Tankless Water Heater Venting

[doc1]

I had one installed about ten years ago, and the biggest change was that the exhaust vent had to be a straight shot up through the roof.  The previous tank water heater had two bends in the vent, and it was not allowed.  This is California.

[Bob10]

well I would suggest you check your math but to be honest I don't think you did or will

[geek2me]

well I would suggest you check your math but to be honest I don't think you did or will

Well, I know it works and we get hot showers. 

Well this is embarrassing.  My recollection was that the 42 in the model number referred to the Kbtu.  When I looked at the manual, it actually refers to 4.2 gpm.  Oops.

...Nameplate says it is 118 kbtu max, and it used the same gas line as my 40 gallon tank heater.  Vented it through the wall with a single double walled pipe.

[woodworker2000]

A high-efficiency/condensing tankless heater can use a single concentric pipe for intake and exhaust  The exhaust gasses are "recycled" (i.e. additional heat extracted) before they exit the unit so the intake/exhaust pipe can be PVC instead of stainless pipe.  We replaced the water heater in our basement (direct vent to the outside) with a high-efficiency/condensing Rinnai tankless unit a few years ago.  The hole in the basement wall for the water heater vent was re-used for the tankless intake/exhaust vent.  We did need to have a larger gas line installed (easy enough since the gas meter was outside and within 10 feet of the tankless unit) as well as a condensate drain (drilled a hole through the basement wall above grade and ran a pipe).  You can get more information on the Rinnai website:  Rinnai Tankless Water Heaters

[MikeBob]

Depending on family size and usage, water quality, the payback for a tank less is about 15 years, about there life time!!! They should be acid cleaned and serviced about every year. They have a minimum water flow thru them b4 they fire up, they use more BTUs to take a bath or shower then a tank heater that is on ready stand by that is at temp all day.
Example, my bro has one in the garage in CA, 2 story house, bathrooms in the upper level, at the opposite end of the house, shower time, I have to run water for almost 4 minutes to get hot water for the shower, wasted water, then wait for hot water.
Put in a 40 gallon tank with a recycling hot water pump on it, that uses almost nothing in electricity, instead hot water at the source almost instantly, no water waste. Cost for all will be way less then a tankless that takes 15 year cost return and by that time it needs to be replaced so no cost savings. Plus annual servicing to keep it efficient.

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What  I did at my bros after my experience out there with his tank less water heater and the kitchen, laundry room and a half bath is the closest to it, and wants instant hot water for just washing hands, some hand dish washing ect.
I put in a 5 gallon electric water heater tank b4 the the tankless water heater, so if they need just a bit of hot water in the kitchen, half bath, it flows through the tankess, temp sez fine and does not fire up, after a water flow. Then on high demand they have hot water, then the tankless fires up, AFTER the instant hot water runs out of the electric heater but over all is more convenient. Just made the best of a bad situation.

[geek2me]

MikeBob
Depending on family size and usage, water quality, the payback for a tank less is about 15 years, about there life time!!!


It does depend on family size, usage, and geographic location (weather), but in my experience the payback is much less than 15 years, and the lifetime of a tankless is more than 15 years.

The tankless I installed cost about 20% more than a tank heater, due to the venting changes.  With careful measurement and analysis, I determined my energy savings averaged 43% with the tankless, which made my payback around two years.

They should be acid cleaned and serviced about every year.

My manufacturer recommends cleaning every five years, and cleaning the reusable filter every year.  No other servicing is required.

They have a minimum water flow thru them b4 they fire up, they use more BTUs to take a bath or shower then a tank heater that is on ready stand by that is at temp all day.

They do have a minimum flow, but it is so low most users will never notice.  A tankless uses much LESS energy to take a shower or bath.  The same amount of energy is required to heat a gallon of water to a given temperature - and that is all the tankless uses.  A tank heater uses that, plus all the energy savings required to reheat the water to maintain that heat for hours on end.  That's why energy savings with tankless heaters are so high, you are only paying for the hot water you actually use.

I have to run water for almost 4 minutes to get hot water for the shower, wasted water, then wait for hot water.

Sounds like something is wrong with that install.  With tank or tankless, you still have to flush the colder water from the house lines before you will get the full hot water.  With a tankless heater, you will have to wait another 20 seconds or so before you will get the full hot water, as the heater has to flush the cold water in the tankless heater and it takes 10 to 20 seconds to start the burner.

Put in a 40 gallon tank with  a recycling hot water pump on it, that uses almost nothing in electricity, instead hot water at the source almost instantly, no water waste.

A recycling pump will provide instant hot water with either a tankless or tank heater.  However, the pump will further increase the energy usage due to the need to constantly reheat the water, for both types.  It will diminish the energy savings of the tankless, but given the significant increase in energy for a tank heater with a pump, the tankless will still be much less expense to operate.

Bottom line, unless you live in a warm climate where the standby losses for a tank heater are minimal (e.g. like Phoenix AZ), a tankless heater will save cost and have a fast payback.  In climates with winter temperatures, it's essentially a "no brainer", tank heaters are so much more expensive to operate that it make no sense to install one.