Central air conditioning -- natural gas?

The Pub
1

In my new house, I am going to put in two furnaces.

The primary heating system will run from a hydronic furnace.  This furnace draws water from the hot water tank through a coil in the forced air system, which then cools the water and vents the heat into the HVAC duct work to heat the house.  In other words, it dumps heat from your hot water tank into your house.

The secondary heating system will be a gas furnace sized to heat my whole house.  This will exhaust into the secondary heating coil on my hot water tank as a back-up heat source.

My hot water tank will be over-sized and use a large collector array.  The closed loop runs at 200-300F, while the large tank (300L or 400L, to act as a heat dump and also to store extra heat to drive the hydronic heating system) will run at 160-190F.  This 160-190F tank water will circulate through the hydronic coil, while output to the hot water piping will pass through a thermostatic mixing valve that mixes in cold water to set an output temperature of 130F.

In short, my heating system and my water heater will run primarily on solar energy, directly, full spectrum.  This is with evacuated tube black body collector, 100% absorption efficiency at the tubes.  Solar panels convert 20-30% of the solar energy that actually lands on the panels into electricity.  Loss is experienced in both radiative heat and conductive heat; the piping system is stainless steel (copper tubing loses heat 40% faster) wedged tightly in thick foam insulating jacket.

The water tank will run on a gas back-up.  If the hydronic system cools the tank or there is not enough sunlight, a gas furnace connected to the tank’s thermostat will activate and direct exhaust through the water tank.  This supplies both hot water and forced air space heating.

That leaves one piece:  Air conditioning.

I am looking into natural gas driven air conditioning units.  In the summer, electricity grid use peaks due to hot summer days and air conditioning power draw.  Natural gas infrastructure is highly underutilized in the summer.  My electrical panel is 100 amps, so driving heavy appliances is non-ideal anyway.

What I would like ideally is a natural gas air conditioner unit that can accept a secondary heat source.  I would attach a plate exchanger to the unit which would run hydronic heat either from the tank or directly on the closed loop.  A thermostatic valve would activate when the air conditioner turns on AND the loop is above adequate temperature to drive the AC, allowing hot water flow to the AC compressor.  Failing this, the loop would close and natural gas would supply a secondary back-up.  We can’t sum both:  if the hydronic loop flows while the AC runs on hotter natural gas, it will act as a cooling system and draw heat away from the AC.

Anyone know anything about natural gas AC?

2

Wow.  I think the simplest response is this.  The system you’re talking about (or at least the parts of it that could actually work) would probably cost more than your house.  When talking about energy saving modifications to building systems, one of the most important calculations is the Return On Investment (ROI), or payback period: how long will it take for the energy cost savings to pay for the initial cost of the system.  A desirable payback period for most such projects is less than five years.  The payback on the system you’re talking about would likely be measured in decades.  If course, it’s possible your local NG utility has incentive programs, low to no interest financing, etc, that might make it economically feasible.

3

I just know that there are propane powered refrigerators that are used for camping and by the Amish (no moving parts).

Natural Gas would work for energy source too in the cooling loop.

4

Well, a solar water tank is an easy install:  there’s no need to hook up gas utility to it, and you basically run flexible stainless steel piping out a conduit in the back wall and up to the roof in another conduit.  The entire system costs about $2500 for a 300L tank, $3300 for a 400L tank, plus some tubing that runs $3 a foot and a.  The government gives about 50% back on this, but then taxes that as income, so about 30%, making this $1750-$2000 to install.

Further, Maryland will grant 1SREC per 1MWh power generated by water heating up to 5MWh; 3MWh is typical for a 150L tank, so I’d exceed the cap.  Utilities that burn fossil fuels must pay $400 per MWh generated or supply a credit; these SREC sell on the open market for about $200 at the moment, so for the next 2-3 years at least (i.e. until the penalty is lowered from $400) I can generate $1000 per year in taxable income, about $700/year, offsetting the cost of the solar water system.  For just water heating, the tank size of 150L would really pay for itself entirely in 1 year; the system I want to build has to generate space heating in the winter, so 2 years.

A hydronic heater costs about as much as any other heater, although it’s got a longer life.  It’s easy to hook up–no gas hookup, just water piping and attach to the HVAC, which is work I can do myself.  It runs a pump and thermostat on regular utility electric.  Mind you the furnace I have is from 1992 and shows visible signs of age, so requires replacement.  That makes this a non-cost.

The only additional cost is the cost to hook up a secondary gas furnace as a back-up on the water tank, which again costs as much as any other gas furnace.  This furnace will usually never run, anyway.  I can put the furnace in place myself, but a certified technician must do the gas hook-up.  This will be around $2000 total cost (the furnace itself would run $1000-$1500, installation will run under an hour of labor… although I have a cousin who works for the local gas/electric utility and can do both main electrical box wiring and gas utility/appliance hookup).

There is no AC compressor currently, so I’d have to buy that anyway.  A natural gas driven AC has very few moving parts (it’s an absorption heater, there’s no compressor; there’s fire and a thermostat, no pumps or anything) and to my understanding they’re generally inexpensive.  It just needs a natural gas hookup–again, labor that I’d have to outsource; I’m not hooking gas appliances up by myself, that’s insanity.

You really think this is going to cost more than my house?  It looks to me like the only additional cost is a secondary gas furnace for a water tank backup.  Overall the total cost would come to some $6500 for the units ($2000 water heater, $1250 for each of 2 furnaces, and $2000 3 ton natural gas AC unit–you think I need 3 ton?  They just come that big!); but to install a normal furnace, AC, and water heater would come to the same, and I need at least two.  The water heater I can probably avoid, but the ROI on the solar water heater will be extremely fast.

5

Do you have any links for the NC AC unit? FWIW I have a 3bedroom 2 full bath house~ 1200sq and the AC is 3.5 tons. I would look into ductless AC where you get over 20 SEER.

Myself? I want an Earth-ship-type house where ypu don;t need AC or heating, and if you do- a wood burning stove to burn all the junk mail. Subscribe to all the mailing lists and  heat your house for free…

6

Haha.  If we could burn the gmail spam folder, we’d have an endless supply of clean energy (electrons don’t emit CO2)!

I mean all I see is stuff like this or whatever comes up for “natural gas AC” on Google.  You need 3.5T for 1200sqft?  The biggest residential unit I’ve heard of is 5T (Trane gave the impression that 5T is a “Big” unit) and I live in a 700sqft apt and it’s small…

The relevant “people trying to convince you to buy their crap” site is http://www.gasairconditioning.org/ so do your own research.  I have, at least I’ve researched designs and some of these (the absorption ones) are EXACTLY what I’ve been wanting.

Site lists a lot of up-side.  Down-side is you need a plumber (gas piping) and electrician (electrical power for thermostat and other controllers) to hook it up, rather than just an electrician.  Also natural gas prices aren’t guaranteed to remain stable; they’re depressed because of mass fracking lately, which has been backed off lately in an attempt to artificially inflate the price.  The “plentiful supply” assumes 100 years of gas reserves “under current usage,” but if everyone went to natural gas AC the natural gas usage would go up and cut that down by a bit.  The brief mention of “hybrid” systems is their nod to this concept.

For me the up-side is I have a 100 amp line and a 100 amp main electrical panel.  Running an electrical AC and dryer unit would strain those limits too much.  The fact is I don’t want my vacuum cleaner (11amp) to throw the main breaker when I’m drying clothes (3kW on a 110V line = 27 amp) and the AC kicks on.  God knows what an AC uses; my window unit uses 9A to cool one room, so a whole house is like 30A?

While I shouldn’t exceed 100A in any normal condition, permanently dropping a clothes dryer or an air conditioning unit off the list helps.  The two of them look like they’d consume over 50% of available electricity, and summer electricity rates are 30% higher here on-peak than winter rates.  (Gas prices fluctuate, sometimes wildly)

I don’t think I can use a secondary heating loop to run these though, otherwise… solar.  Photovoltaics just don’t pay off.

7

You could always move to northern Europe. Average daily high in the summer is 75-78, and then it cools down to around 55-60 in the evenings. We keep the windows closed at night.

I think you’d be better off with super-high R insulation and an attic fan. Windows open early in the morning, attic fan on, seal everything up before going to work.

8

The house is very well insulated, and that’s actually how I do it at the apartment.  Two months of the year I used 520kWh at 74 degrees average daily temperature, April and May.  I made it to mid-June with just a whole house fan shoved in my window at night and shut the windows in the morning, and burned about 240kWh for April and May.

September is back to the same.  It’s too hot in the day, but in the morning it’s freezing.  I want to shower in the morning with the fan pumping air in (pushing shower steam out the bathroom window), when the outside air is the coldest just before dawn, and then shut all the windows and let the apartment retain heat.  Likewise, the house.  With any AC, I’ll probably shut off the thermostat at night and run the fans; then close up the house in the morning and turn the AC on to maintain temperature.  But you know, with a solar/hydronic heating system it’s pretty much the same automatically:  sunlight to heat during the day, maintains at night with gas if the tank cools too much.  And why not?  Why pay for winter heating when you have a big friggin’ fireball above your house?

9

I have a window-fan except it has two fans. Put it at one end of the house and open a window at the other. It’ll suck all that warm air out and pull in the cool air. I’ll be using it in a month or whenever it cools down enough in the evening to make it worthwhile.

http://www.mitsubishicomfort.com/en/consumer/product-solutions/product-showcase/indoor-units

How does 23 SEER sound?

10

Excessive for a house my size.  Interestingly, when you get down to a certain size living space, the difference between 14SEER and 24SEER is non-existent; whereas you get a house twice the size and a 14SEER unit costs you 3 times as much to run than a 24SEER and burns a ton more fuel/electricity.

Probably works for you.  20+SEER works for both my parents’ houses.

11

Dude, I’ve done HVAC for the last 20 years. In sizing a/c, you figure 1 ton per 500 sqft. If your place in only 700 sqft, I say throw a window unit in for 150$ and call it a day. To cool a place that size, you can run a couple of these for minimal cost. They don’t use that much juice. As for running a/c on 100 amp service, its no problem. That’s all most home have. Some have 200 amp, but few. A unit 2 ton unit runs off a 20 amp breaker, a 3 ton on a 30. But, they do not pull that many amps. That’s just where the breaker kicks. At start up, to get the compressor moving, a 3 ton may pull 20 amps, for a split second. You can dry your clothes, cook your dinner and sweep your floor at the same time with the AC on and still not pop the main.
  From  the size place you say have, you’re way over thinking this, IMO. To much work, to much to go wrong. Take it from a guy who’s seen it. People try to put in all this “money saving” stuff, and it never does work right. Youll be farther ahead and more comfortable to to put in a 90% + furnace, and a 12 SEER A/C, or even a window unit.

BTW, SEER has nothing to do with the size of the house. Its all about efficiency. The higher the SEER, the more efficient the unit. Not sure where you get the idea a 24 SEER cost 3x to run than a 14 SEER.

12

There are adsorption chillers (that’s ADsorption) that can cool buildings using hot water. They are expensive though.