hot water heat + forced air?
davidmeiland
| Posted in Energy, Heating & Insulation on
I’m looking at a job that has a slab on grade addition to a crawl-space house. Existing heat in the house is electric baseboards. Drawings show forced air for the entire house, including duct work under the slab to outlets in the addiiton. I want to push for a heated slab instead, but would like to stick to one boiler for everything. I am vaguely aware of forced air heating that uses hot water (loops thru the air handler, I believe) to generate the heat. Can anyone shed some light?
Replies
Heating the slab (high-mass, slow response) can be nicely combined with hydronic forced air (instant response). A single hot water source can supply both, certainly if you use one or two "mixing" or "tempering" valves. Hot-water baseboard like water at 180F or so. So do hydronic water-to-air heat exchangers.
In-slab heat (RFH) is done at 80 to 110F, depending on floor coverings and outside temps. Ideally, it should be adjustable and set to 80-90F in mild conditions and 110F-ish when it get sub-zero outside.
One low-impact way to add a bit of hydronic heat is with kick-space heaters below the cabinets. Run the hottest water you have through those, control them with a t-stat, and let them add to the heating when the RFH isn't keeping up. By not using the RFH quite as much, you avoid over-shoot when a cloudy, cold day switches to a sunny day and your slab is too hot. Yet the RFH provides the base load of heat and keeps the whole house very uniformily heated because of its massive surface area. It's thermal inertai also provides a buffer in the event of a power failure. My slab-on-grade, RFH house doesn't get cold during a four-hour power failure at -10F outside temps.
I would stay away from ductwork in or below the slab. Great breeding grounds for molds and other nasties,
My question probably wasn't clear. What I want to know is this: what does the installation look like? The house has 7 rooms that now have the electric baseboard. There is a 4' crawl under the whole place that can easily accommodate ductwork. They do not want Runtal rads on the walls. I would rather have my plumber install a single boiler and the hardware to run the various water temps, and use hot water to heat the forced air as well as the slab. One appliance, one vent.
Of course I could just install a furnace next to the boiler and heat the air that way, but I know I have seen info on air handlers that have water coils thru them. I believe I've also seen coils right at the register mouth. Been a while since I looked into this, and I will certainly talk to my sub this week, but I'm thinking about it today in preparation for a meeting.
Ductwork under the slab.... what could be worse?
I'm currently doing a daily inspection at a commercial office building (83'x263') with a massive HVAV system that will use boilers to provide heat to the various sections of offices.
The tin man called the hot water exchange units "PAF"'s....maybe it means "perimeter air furnaces", I'm not sure. These PAF units are about the size of a medium sized window A/C without the outer box/shell, and are installed into the supply duct at office space drops for ceiling HVAC.
They are plumbed with copper pipe and are accessable for repairs/replacement via a removable duct section. The unit is basically like an A/C cooling coil with lots of coppper tubing with finned radiator.
I'll be back on jobsite Monday AM, If you like I will get more details from "Kevin the tin man".
........Iron Helix
I'm not an HVAC guy, but have had this done.The one boiler supplies all the heat. tempering valves adjust it down to about 111°F for the slab and under floor heat ( Some loops of PEX staple-up on some floors from under in the crawl) And each zone that gets hot air heat has an air handler inside the main trunk ducting. It is essentailly a radiator fed by the boiler supply line, with a fan on it. All triggered by the thermostat for that zone. Cold air return register can have the filter there, or a slide insert in duct immediately before the air handler unit can harbour the filter.You can also add humidifiers, dehumidifiers, and/or AC to same ducting, but it has to be designed to work as one unit can restrict flow of air for others.Dave refered to toekickspace heaters under cabs. Those and wall surface mounted convection heaters are all basicly the same idea as the in duct air handler - an automobile radiator and fan to move air through it.
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The ones I dismantled had standard heat pump ducting interrupted by a good-sized radiator, about like one in my Cat. In that case, it was for solar systems, defunct when I got there. 6 townhomes (rental).
Unfortunately, some genius had installed the air filter downstream from the radiator. They'd wondered why the filter never needed to be changed. Wasn't a whole lot of air getting past the radiator. Electric bills decreased after I removed the radiator, which made a pretty good filter. Kinda hard to clean.PAHS Designer/Builder- Bury it!
David,
Many manufacturers sell air handling/fan coil units that have a DX (i.e. A/C) and a hot water coil and the requistire controls installed from the factory. These units cost less than a similar sized furnace. Operation is practically identical to agas furnace. On a call for heat, control valve opens, fan blows warm air. Extra benefit is that in a ventilating mode (t-stat satisfied, air moving slowly) during the heating season, they can provide slighlty warmed air, where a VS, 2-stage furnace cannot.
What would the installation look like? Depends on the controls, and zoning for the radiant heat. As a designer, I like simpler systems for residential applications. 3-way mixing valve for tempered (low temp water), circulators zone control. Each hydronic AHU would be a separate zone. The air side will look like any other forced air system. The water side will look like any other hydronic system except that instead of high temp water going to a radiator of finned tube convector, in one instance, it will go to/from the AHU.
Thanks for the info.
I'm not sure what DX is.
Is the AHU a large item or small enough to be one per room (maybe installed in the ductwork)? If I understand correctly it contains is a fan and a coil. Does this system come with the type of 'brain' that is used with radiant, like a Taco?
DX is short for direct expansion and is the inside or evaporator coil portion of a split air conditioner system.
The Air Handling Unit or as some call them, blower coil or fan coil unit, is about the same size as a furnace. For instance, a Goodman AH3645-1FR, which is good for 3 tons (36,000 btu/hr) cooling and 45,000 btuh heating, is 24" wide X 21" deep X 44.25" high, and has a retail price of $738.00. It is basically a furnace without a burner, gas controls, vent motor, heat exchanger, etc. It has a control board, control transformer, multi-speed, direct drive blower, drain pan for the AC condensate, filter rack, 2 coils (one for AC and one for heating) and an insulated cabinet. It is intended to be a direct swap for a furnace and is intended to be installed with ducted supply and return.
It will not provide any output to the hot water side of the system other than a call for heat, based on the input of a room thermostat. The AHU is a a "smart" component of a system. Controls for a hybrid system can be simple or complex, depending on the designer, the installer, the budget and/or the homeowner's preferences.
This type of system: RFH and hydronic/DX air handlers, coupled with and indirect water heater and a highly modulated, high efficiency boiler, like a Weil-McLain Ultra, is very flexible and desirable system to have. Wish I had put this in my home.
Great explanation, thanks. I bet I can talk the owner into this. All he has at the moment are some ancient electric baseboards and a 30 gallon electric WH.