My old house came with a gas water heater that isn’t venting
dneil
| Posted in Green Products and Materials on
My old house came with a gas water heater that isn’t venting; I need to replace it. Getting the venting right, even with a high efficiency, power exhaust, heater may be quite expensive. I am considering a heat pump electric. Has anyone done a comparison?
The house is big. Three floors with three bathrooms per floor. Our first-hour demand is maybe 50 gallons.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
David,
I'm not sure what you mean by, "Has anyone done a comparison?"
Here is a link to an article with more information on heat-pump water heaters: Heat-Pump Water Heaters Come of Age.
Sorry. I mean a price comparison, so I could get an idea of how much more expensive running a heat pump water heater is compared with a 95% efficient gas water heater. As I said, installing the gas water heater is going to cost more given the need for venting. I need to figure out "how much more" is worthwhile.
Your house really has 9 bathrooms? What's the square footage? I imagine that everything related to such a house would be "quite expensive."
David,
The article I linked to includes information on heat-pump water heater prices and anticipated savings.
To learn the installed price of a heat-pump water heater in your area, you will have to call up a few contractors.
What are your local gas and electric rates ?
You state a 50 gallon first hour rating . Are you stating that when you are living there your consumption will be 1.2 gpm or less ? Highly unlikely , the average showerhead will require more hot flow than that . Your most important thing to look at will be recovery rate .
Sorry, this is my first time posting a question - I'm not stating the facts well. I based the first hour rating on 2 people taking showers of 15 mintues apiece. We do have low flow shower heads that are about 1.2 gallons per minute. There was a typo earlier, we have four total bathrooms on three floors. We live in Boston, electricity is about 18 cents per kwh, gas is about $0.80 per therm in the summer, $1.30 in the winter.
My goal is to decide between a high efficient gas water heater (given the cost of creative venting) or a high efficient heat pump. I was wondering if any one knows of studies comparing the two types of heaters. I noted that a 50 gallon heat pump unit (GE) had a 60 gallon recovery rate. That is cutting it close.
The 80 gallon GE would give you significant margin on first-hour gallons, and you'd be less likely to need to put it in hybrid mode, and you could run it solely in heat-pump mode.
If you normally have to run a dehumidifier in the basement in summers to keep the must-basement smell at bay (common in eastern MA), a heat pump water heater would take a large chunk out of the operating duty cycle of the HPWH. A dehumidifier is converting a latent cooling load (humidity) into a sensible cooling load (heat), whereas the heat pump water heater removes that moisture in the process of moving heat from the room air into the water, which is a much more valuable use of that latent heat during the cooling season.
If showering performance is your primary concern, a sufficiently tall & fat drainwater heat recovery heat exchanger dramatically increase the "apparent size" of the hot water heater. For low volume users it can take awhile to rationalize the cost on energy savings alone, but if it avoids the higher up-front cost of a larger hot water heater the economics look pretty good. A 4" x 48" or a 3" x 60" returns about 53-54% of the heat going down the drain to the incoming water flow at 2.5gpm, and north of 65% at 1.2 gpm. By pre-heating the water going into both the cold side of the shower and to the hot water heater to above room temp, it increases the fraction of cold water going into the shower mixer/decreases the rate of hot water drawn from the tank, and decreases the recovery time of the water heater since there's no slug of 40F water in the bottom, it's ~75F. But they don't do a thing for tub-filling capacity, since it can only recover the heat when the drain is flowing. (You can get them at the wholesale price from EFI by opening an account over the phone with a credit card, which is cheaper than buying direct from the manufacturer or through the big orange box store: http://test.efi.org/sites/default/files/power_pipe.pdf )
A 15 minute shower is about 2x the national average, but my kid could beat that without even trying when the hot water delivery system is truly endless! :-) Since that's the case at our house, I put the bathroom lights on a vacancy sensor set to time out at about 10 minutes as the visual cue that maybe it's time to finish up. It seems to work. Unless you really insist on 15 minute back-to-back showers a 50 gallon heat pump water heater would probably serve your needs, and it definitely would if coupled with drainwater heat recovery. If it's big enough to fill the biggest tub in the house, you'll be fine.
Cost-wise an EF 3.0 heat pump water heater at 18 cents/kwh is like a standard electric tank with 6 cent electricity, which is pretty cheap. And if it's offsetting dehumidifer power use it's even cheaper still, since that's power you would have already spent. At 0.9EF water heater delivers 0.9 x 3412 BTU/kwh= 3071 BTU of heat into the water and it takes 326kw to deliver a million BTU. At the effective ~6-cent/kwh that's about $20 / MMBTU.
A pretty-good Energy Star power vented gas unit will have an EF of about 0.7, so at 100,000 BTU/therm that's delivering 70,000 BTU/therm into the water, or 14 therms/MMBTU. At $1.30/therm that's $18 / MMBTU, so it's pretty much a wash. It really depends on where electricity prices and gas prices are going- both of which are fairly volatile in New England.
In MA you have supplier choice, and you may be able to lock into a PPA contract at less than the standard mix rate if you shop around. I bought into a 3 year fixed rate contract for 100% wind power just before the price spike a year ago, and it's been a money saver relative to the standard rates. Longer term the rates are going down, but in the near term the wintertime gas supply issues have led to volatility in the electricity markets due to the heavy dependence upon gas-fired power generation competing with space heating uses during cold snaps for limited storage & distribution capacity. (There are several non-hardware very low cost fixes to that, but there are state-by-state regulation issues to iron out first. NH is considering allowing power generators to buy pipeline capacity and pass the cost onto the ratepayers, which seems like a more expensive solution, but a solution nonetheless, one of many.)
If you are heating the house with a condensing hydronic boiler there may be better marginal cost economics by heating your hot water with an indirect fired tank running as a zone off the heating system, since 8 months the year you'd be getting an effective 0.9 EF+ out of the indirect, and it'll improve the as-used AFUE of the boiler. You'd also have much higher first-hour gallons than with any standard standalone tank, and maybe even more than the low-end condensing tank type hot water heaters, depending on the BTU output rating of the boiler.
Thank you very much for the information. And I apologize for asking one more question: the gas water heater we are considering is the HTP Pheonix light duty water heater which claims to be about 95% to 97% efficient. Does that change the comparision costs that much?
PS We considered indirect heating (i.e. tied into our space heating boiler) but that system is pretty shakey right now and is on the opposite side of the basement.
David- I showed the arithmetic- you can do the marginal cost math yourself. If it's only used as a hot water heater for 2-4 people, assume the HTP will have an EF of something like 0.80, 0.85 at best, since you'd be running that 76,000 BTU/hr burner at some ridiculously low duty cycle, not at steady-state where it would run over 90%.
The marginal operating cost of heating hot water with the HTP will be somewhat cheaper than a GeoSpring, at least for now, and you'll NEVER run out of hot water. A 1.2 gpm flow with a 70F temperature rise (35F in, 105F at the shower head) is only ~42,000 BTU/hr, and the Light Duty puts out over 70,000 BTU/hr. But the upfront installed cost is going to be quite a bit higher- especially if you have to upgrade the gas plumbing to the hot water heat to support a modulating 76,000 BTU/hr burner. With that much burner and 3/4" gas piping you can only go about 80 "equivalent feet between the appliance and the gas meter, with no tees off to other appliances according to most charts:
http://www.industrial-equipment.biz/assets/images/information/gas-natural-piping.jpg
The "equivalent length" of every tee & ell along the path has to be added to the pipe lengths:
http://www.engineeringtoolbox.com/resistance-equivalent-length-d_192.html
If you cheat those numbers, odds are pretty good that it'll have flaky behavior, and may turn itself off, spitting out an error code. Modulating burners are more sensitive to pressure fluctuations than dumb on/off burners. It's generally better to give yourself a 25% capacity margin on the gas plumbing relative to what the charts say if it's a modulating burner, especially if there are tees off to serve other appliances. You may be able to just drop in in the same place as your dumb atmospheric drafted hot water heater, but don't count on it. Take the time to measure up the equivalent lengths first, and KNOW you have a lot of margin, or you'll be kicking yourself later.
The Phoenix Light Duty can probably operate as your space heating boiler as well, since most homes in Boston area have heat loads less than 50,000 BTU/hr. But that depends on a lot of other factors (like the amount & type of radiation relative to the room loads, etc.) Using a Phoenix Light Duty just for your hot water is really overkill unless you are using a LOT of hot water. The "Light Duty" is only relative to commercial applications. It's MONSTER duty for a typical residential application. If the old heating boiler is on it's last legs, maybe it's time to take a look at the bigger picture. Installing a heat pump water heater wouldn't affect the bigger picture, but a Phoenix Light Duty definitely would.
The Light Duty will in all probability never use more than your old heater did if you had no problem with the delivery . Taking this into consideration the Lght Duty will use between 25,000 BTUh and 76000 BTUh . Your installed gas piping will almost certainly provide plenty of fuel even at the high end of 76000 BTUh . Gas pipe by nature and well , stupidity is inherently oversized . Longest run method is the norm when sizing gas pipe as opposed to series summation . Series summation method takes all parts and Physics into account which allows pipe sizes to be smaller and still more than adequate , and the codes recognize this engineering method .
Dana's area has an average delivery pressure of 5" w.c @ the meter , the Light Duty requires a minimum of 3.5" w.c . If you take into account a greater delivery pressure than required as per 402.5 of the IFGC you have an allowable pressure drop of 1.4 " . At 80K 3/4" gas pipe has a P.D of .003 " wc per foot . Since Dana used the meter to fixture example , I will follow that . I'll let you do the math . 1.4 / .003 = 466.66'. Now we know you do not have a line directly from the meter to the appliance , but if you did that would be your maximum gas pipe length allowed while still having proper operation without a hitch .
Let's take a realistic look . If your home has a connected load of 400,000 BTU and all fixtures ran at the same time you would suffer a PD of .062" per foot if all the piping was 3/4" from the meter to every fixture . all the fixtures could be 22' from the meter . Now let's get realistic ,
You probably have greater than 1 " running into the house , because plumbers are stupid and do what they are told without question . That 1' portion of the line would have a PD of .019 with all fixtures firing . If you have 5" at the meter and a 1" main trunk that is 50' the PD in that segment would be .95" . That means at the end of that run you still have 4.05" , that's 50' from the meter . If the water heater was at the far end and you wanted to deliver 3.75" at the heater you could run 25 feet of 1/2 (.012 PD-ft ) iron or 100" of 3/4 ( .003" PD-ft ) to the heater . Boyle and Charles were pretty sharp guys .
In short , if you're looking at the Light Duty it is as efficient as you'll get and only uses what it needs . The firing differential is also adjustable so you can hold the burner out until needed. don't forget to store at 140* and use a mixing valve with pressure balancing positive shut off capability . This will hold the burner out even longer and protect your family from disease .
Hope this helped .
Therein is the peril of possibly not getting what you want or need due to a lack of understanding . Most codes allow this sizing as it is based on physics which applies everywhere .