Oh really?

Anything that is piped or wired into the gas train and not AGA approved, UL listed would void warranties, subject user to safety issues and lead to liability issues.

If you have a plug in power vented water heater, a plug-in wall timer can be had at HD for about $10.

With todays EF of water heaters, shutting it off for 8 hours at a time is inmeasurable on a power vented unit with minimal flue loss.

I'm with ZL700,

I'm very skeptical of the interface with the water heater and very, very skeptical of the savings claims, despite what the manufacturer might claim.

The big inefficiencies in a standard gas water heater are introduced from two places: 1. the flue temps must be high enough to naturally vent the flue gases -- that means wasting a lot of heat so that the gases exit the house. 2. the flue up the center of the tank is uninsulated and acts like... well, like a flue, even when the heater is off. Convection air currents keep cool air entering the bottom and exiting at the top, effectively cooling the water in the tank.

A timer would only have a slight impact on the second issue-- and there it would only work because of the reduced temperature differential. The water in the tank would cool off because the burner is shut off by the timer and thus less heat would be lost once the water did cool off.

Tankless gas heaters which solve the above two problems, as well as heat loss through the tank and a standing pilot achieve a savings of about 35-39 % over a standard gas tank heater. You aren't going to get those same savings with just lowering the tank temp for part of the day.

Tankless gas heaters which solve the above two problems, as well as heat loss through the tank and a standing pilot achieve a savings of about 35-39 % over a standard gas tank heater.

Please share with us the source of this energy/cost savings data.

Thanks in advance...

fixizin,

The energy factor of a typical gas tank unit is .58-.62 (the .62 is a "high efficiency" unit.)

The energy factor of a gas tankless is typically .82-.84

(I used Rheem numbers but they really are typical- I've checked)

If we use the middle numbers for both (.60 and .83), we see a 38% increase in efficiency with a gas tankless over a standard gas tank water heater. Surprisingly, a power vent unit isn't much better.

I hear the msg about teens not being driven out of the shower by cold water... I'm not at all convinced that's a reasonable argument. On the other hand, turning the tankless unit off with the remote is a sure way to say that "Time is up".

And better yet a Navien unit comes it at .95-.98 EF (DOE)with a thermal efficiency of 97.3 to 98.8% depending on model, now that is savings!

Here is a link that might be useful: Navien on-demand

Re: Navien unit

The last condensing tankless that I looked at was so far into diminishing returns that there's no way that it would be a reasonable investment. Standard tankless right now are borderline for payback... and I say that being a supporter of them.

The Navien unit looks like it could use PVC pipe for venting which could change the equation a bit. Stainless flues drive up the price of a tankless installation and make them prohibitive in many cases (venting out of a roof instead of a sidewall, for example)

It will be interesting to see what happens to this market.

However, this is exactly the type of retooling that this country needs to do to reduce our dependency on foreign oil. (and yes, I know that we are talking natural gas, but it's all substitutable)

We should be building these things in this country, getting the price down and writing better energy efficiency into code. Where I live, you can still buy and install a 80% gas furnace.

Yes Jake, Navien is PVC Sch40 vented. With venting capabilities of from 60-100' depending on model, the cost of special venting is no more or the need to relocate piping to mount a standard on-demand at an outside wall. Another advantage is the A model incorporates a built-in circ and mini buffer tank so that minute flows still have hot water along with built-in hot water recirc if extra line is run. Thus the "cold water sandwich" is eliminated.

Best of all, the price is nowhere near as high as the initial condensing on-demands that were introduced years ago.

Being the makers of the heating box, they are in development stages of a dual unit, domestic and heating in one cabinet.

I hear ya on the 80% furnaces, yet in the south anything over 80% is difficult to recoup, as they are used as air handlers with a bit of backup. .

This is very cool and a huge step in the right direction. Any idea on the cost of say a 240A?

BTW: I'm in Central Indiana... not the south. It's completely unreasonable to still allow 80% efficient gas furnaces.

In the Metropolis areas, around $1,400 across the counter to a professional.

Not bad for simple inexpensive venting, 17,000-199,000 BTU/H with buffer tank and circ included

A timer for a gas water heater? Well in my case the cheapest wall electrical timer ($10 at WallyWorld) will work. If there's no power for the forced vent, the thing won't turn on the gas. The thing is plugged into a regular 15A outlet.

That's a great price point. My Rheem 175000 BTU/H was about $850 + $220 or so for the basic SS vent kit that does most through-the-wall projects. Most 199,000 BTU/H units are around $1200-1400 for just the box and they require the expensive SS vent + are still about .82-.85 on the energy factor. And as you said, the "A" models throw in the circ pump and small buffer tank.

Have you seen a published Energy Factor on the Navien?

Yes the EF (DOE)is

.98 on the 180's (15,000-150,000 BTU)
.95 on the 210's (17,000-175,000 BTU)
.95 on the 240's (17,000-199,000 BTU)

200 watt (1.7A) current draw (Way less than a tank with power vent)

Performance at 55 degree F rise:

180-5.9 GPM
210-6.9GPM
240-7.8GPM

Residential unit 140 degree F max
Commercial unit 185 degree F max

* 3" PVC Vent, up to 100' w/6 ells
* both heat exchangers are stainless steel
* largest unit, A model with circ and tank is 86 lbs.

That condensing unit looks interesting, I would like to see it in wider use before I take the leap.

I decided go a different route for water heating energy efficiency. I'm a high water user (have teenagers) and don't want to go tankless because of gas and venting needs. I just ordered a drain heat recovery unit, now that copper is back to more traditional levels. I like that it is very simple and even an efficiency of 40% would provide a good ROI. A WH with a DHRU could yield an efficiency greater than 100% under the right conditions (with a better ROI and no maintenance requirements). I haven't installed mine yet so, it will be a while before I can provide real world usage data. But, its worth loking into if your interested in saving energy costs.

I think ultimately thermal solar is the answer, its just a matter of time before it is more widespread. I plan on adding a system in the next year or so.

I have to ask, you will install this drain heat recovery unit where, shower or whole house?

Showering time is limited, whole house may have a average temp of drain not much above 75, how much can you really recover?

You would gain more by wrapping your flue with fins, anyone remember that gimmick?

According to the same webpage you referenced earlier in the thread (EERE), 80-90% of the energy used to heat the water goes back down the drain,DWHR. It also gives a payback of 2.5-7 yrs. Even at 7yrs, that is an ROI of over 10% per year, not too many investments can give you that.

The DOE also supports it ,DOE-OIT.

Shower drain water is 85-95, the avg cold water temp is 55 in the midwest(probably 45 now). For equal flow rates (drain and supply), at 50% recovery the water would be 72. In effect, your recovering ~40% of the original energy used to heat the water. You save both by heating the water going to the WH, but you use less hot water since the ratio of cold to hot is higher at the shower (since the cold water is now 17 degrees warmer).

Its far from a gimmick, its like saying a heat pump can't work when its 45 outside or 55 in the case of geo. It may not be instinctive, but it still works and its almost idiot proof.

Heat recovery units are legit - you do need to install them as they were designed (many must be installed vertically). The other issue with them is that they work for showers, for example, but not so much for batch uses such as dishwashers, washing machines, bath tubs, etc. They would be most effective if they could be installed near the point of use and the drain pipe prior to the unit was PVC (or something other than metal)

Also, take a look at whether it's best to pre-heat both the water supplying the heater and the cold to the faucets or just the feed to the heater. It seems to me that you would be wasting warmed water in some applications (such as the john)

There are definately limitations such as the batch uses. I think I have the ideal setup, 4 people taking back to back showers in morning. It will also help greatly with capacity given the tank WH.

The numbers I've seen definately favor feeding both the hot and cold from the exchanger. This is the optimum sitaution when in the showering mode. I'm sure its just a matter of better heat conduction, since the average temperature differential will be greater if both supplies are feed from it.

Another aspect/limitation is friction pressure loss on the supply side. Although this shouldn't be a problem with most systems that have reasonable working pressures.

I tend to kid around a bit.

I have seen full blown drain recovery units in Europe, where the water flows through a receptor, similar to a big poly grease trap, with baffles and piping as the exchange medium. Not sure how they go away with that with the risk of cross contamination.

Jake, I'm not sure about your comment about the drain pipe should be PVC (or something other than metal)?

PVC has inherant insulation properties while copper for instance has a emisitivity much higher than PVC, thus a better heat transfer material.

I believe he meant all of the drain prior to the heat exchanger should be non-metal, presumably to prevent heat loss prior to reaching the exchanger.I would agree with that.

The thermal conductivity of copper (400 W/mK) is about a 2000 times higher than PVC (0.19 W/mK).

I wish I had the thing installed this morining, the cold water was absolutely frigid (probably