Tankless water heaters. Size vs operational costs

[Michael Weber]

There is a possibility I may be able to get a tankless water heater at no cost assuming I do the installation. The choice is between two different models from the same manufacturer. One is adequately sized and the other is over sized. They both have the same minimum BTU input (11,000 BTU's). The maximum inputs are 150,000 and 199,000 BTU's. I would never need the larger units capacity as my house is configured now and is unlikely to change. The energy input to the burners of both units is modulated to the demand for hot water. Since the modulated energy used is dependent on demand, not on the maximum rated input won't the operational costs be the same for both models? That is, in the event of a demand for say 3 gallons of water per minute won't both fire at the same rate thus being equal in ongoing operational costs? Am I missing something?

[John McClanahan]

When we replaced our gas dryer with electric, our electric bill went up but our gas bill didn't change during the months of warm weather. When we don't use the furnace, the gas bill is mostly "standard customer charges" not related to gas usage. So, when it was time to replace the water heater, I could not see where the additional cost of a tankless heater would ever be offset by any gas usage reduction. I found a tank type heater on sale and never looked back.

John

[Chris Mahmood]

That sounds correct to me but if you have the specifications for two you can compare their energy factors which, at least theoretically, tells you what you want to know. Don't forget to factor in the cost of one needing a possibly larger gas pipe (my Takagi TK-3 required 1" from the meter) although a 150K BTU unit will probably need a big gas line too. Just from my experience I'd err on the side of oversizing even if the larger unit is slightly more expensive to operate...cold inlet water temperatures can really reduce the flow rate, you might want to get a dishwasher that doesn't have an internal heater, decide that you really like taking 3 hour showers, etc.

[Glenn Vaughn]

We just installed a 180,000 BTU unit a couple of weeks ago. We are happy with it but there is a downside to the tankless systems; Demand is based not so much on quanity as it is on temp rise. Temp rise is based on the target temp - the temp of the incoming cold water. In our case we want 130 degree hot water and our cold water is coming in at less than 40 degrees this time of year. For a 90 degree rise the heater throttles the flow way down. Sink faucets and showers are not affected much but the tub is really affected.

The 180,000 btu unit needed a dedicated 1" gas line and a meter upsize. We have been told that we should see significant savings over a year in gas usage vs the tank water heater we replaced. The unit only uses gas when we have hot water on - it does not use gas to keep the water hot.

I would have opted for higer BUT's but that was not an option avaible when I bought the unit (on sale at Home Depot - a GE unit manufactured by Rennai (the installers could not tell the difference from Rennai, even the manual is the same).

The nice thing is that even though filling the tub is slower, it is nice having hot water at the same temp the whole time it is filling. It holds 70 gallons and wiped out the tanked heater in short order.

[Alan Trout]

Glenn is 100% correct. Temp rise is the key on usage with a tankless unit. I have the large Electric Seisco unit and it works exceptionally well. But rarely if ever is our incoming water temp in South Texas below 60 degrees. I would have gone with the gas but at the time I did the install almost all the gas units on the market needed horizontal run on the vent flue and that could not be provided with my situation. I would personally in your situation go with the higher BTU unit if you have the proper capacity gas line to support the Extra BTU's.

I have been thrilled with the operation of my Siesco unit. I had to have adequately sized electrical for the unit. Mine took another 125 amp sub-panel with four 30 amp breakers and #8 wire for each element to the unit. It is like the gas units, it only uses energy it needs to bring the volume of water to temperature. They really do save money and I bet you will really enjoy your gas unit.

Good Luck

Alan

[Mike Cutler]

All things being equal, yes they will expend the same amount energy in BTU's to raise the same amount of water at the same flow the same amount degrees.
Will both fire at the same rate? That's dependent on the internal programming. The 200K unit has the ability to heat the initial surge faster, but once the flow rate is established and the temp is stable the energy is the same minus any losses due to design.
One additional point to consider is the minimum flow rate to start the unit. Most are .5gpm. If you have flow restrictors and water savers in faucets, they may not satisfy the requirements of the unit to start. Some "systems" are using a surge tank with a recirc loop to handle the initial demand and low flow requirements. AO Smith just relaesed a Hybrid unit last year that combines an on demand feature with a reserve volume, that uses the exhaust gasses, heat, in a recirc system to compliment the burner. Nice unit, but $$$$

I'd really like to install a tankless system. However. here in CT, with propane prices over $4.00 a gallon, and electricity at 19 cents a KW/HR. It's kind of a wash.

[Michael Weber]

Thanks for all the great feedback. Both models have the same energy factor (.82) and identical activation flow rates (.4 GPM). Another possibility is, since I have a two story house, is to get two smaller units to eliminate long water lines. One for upstairs and one for downstairs. However, the point Glenn brought up about taking longer to fill a tub makes me concerned that situation might cause a problem if someone is taking a shower in one bathroom while someone is filling a tub in the other. The large unit is rated for 3 bathroom homes and the smaller one for 2 to 3 bathroom homes. However, reading the fine print on the specs shows that rating is for simultaneous showers using 2.5 GPM (dependent on cold water temperature) so if someone is filling a tub which I assume uses more than 2.5 GPM the smaller unit might not be adequate for a second simultaneous shower. Seems the ratings are kind of nebulus. Sounds like one honking big unit is the best solution.

[Chris Mahmood]

Another possibility is that some models (the Takagi TK-3 is one but I'm sure there are others) can be connected in series so that the second unit only fires when the demand exceeds what a single unit can provide. A friend has a setup like this for a 6-plex he owns and it seems to work really well. If yours is one of these you could always add a second later on if you needed it but having the heaters close to high-use faucets like sinks really makes a big difference in the amount of cold water you waste if you aren't using a recirc pump.

[Glenn Vaughn]

Thanks for all the great feedback. Both models have the same energy factor (.82) and identical activation flow rates (.4 GPM). Another possibility is, since I have a two story house, is to get two smaller units to eliminate long water lines. One for upstairs and one for downstairs. However, the point Glenn brought up about taking longer to fill a tub makes me concerned that situation might cause a problem if someone is taking a shower in one bathroom while someone is filling a tub in the other. The large unit is rated for 3 bathroom homes and the smaller one for 2 to 3 bathroom homes. However, reading the fine print on the specs shows that rating is for simultaneous showers using 2.5 GPM (dependent on cold water temperature) so if someone is filling a tub which I assume uses more than 2.5 GPM the smaller unit might not be adequate for a second simultaneous shower. Seems the ratings are kind of nebulus. Sounds like one honking big unit is the best solution.

I( assume that the ratings of the 2 units you are speaking of are for a specific temperature rise - the smaller unit delivering 5 GPM and the larger delivering 7.5 GPM. The documentation will tell you what the rise they are talking about is.

Determing if a unit will work well is complex. Here is a link to a pretty good explanation of what is involved: http://www.tanklesswaterheaterguide.com/.

The important thing to remember is the wider the temperature spread betrween the incoming cold water and the heated hot water, the lower the gallons per minute of hot water you will get. The tankless heaters control the flow of the water to maintain the desired temperature. Reducing the amount of water at the faucet does not affect the flow from the heater until the demand is less than the flow being provided by the heater. I suspect thaat where you are the rise will need to be less than here in Colorado where the water supply is from snow melt.

The Energy Factor rating is a rating of the efficiency of the unit use of fuel - not an estimate of savings. A unit that burns twice as much fuel to produce 200000 BTUs as a unit that produces 100000 BTUs will have the same efficience factor.

If the water flow is at the maximumk the unit can provide for a given temperature rise, it will be burning the maximum amount of gas. Savings come in when the demand from the faucet is less than the maximum flow for the temperature rise - the heater needs to burn less gas to heat the water.

The concern of running a shower and filling a tub is valid but the effect is not like it would be with a tanked water heater. The result is reduced flow - not reduced temperature. Faucets are usually not a problem other than a tub.

One last thing to consider is the temperature drop in the delivered hot water caused by the cooling effect of the water lines. The further the run, to more cooling. The temperature at the heater has to be adjusted up to compensate for any drop. In my case we are seeing about a 5 - 7 degree drop from the heater in the basement to the kitched but only a 1 - 2 degree drop in the bath rooms (which are much closer).

[Michael Weber]

Great info Glenn. thanks a bunch. I did not realize that they would throttle back on the amount of water once they could not maintain the desired setpoint.

[phil harold]

another way to avoid the throttle problem is to temper your water first
this can be done with gray water heat exchanger which would help with showers but not filling a tub
a water tank before the tank-less heater, cold water comes into the tank and get close to room temp before heading to the tankless

I Installed a 80 gallon marthon electric water heater super insulated
turned of the breaker off by accident but did not notice till 3 days of showers later...

some other options

http://www.ecohomemagazine.com/green-products/think-tank.aspx