Install gas furnace inline with existing heat pump package unit.

Discussion in 'HVAC Heating & Cooling' started by mrbeing123, Oct 25, 2017.

  1. mrbeing123

    mrbeing123 New Member

    Joined:
    Dec 5, 2012
    Location:
    Louisiana USA
    Let me try to be as clear as possible. I have a relatively new package heat pump installed on my house. At the time it was installed, a natural gas main extension would have been $30k. Now it is slowly rolling out on my street and has become cheaper, but still not free. I understand that I could simply replace the package unit with a dual fuel unit. But, beings that it is only 3 years old, could I simply add a properly sized standard gas furnace inline with my packages return air duct in the house and wire thermostat accordingly? If I would, I would have to remove one of the blowers. Which blower should I remove, probably the furnace blower? Obviously not ideal, but is this configuration done in the field? If not, other than a new PU, is there any other configuration you would recommend?
     
  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Usually heating with heat pumps in temperate southern winters is cheaper than heating with natural gas, but being a gas-producing area maybe you have ridiculously cheap gas (other than getting it to your house, that is)?

    Dual-fuel heat pump/gas systems almost never save money, and where they do it's in places where it stays well below freezing for more than a month at a time at temperatures where the heat pump operates less efficiently and at much lower capacity. That's not the Gulf coast states. A right-sized heat pump with auxilliary resistance heating strips is usually a better compromise in your area.

    What's the attraction of gas? Does the heat pump fail to keep up during colder weather?

    The design heating loads of most houses in LA are low enough that there's almost no such thing as a "...properly sized standard gas furnace ..." in the first place. In fact, most homes in LA could be heated with a gas water heater and hydro-air coil, with plenty of margin on burner capacity for heating the hot water too.

    What is the manufacturers' stated HSPF of the heat pump?

    How many tons?

    How big is the biggest winter month's power use (in kwh, not dollars) compared to the lowest power bill of the year?

    Many AC installers use a crummy "ton per 500 feet of floor area" rule of thumb for sizing the AC. Most houses have a real load/area ratio of 1000 feet or more, if it has at least some insulation. In LA the design heat load is usually less than the design AC load, and a typical heat pump will deliver the same or higher BTU/hr at an outdoor temperature of +17F (one of the HSPF testing tempertures) than the rating cooling output. Bottom line, if it keeps up with your cooling load, it's probably oversized for the heating load, but at that oversizing level should be able to heat the place even when it's 0F outside (which almost never happens near you.)

    That's a lot of assumptions on my part, but some real information about the house, location, insulation levels, size, model number of the heat pump and the power use could put it in much sharper focus.
     
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  4. mrbeing123

    mrbeing123 New Member

    Joined:
    Dec 5, 2012
    Location:
    Louisiana USA
    I am in the heart of an area where natural gas is really cheap. We have countless natural gas fields in the area. Residential gas rate here right now is not much more than a $1/CCF! I think it is less than that actually.

    I'm not so interested in a dual-fuel system. But I am interested in having natural gas heat. I am the type that always like to have a backup source though when SHTF.

    My house is very old, drafty, and short of me foaming the attic, mostly uninsulated. We plan to insulate the walls when we re-side in hopefully 3 years. The heat pump was sized for cooling here. It doesn't keep up in really cold spells though. I understand that when we insulate/air seal, the unit will operate great. But NG is so cheap here.

    When I said properly sized, I meant the cabinet properly sized for the existing ducting so as to not cause too much extra resistance in the ductwork.

    3.5

    4,500 During the summer, we use less than 2000. When the temp(which rarely does) drops below 20, the heat pump is basically useless and when I switch to em. heat, we are using at least double the kwh.

    There was a manual J done, but the comfort level during the humid summer months was more important than heat at the time. Any larger of a unit wouldn't get rid of the humidity without a dehumidifier running constantly.

    I really appreciate your response. I really wanted to know whether the configuration of a furnace and heat pump in separate cabinets is "normalish".
     
  5. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    If the place is air-leaky and basically uninsulated, doing the air sealing and insulation would provide more comfort than any heating system (short of radiant floors or something. You don't have to wait to re-side to fill the wall cavites with blown fiberglass or cellulose. Wall insulation doesn't have a big effect on the sensible cooling load, but the sometimes reduction in air leakage that comes with blowing insulation in walls will reduce the latent load, usually by quite a bit. Wall insulation makes a much bigger difference in the heat load than it does the cooling load.

    Talk to some contractors, get some quotes. It may be less than hacking in a gas furnace, and it won't tear your house up. You can fix any window flashing details that need attending to when you re-side.

    BTW: What did the Manual-J come up with for a heat load? Did they spell it out room-by-room?

    So your background power use is on the order of 2000 kwh (?), or is that during the heavy cooling season (seems more likely). At some point in the spring or fall you get a bill with very little heating or cooling use on it, and that's what we need to compare the 4500 kwh to. Unless you have a pool pump going 24/7 during the shoulder seasons your minimum bill is probably fewer than 1000 kwh, but whatever it is would be important to separated out the heating or cooling power use vs. the general power use.

    The minimum legal HSPF of a heat pump sold in the US a few years ago was ~7 BTU per watt-hour, or 7000 BTU/kwh. (It's now something like 8.2 minimum.) If 3500 of the 4500 kwh was for heating, at 7000 BTU/kwh that's ~24,500 ,000, or about the same as 255 CCF burned in a condensing boiler or water heater. The mean January temp in say, Alexandria LA is about 50F, so the average day is about (65F-50F= ) 15 heating degree-days, so over a 30 billing period that would be 450 heating degree days. 24,500 ,000/ 450= 54,444 BTU per HDD, or 2,268 BTU/degree-hour (which is quite high implying a really large or really leaky house, or a woefully leaky duct system.) The 99% outside design temp in Alexandria is +30F, which is (65F-30F= ) 35F heating degrees below the presumptive heating/cooling balance point, for an implied heat load of 35F x 2,268= 79,380 BTU/hr, which is just crazy if it's an average sized house, and yes, a 3.5 tonner won't really keep up with a load that high without resistance heating backup, which means the "as used HSPF" and heat load is actually a lot lower, due to the low efficiency of the resistance heating.

    For reference, my somewhat tightened up mostly-insulated 2400' 1.5 story 2x4 framed 1920s bungalow with 1500' of insulated basement comes in at about 21,000 BTU/hr @ +30F. If the heating system failed a four 1500 watt (5100 BTU/hr) oil filled radiator type space heaters can fully heat the place @ 30F outdoors while waiting for repairs. (But we also have a wood stove, so...) Just about any 3.5 ton heat pump could fully heat my place even at +10F.

    Buck a CCF isn't as cheap as I had expected- a lot of that has to be paying for the new distribution infrastructure. There are some places in the US paying only a bit more than half that much. At 102,000BTU/CCF (it varies depending on the mix), burned in a 97% condensing furnace delivers 99,000 BTU/ccf into the ducts. Normalizing to million BTU (MMBTU) it takes 1,000,000 /99000= 10.1 ccf/MMBTU, at a cost of $10.10. In a minimum-legal efficiency furnace (which could be more expensive to install if the exhaust venting is complicated) that would rise to about $12/MMBTU.

    The average price of electricity in LA is about 9 cent/kwh. With a minimum legal HSPF of 7 that's 1,000,000/7000= 143 kwh/MMBTU, at a cost of $0.09 x 143= $12.87/MMBTU, not a dramatic difference. At an only slightly better HSPF efficiency it's still cheaper than minimum-efficiency gas.

    If you want gas as a back up heating system and want to use a gas furnace, put in in parallel with, not in series with the package unit, and figure out how to vane-off the two systems when not in use. It's a hack, but it'll probably . Gas furnaces have minimum flow requirements, so do heat pumps, and the system really has to be designed to factor-in the impedences of both the cooling coil and gas heat exchangers to have a shot at making the flow correct for either. The alternative is to install an oversized hydronic coil in series with the package unit using a water heater to the coil, but even that has to be designed, not hacked.
     
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