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Forced Air: Nat Gas Furnace or Hydronic Air Handler

17903 Views 136 Replies 6 Participants Last post by  supers05
I've got a good mechanical designer doing load calc's right now and we're discussing what sort of heat to use for the Forced Air - Natural Gas or take advantage of the boiler being used for the radiant floors.

Radiant is only in basement slab, main floor mudroom, and 2 upstairs bathrooms.

All mechanical will be completely including duct work. Home will be spray foamed and new doors/windows.

Home is in Toronto and is 1100 sqft/floor (3300 total incl basement).

I assume about $2k savings of hydronic air handler over HFE Multi Stage.

Is either the better solution?
Service Life

Having this conversation with the designer but wanted to draw from other sources as well. Thanks in advance.
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go with the hydronic coil in the air option all way around..
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Just completing the process of selecting a boiler for an existing home in Maine that is a 2200 ft² ranch with a 1250 ft² basement. Part of the ranch has 400 ft² of slab with radiant heat. Well built home, but not as well insulated as spray foam, and we struggled to get a boiler small enough to not be an elephant. With what you list as radiant off of the boiler, it will have a huge amount of excess capacity. If we ignore most of the basement and use 25 btu per ft² you would need 55,000 total. You are talking two heating units, way overkill.

Is your mechanical designer going to do a Manual J calculation and is he the one selling the two systems?? If you note a hint of caution there it was intentional.

Is this new construction or remodel of an existing building, what stype and age?

So far we have just been talking about options. He is doing full set of 'computerized' calcs. I assume that will be a good analysis.

He's taken in a lot of information so I'll know shortly.

I've seen a few people with gas forced and combi boiler for radiant and DHW. Some have argued that gas and boiler give you heat should one go.

Van G
Backup is always nice, but I'd go with the hydronic coil. You already have the boiler. (I assume that your engineer has already calculated that you have the excess capacity) The more parts you have, the more that can break and cost in repairs.

PS. For your comparisons: I'm also near Toronto, with a 900ish sq. Ft detached home, at least 60 years old. Original insulation/plaster/brick, decade old windows. Almost never use the second stage of the 40kbtu 2 stage furnace, recently installed. (even at the recent -26*C and strong winds)

Interesting. We are adding onto a 1950's semi which is double double brick wall with cavity. Addition will be about 60% of the total space.

Van G
If you have a gas boiler now, what size is it?
I audited one well insulated home that had radiant heat in the basement and the owner was one who liked to track his heat use. He said his radiant floor in the basement almost never came on. His is an open unfinished basement (insulation on outside of foundation) so the boiler gives off more than enough heat for down there.

No system in place right now. It will be all new system and well insulated including under the slab before we pour.

Van G
Air handler is the best choice if you're getting it anyhow.

You can also get an indirect hot water tank and skip the water heater. Pro is that it's more efficient most of the time, especially with a condensing boiler. Con is that it may not be very efficient to have a 80 to 100 000 btu boiler come on in short cycling during the summer to keep water hot. Increases wear too, having to fire the boiler year round.

Do not let your designer treat the tight, high r-value spray foamed house as a regular house. The heat loss will be very low for the square footage, could even be under 40 000 btu/hr. A/c will be less than 1 ton per 1000 sq ft. This is under the assumption that the original double brick structure will be insulated as well. You're splitting hairs, spending extra money on spray foam for the addition if the original house is to remain leaky and uninsulated.

Look into a HRV.
Spray foam on all exterior walls and flat roof, blow in loose in the existing attic, and insulate/vapour before new slabs poured.

Will discuss this with him tomorrow and post the calc when I have them.

Going with LifeBreath HRV, likely a simplified setup as existing framing is 2x8 timber. Would be bulk heads everywhere.

Van G
Whether to use the boiler or a separate gas furnace, depends on what type of boiler you are looking at. A high efficiency condensing boiler use on a hydronic coil will need to go above 140°F when the hydronic coil is heating. Unless you really over size the coil.

The efficiency of the boiler drops as you increase water temp. So you need to decide what you want to trade off on. High efficiency. Or convenience(1 zone will always work as long as you have gas and electric) and cost of 2 systems.

Next, on a spray foam house. You will need mechanical ventilation. So how are you addressing that. Most add it into the duct work. With a hydrocoil, you would also want an antifreeze stat in case the boiler failed, so you don't freeze the hydrocoil and burst it.

PS: If I wnet with a hydrocoil, I would want a heat pump instead of straight A/C in case the boiler failed.
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Setting up call with mech designer but his initial reply:

The gas furnace will have better features and will give you a second heat source in case the boiler goes down. The hydronic air handler will be cheaper to buy and will run off the boiler's hot water so we would need to upsize the boiler to be able to handle the extra load. The airhandler will not have the extra features of multi stage or modulating fan.

Van G
Hi Van
I'm beginning to question the capability of your mech guy.
"Radiant is only in basement slab, main floor mudroom, and 2 upstairs bathrooms." That is a very small load unless you are heating other areas and even if that, he says up size the boiler to handle the hydronic air handler.

Not making sense here. Installing a second heating system just to have back-up would be an expensive approach.

I'll wait to hear what his final heat loss numbers and what he is recommending for equipment.


Seems like a straight shooting guy. He was more money than the other 2 designers.

Basement is 1123 sqft inside dimensions less areas that won't use it like mech room, stair foot print.

Add another 180 sqft for the other rooms.

Van G
You can get an air handler with a VS blower. Wired right, you can have it move a lower CFM when its mild outside. And a higher CFM when its real cold outside.

What size boiler is he intending to use if you get a furnace. What will be the total sq foot of hated space?
Will the boiler be heating the dhw?

You can get a water heater designed for heating and kill two birds with one stone. Would be cheaper than getting a boiler and water heater, or boiler with direct tank.

Truth be told the hydro-air won't be as energy efficient as a gas furnace.

Even with a condensing boiler, it needs a low water temperature to actually run in the 90%+ range. The warmer the return water the less efficient the boiler becomes.

With furnaces the return air temp is room temperature, so the secondary heat exchanger's heat transfer to better on a furnace vs boiler. Exhaust from a high efficiency furnace can be cooler than the returning water to a boiler, especially with the air handler running at a low fan speed.
Truth be told the hydro-air won't be as energy efficient as a gas furnace.

Even with a condensing boiler, it needs a low water temperature to actually run in the 90%+ range. The warmer the return water the less efficient the boiler becomes.
Heat pump with hydrocoil is more efficient then a gas furnace.
Run a heatpump in Ontario most of the winter and it'll burn a hole in your wallet.

Efficient? Maybe. Cost effective? No.
...also central air style heatpumps are rare in Toronto. Which means finding a tech to work on one could be a challenge. The lack of capacity below freezing, high electricity costs make them unattractive.

In areas with no natural gas or cheap electricity, the heatpump is worth considering.
Run a heatpump in Ontario most of the winter and it'll burn a hole in your wallet.

Efficient? Maybe. Cost effective? No.
I thought Ontario had TOU billing. If so. Then a heat pump used off peak would be very cost effective.
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