old tank water heater (yikes!). Since they are both well beyond their typical lifetime, we were getting estimates for replacements.

The furnace and tank water heater are currently in our laundry room, taking up ~1/3 of the space. We're interested in capturing more space in there to potentially add a half-bath one day. So that's what started our journey into tankless water heaters.

One HVAC technician provided two options, with the latter saving more space:

1. High Efficiency Furnace + Tankless Water Heater
   
2. Combi Boiler + Hydronic Air Handler ($$$$ more to install than #1)
   

I understand #1, but #2 was brand new to me. I had a few questions?

1. Is either more energy-efficient?
   
2. I read in another thread the combi can be loud. Louder than a furnace?
   
3. The technician mentioned that I may not be able to use many water sources + heat at once. Is that typical? -- This person claimed they didn't have issues during a cold Chicago winter
   
4. Living in northeast US, would most technicians be able to service this type of solution?
   
5. Any other pros/cons of the combi/hydronic approach?
   

(Note: I did find this thread on a related topic, but that was for a boiler instead of forced air)

Thank you!

ThomasZ

One addition:
This is a home built in 1960s. It tends to be a bit drafty (i.e. not a great overall seal).

Jamie Hall

Just my own opinion -- and others will, no doubt, differ, but -- since you already have ductwork, however good or bad it may be, and since both of your options appear to use the ductwork, my inclination would be to the high efficiency furnace and the tankless water heater.

Not because it's less expensive, but because you will be able to size both units correctly for the load. This will give you higher overall efficiency.

On the amount of domestic hot water... that's very much a function of how big a tankless unit you decide to put in. It's quite true that with a combi you are using either hot water or heating the house, but the loss of the space heating while you are taking a shower or whatever is minor. Either one can produce the same amount of hot water -- but they must be sized for the anticipated load.

TAG

A nice modulating high efficiency furnace is the ticket --- going low and slow with constant flow of heat. Much more comfortable especially in an older house. For average hot water use (are you running out of hot water?) --- tanks are hard to beat with Natural Gas available.

What are the rebates in the area .. That often drives decisions

ThomasZ

Jamie Hall said:

Not because it's less expensive, but because you will be able to size both units correctly for the load. This will give you higher overall efficiency.

Thank you for the perspective! This is very helpful. I somewhat like the idea of problem-isolation too. That is, if the water heater goes out, air heat does not. And vice versa.

ThomasZ

TAG said:

A nice modulating high efficiency furnace is the ticket --- going low and slow with constant flow of heat. Much more comfortable especially in an older house.

Good point. We also have pretty terrible upper level / lower level balance/circulation as it currently stands. I wonder if what you mentioned makes the furnace more suitable for balancing.

TAG said:

For average hot water use (are you running out of hot water?) --- tanks are hard to beat with Natural Gas available.

Only two of us and a toddler currently. We've never run out of water yet. So capacity hasn't been a problem yet.

It would be nice to reclaim some of that tank space. We need to fit a water softener in the laundry room too, so ideally that softener will take the place of where the water heater tank currently is. I suspect that is why the technician proposed both options from a space standpoint.

TAG said:

What are the rebates in the area .. That often drives decisions

Rebates will be between $700-$1000 I believe.

Jon_blaney

The air temperature coming out of a hydronic air handler will be cooler than what comes out of a furnace. Your duct work may be inadequate for this application.

Your furnace also may be rebuildable. The heat exchangers can sometimes be replace. Are the savings from a new unit worth the cost of new? Or maybe it is just a comfort thing.

GroundUp

Jon_blaney said:

The air temperature coming out of a hydronic air handler will be cooler than what comes out of a furnace.

What gives you that idea? Do you know what water temp the technician plans on using to supply said air handler or what the CFM may be?

HVACNUT

GroundUp said:

The air temperature coming out of a hydronic air handler will be cooler than what comes out of a furnace.

What gives you that idea? Do you know what water temp the technician plans on using to supply said air handler or what the CFM may be?

Can anyone answer?

GroundUp

HVACNUT said:

GroundUp said:

The air temperature coming out of a hydronic air handler will be cooler than what comes out of a furnace.

What gives you that idea? Do you know what water temp the technician plans on using to supply said air handler or what the CFM may be?

Can anyone answer?



Exactly. There is no "right answer" without more information

TAG

Guess there are situations where the hydronic air handler works/ makes sense? It seems to be more popular in New England. I guess some of that has to do with lots of oil boilers and no NG gas in many places -- both my brother and sister went this way when they added on to houses outside Boston. No AC in original -- forced air with AC in the new addition. Since the oversized boiler was there making very hot water anyway -- way not pump it to the addition.

It's been proposed to me a couple of times. My new build has a boiler for radiant and DHW and most of it is covered with a ducted AC (HP). When I told the HVAC guys I wanted a furnace as well -- they first said why not just do a coil on the air handler. Well -- when they priced it out the cost for the furnace was not all that much more ..... and the controls you get with a modern forced air system are far superior. Controlling the coil off the boiler is rather crude.

I know some of the wall boiler makers in the USA make combi units showing matching air handlers -- they must have controls, maybe they can modulate the boiler and control fan speed.

hot_rod

TAG said:

Guess there are situations where the hydronic air handler works/ makes sense? It seems to be more popular in New England. I guess some of that has to do with lots of oil boilers and no NG gas in many places -- both my brother and sister went this way when they added on to houses outside Boston. No AC in original -- forced air with AC in the new addition. Since the oversized boiler was there making very hot water anyway -- way not pump it to the addition.

It's been proposed to me a couple of times. My new build has a boiler for radiant and DHW and most of it is covered with a ducted AC (HP). When I told the HVAC guys I wanted a furnace as well -- they first said why not just do a coil on the air handler. Well -- when they priced it out the cost for the furnace was not all that much more ..... and the controls you get with a modern forced air system are far superior. Controlling the coil off the boiler is rather crude.

I know some of the wall boiler makers in the USA make combi units showing matching air handlers -- they must have controls, maybe they can modulate the boiler and control fan speed.

But another vent, maybe combustion air to add a furnace.