THSCC Forums

Donnie,

If you don't circulate the water through the tank, then how does the water in the hot loop stay hot? My understanding is that you either have a dedicted loop with a pump the pumps water through the heater to keep the water hot at all times. Or you have some type of "on demand" button that you push and it pumps water through a standard non-return line that is connected back into the cold water line at some point (basically you are turning on the hot water but instead of it going down the drain it goes back into the cold water supply).

I am asking all of this as I am semi-interested in some type of on-demand system. I believe that the way my house was plumbed was to have both the master hot and cold water run into the crawlspace under the house. But they didn't put the hot water heater there. It is in the attic. So I "think" my hot water to my master bath runs from the attic (above the master bath), down two floors, back up two floors and then into the bathroom. Needless to say, you have to run a lot of cold water to get any hot water even though the hot water heater is about 10 ft away from the faucet.

Richard

_________________
Richard Casto
1972 Porsche 914
2013 Honda Fit Sport
2015 Honda Fit EX
http://motorsport.zyyz.com
Money can't buy happiness, but somehow it's more comfortable to cry in a Porsche than a Kia.

Sorry I wasn't more clear. It does circulate some water through the tank, certainly. But the tank is not "in" the loop. The line comes out of the tank and not very far from where it comes out, it T's into the loop. So while the tank isn't in the loop, some convection does keep the water in the loop hot. So water in the tank is still circulating, just not as much as would be if you had an input and output on the tank. In effect you usually don't really get incredibly hot water to start...but it's warm enough that you don't notice it's heating up as it gets there and thus you aren't stepping into a cold shower, per se.

I thought we'd want a switch to turn the pump in the loop on and off at our mountain house due to the extra energy this system requires. But the effect was we use no more electricity than before.


--Donnie

Bernie, sorry I wasn't more clear, but I think we're both right. I think you had undersized and/or innefficient units that you replaced with better sized and/or more efficient units (and it was probably some of both in each case).

But I stand by my statement that if you have a *good* HVAC guy recommend the *right* size, you are doing yourself a great disservice by then asking him to put a *larger* tonnage unit in. Maybe a small increment, but probably not a large one.

I've got a very good friend who is in the HVAC business. He's a smart guy who keeps up with his industry. It was from him I heard about the studies where oversized units have been known to start problems with mold due to improper humidity control. He has no reason to lie to me. He's corrected numerous problems for me that were created by sub-par HVAC guys that I used before I got to know him. He's been right in each and every case, and like I said, I now know him pretty well. I've got a pretty good bit of experience now, having had one commercial system installed as well as at least eight residential units.

I stand behind my statement that a unit that is oversized will not control humidity. It will not be able to circulate enough air...it will leave "warm spots" that retain humidity and simply cool enough air to get the thermostat temp down which is out on a wall in the open. It will not completely circulate through the entire air volume, and can even leave hot air above the thermostat. I've seen this personally (hot air rises, as we all know).

I'm guessing you're talking about slight oversizing, which can be fine. I still think you'll end up living with some amount of temperature swing you wouldn't have to with a lesser unit, but you may also run the risk of not having enough cooling on abnormally hot days with the "right size" system, so everything is a tradeoff.


--Donnie

One solution to the "make sure it is big enough but not too big problem" is a two stage compressor:

The Lennox website says this about two stage:

"Two-stage cooling means the air conditioner or heat pump has a compressor with two levels of operation: high for hot summer days and low for milder days. Since the low setting is adequate to meet household-cooling demands 80% of the time, a two-stage unit runs for longer periods and produces more even temperatures.

Longer cooling cycles also translate to quieter, more efficient operation and enhanced humidity control. Compared to a single-stage unit, a two-stage air conditioner or heat pump can remove twice as much moisture from the air. This is important because when moisture levels are high, there's a higher potential for mold and other pollutant problems."

Note that two stage compressors may (or do) include variable speed air handlers.

Dick (who just bought a new heat pump but did NOT spend the large amount of EXTRA money for two stage but did spend some extra money for a variable speed air handler which is variable speed just for AC)

_________________
Dick Rasmussen

FS 50 2018 Mustang GT

Donnie, ask your HVAC guy what size unit he'd use in a 1500 sq ft open studio apartment, no walls, one large closet and enclosed bathroom. I guarentee it will 3 tons or less. They all say the same thing and the facts just don't add up to what they say. I had a new 3 ton unit that we changed for five ton and efficency is up elec bill down and is much more comfortable.
The exact same situation down on the coast 1500 sq ft home, neighbors home same square footage he puts in a carrier 3 ton, I put in a trane 5 ton. my elec bill is 30% lower than his. Those are the facts, pure and simple. I know thats wrong according to all the HVAC people I've talked to, but the results I've seen makes me believe I'm correct.

_________________
2004 C5(415whp,390ft/lbs),
1997C5,1997Trans Am, 1986 C4,
1990 Miata, 1976 MGB,1997 Protege, 1989 MR2

Back to the original topic, ever wonder why hotels have "Instant" hot water? They use the "loop" systems through their hot water boiler and are continuously circulating water throughout the building. The loop includes the boiler. All the pipes are heavily insulated, and voila!, instant hot water.

For a single-family dwelling, this is a very economical method, using conventional components. If tankless systems really had a 3-5 yr. payback, EVERY newer hotel would already have them. It would be a given in the design/construction business decision. Should be easy to check.

I think a loop system would be hard to beat if properly insulated, and the circulator turned off during very hot weather. Installation costs of multiple tankless units and the fuel lines to run them would be difficult to justify vs. the simpler, more mature loop method. Operating costs are probably pretty close to a wash, given that each require the same amount of fuel to produce the required therms, and each system uses a certain amount of fuel to maintain a minumum temperature. Then there's the whole life expectancy/depreciation thing. A $700 conventional Gas water heater w/ circulator and plumbing vs. two or three tankless set-ups.

Interesting decision to make.

_________________
Brad

Brad
I think you just mentioned why hotels do not use tankless water heaters. If they wanted to go tankless they would need many units throughout the building because the tankless units have limited flow rates. Also the demand for hot water at a hotel is more constant and the water heaters would be running at a higher duty cycle than in a home. I would expect this would shorten the lifespan of the unit and negate much of the energy savings. Also using a loop system on the hot water lines should cause a similar increase in duty cycle.

Just my $.02

_________________
George Bright Member # 141
One of the youngest of the old farts.
Yes I voted against you joining the club.

The 3-5 year payback on a tankless heater is a simple fact, there no guessing and all it takes is some quick math to prove it.

As for Hotels using them, its apples to oranges here. The requirements for commercial and residential uses are vastly differnet, and comparing products used in one to another are not very accurate. Tankless units use no power when not suppling hot water. However when running they use dramatically more than a std heater. As such, in your hotel example, the peak power demands required if all the guests decided to take a shower would require such a huge amount of current, that the wiring size, current rating, etc would make it impractical.

_________________
91 Jetta GLI STS/DSP 111
85 Porsche 944 ASP 11
http://www.blackforestindustries.com

Bernie,

I work with an engineering firm here in Raleigh that designs PME systems for commercial buildings. While working part time in school and co-oping for the firm I did electrical design work. Upon graduation 2-1/2 years ago I shifted over to mechanical and plumbing design. I can tell you that the majority of what Donnie has said is correct.

I also agree with him that the fact you aren't experiencing the problems mentioned have more to do with the equipment now being more appropriately sized for your application than being oversized.

Residential HVAC contractors often like to adhere to the 500sf to 600sf per ton rule. While this is fine for many applications it isn't applicable to all. There are many factors that weigh into cooling loads. Building/home directional orientation, location, construction type, glass type, amount of glass, shading, etc.

Using your 1500sf studio apartment for example and the 500/600sf/ton rule would in fact yield 2.5 or 3 tons. However you mention that it is all open. I also envision high ceilings and potentially a good deal of glass. This glass may even face SW or W. Given those conditions a 4 or 5 ton unit certainly isn't unreasonable.

The three 5 ton units for your 15,000sf kennel actually sounds on the short side to me. However I don't have any infomation for the building beyond the square footage nor do I know what kind of conditions you were trying to meet. I will use that to further illustrate my point that you can't just assume x-amount of square footage needs x-amount of tonnage. Like stated above this is what a lot of residential contractors do. I doubt many/if any at all actually have software to compute heating/cooling loads.

One last thing regarding what you mentioned with humidity. Yes as the temperature decreases humidity will go down. However keep in mind that with large oversized equipment your discharge temperature off of the cooling coil will be higher. Therefore it will not remove as much moisture yet still satisfy the temperature demands of the space.