Need Help on the Basics HVAC Layout

Discussion in 'HVAC Heating & Cooling' started by macher, Apr 24, 2020.

  1. macher

    macher New Member

    Apr 23, 2020
    Hi have a fully gutted house row house. It’s 2 story, 2 bedroom 840 sq ft. House already has a newer furnace for heat and there’s a chimney so looks like no high efficiency.

    My contractor who isn’t that familiar with HVAC wants to try to do it because the house is straight through on 1st floor, 2nd floor has 2 bedrooms and a bathroom. Basement is just a cellar for now(down the road we are going to dig it out to make it a livable space.

    Contractor says we might be able to use flex duct since house is small. Trunk line from basement will be hard pipe and some hard pipe in between.

    Can anyone suggest a basic HVAC layout for this? Like location of registers etc? He thinks we can make soffits in the first floor for the heat and central air. That means heat and cooling will come from about 8-9 feet high on first floor.

    Why he’s trying to figure this out I figured I would do some research myself.

  2. fitter30

    fitter30 Well-Known Member

    Feb 2, 2020
    Retired service tech
    Peace valley missouri
    Since you or your contractor have very little idea how to make a system work properly call a couple of hvac contractors in. Is there a city inspection, does the installer have to licence and just because there is a existing furnace doesn't mean its the right size. There just to many varibles gas, heat pump, mini splits and there could be rebates from utility companies.
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  4. WorthFlorida

    WorthFlorida The wife is still training me.

    Oct 28, 2009
    Orlando, Florida
    The first factor is budget and what do you want to do. New furnace but what type, hydronic or forced air and type of fuel, gas, oil, or electric heat? Want AC mini split or an a coil over a forced air furnace for central air. The most important part is the proper size duct work or properly laid out radiators. It is a big factor to get the efficiency’s out of HVAC systems. There is no one size that fits all. As mentioned above, your GC probably cannot get a permit for HVAC work.
    Your GC is better off making a deal with a HVAC one man company and helping with the labor, a good way to learn from. The HVAC guy would need to do the connections, call for inspections and a source for materials.
    Last edited: Apr 24, 2020
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    From a comfort aspect, it's hard to have heating ducts in the ceiling or high up on the wall and get even heat through the room...the hot air wants to rise, so by the time any, if it gets there, gets to the floor, it's already cooled off a fair amount. High up outlets, OTOH, work much better when coming out from above if you have a/c, as that's where the maximum heat is, and the cold falls down, so your head is cooler, and your feet don't get cold. One way to do this is to have two registers on a run, one down low, and a second one up high, and close one or the other each change of seasons.

    For maximum comfort, you'd do in-floor radiant heating, but if you have a furnace you want to keep, that would mean a total replacement. In-floor is also nice in that you have a lot more flexibility of where you can put furniture otherwise, you may just block off a vent.

    You also want to deal with cold air returns, and those, too, might work best with one high and one low and opposite of the conditioned air outlets.

    While things are gutted, you want to go over things and air leak seal and insulate. That will also affect what you will need for HVAC. Older houses tend to be quite air leaky. On a major upgrade, most codes will treat things more like a new house, and the insulation, plumbing, and electrical will need to meet current codes, as it's not considered just a remodel.

    Your system MIGHT support high velocity outlets, and then, you can use small ducts. That, generally, may require changes to the blower, if it's supported. Those ducts might be able to be run through the wall without building a soffit.
  6. Dana

    Dana In the trades

    Jan 14, 2009
    The heat load of a row house is pretty small since it has much less exterior surface area to lose heat from than a standalone single family house, making all but the very smallest gas fired (and literally all oil fired) furnaces WAY oversized.

    If this is a middle unit rather than an end unit and in the full gut remodel the insulation & windows are being brought up to current code minimums you are probably looking at a design heat load of less than 10,000 BTU/hr @ +70F indoors/+17F outdoors (Philly's 99th percentile temperature bin). ASHRAE recommends a 1.4x oversize factor for the load at the 99% design temperature as the best compromise on having sufficient capacity to cover Polar Vortex event cold snap coolth, yet still have a high enough duty cycle during normal cold weather to have reasonable comfort. At 1.4x oversizing the furnace would be running (1/1.4 =) ~70% of the time when it's +17F outside, for very long "warm summer breeze" on-cycles, and with relatively short off cycles.

    When the furnace is 3x oversized for the load it only runs 33% of the time even when it's +17F outside, which is OK for efficiency, but terrible for comfort, delivering the hot-flash with a temperature overshoot, followed by the extended chill & draft. Look at the ratings on your furnace. If it's likely to be crazy oversized, it may be worth running an aggressive Manual-J load calculation using CoolCalc or Man-J-ish calculation using LoadCalc. Either of those tools will oversize from reality, especially in the hands of a newbie. To avoid crazy oversizing with those tools be very aggressive on air tightness & ventilation assumptions and duct losses (zero infiltration, zero ventilation, zero duct loss), and DON'T upsize from there.

    More on how oversizing detracts from comfort see Nate Adams' freebie book chapters & videos.

    A 1-ton or 1.5 ton hydro air handler(s) running off a condensing water heater is one way to right-size (or even zone) low load houses/apartments. Ducted modulating right-sized mini-split heat pumps is another (usually more expensive, but more comfortable too.) Yet another is to calculate the heating & cooling loads and use a reasonbly-sized Dettson Chinook, which uses tiny plastic ducts small enough to run horizontally through 2x4 studs (on non-structural studwalls.)

    Are you intending to heat and air condition with the same ducts?

    With the right types of diffusers it's possible to heat and cool just fine with soffited ducts 9 feet (or even higher) from the floor. On a project I was involved with a handful of years ago in a split level "mid-century modern" type house the diffusers were ceiling mounted more than 15' off the floor of the lower level of the house. With a right sized 2-stage gas furnace the duty cycle was high enough to keep the air mixing with relatively high-throw diffusers (even at modest cfm), with very little temperature stratification effects.

    That house was about 4000' of conditioned space (2000' of which is walk-out basement) with lots of window area, and is served with 60,000 BTU/hr 2-stage furnace and a 3 ton single-stage AC, at an outdoor design temp of +5F, and the house does just fine when it's -10F outside. It replaced 200,000 BTU/hr of gas heat and 9 tons of AC that had been making it a wretchedly miserable place to live (even though it's a very pretty house.)
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