Tankless Water Heater Venting

[crokett™]

my sister has a natural gas WH that the tank is starting to leak. she is considering tankless. I know there's a vent already for the existing WH, I believe it is 4". The room that the tankless would go in is about 7'x12'. Is this large enough to provide combustion air to use a power-vent heater? We could do direct vent but I want to avoid cutting another hole in the wall of her house if we can. Does the tankless typically require more gas (bigger pipe) and/or different pressure regulator than the tanked?

[Bob10]

any chance you can mount it in an exterior wall so it is direct vent?

[bottd]

Can't answer all of your questions but, yes when firing they do burn more gas than a tank heater. Whether or not that means you need to repipe or replace the regulator depends on the specifics of what other gas appliances she has, BTUs of the old water heater compared to BTUs of the tankless heater and what size and length of run of the pipe is there right now.

Just for something to do just did a quick check - Looks like most tankless are running well north of 100,000 BTUs with many of them close to 200,000 BTUs on natural gas, while my 50 gal runs at 40,000 BTUs. Would not be surprised if larger pipe would is needed.

[crokett™]

Bob, we could. Is that enough of an improvement to put a hole in the wall?

There's a condensate drain there already for some reason, I Can't remember why. I don't think it's being used but I will have her clean it out.

This thread may be moot, she's now thinking just replace with a tanked model again. She's not sure she will be in the house long enough to get back the extra expenses.

[Bob10]

The unit we installed bolted between the studs vented outside so all the was required was supply lines and an electrical outlet

[geek2me]

The room that the tankless would go in is about 7'x12'.  Is this large enough to provide combustion air to use a power-vent heater?  We could do direct vent but I want to avoid cutting another hole in the wall of her house if we can.   Does the tankless typically require more gas (bigger pipe) and/or different pressure regulator than the tanked?

Generally speaking, a properly sized tankless heater will not require a larger gas line.  Installed cost is usually about the same as a tank heater, give the increased cost of tank heater due to the additional safety requirements.

As a reference, I installed a 42KBTU tankless for a family of four and a 2200 +/- square foot house, in a winter area (subzero winter temperatures).  Cost less than $1000 installed, works great, energy savings is in the 40% range.  I would not want to go back to the lower comfort and higher costs of a tank unit.

[crokett™]

Thanks Terre. I came to that conclusion also on needing someone more experienced than me. All my water heater experience is with electric ones, including some smaller single-point of use tankless. She's decided to just replace it with one with a tank.

[Bob10]

the exterior unit was pretty easy IIRC 1" supply for gas, 3/4" for water and a dedicated 15 amp circuit

[geek2me]

Yes, they burn more fuel than a tank type water heater.  Tank type = 40k BTUs.  Tankless = 180k BTUs.  No, the room cannot provide combustion air.

Also, the flue piping will likely be 6" or 8" and the combustion air piping will likely be 6".  You're going to have to cut a larger hole.  They have concentric vent kits for these, also.

Also....it is probable that the current gas pipe size is undersized.  This could be remedied possibly by a couple of ways- increase the supply pressure at the meter, which will require new regulators at each appliance along with regulator venting (usually 3/8" soft copper to the outside) or have a new line run to the water heater that is medium pressure from the meter.

It sounds as if you need a fairly experienced contractor to install this.

[For a 180K btu unit, probably does need a larger vent and gas pipe - but that is really, really huge for a residential unit. ]
Edit - misremembered size of my unit, 180K is not that "huge".  Sorry for the confusion.

 The 4.2 gpm I installed is a more typical size for residential use, at least in this area.  Winter temperatures get down to 0 F or so, but if one lived in a colder area, a higher BTU unit might be needed.  In addition, if one wants to run more than two fixtures simultaneously, a larger unit would be needed.  Cost wise, installed cost was a little less than a tank heater - so it was pretty much a no-brainer.

A lot of contractors seem to oversize tankless heaters, not really sure why.  The important information is minimum incoming water temperature, volume of water consumption, and the temperature rise the heater can provide at that flow rate.  For example, our minimum incoming water temperature is around 43 F, our heater provides a 60F temperature increase at 2.5 gpm - so we can easily get 103F water (which will feel hot, anything higher than body temp will feel warm to hot) when running two 1 GPM fixtures.  Typical 1 GPM fixtures include a shower head, so bottom line is we can have two hot showers going at the same time.

[blackhat]

For a 180K btu unit, probably does need a larger vent and gas pipe - but that is really, really huge for a residential unit.  The 40 Kbtu unit I installed is a more typical size for residential use, at least in this area.  Winter temperatures get down to 0 F or so, but if one lived in a colder area, a higher BTU unit might be needed.  In addition, if one wants to run more than two fixtures simultaneously, a larger unit would be needed.  Cost wise, installed cost was a little less than a tank heater - so it was pretty much a no-brainer.

A lot of contractors seem to oversize tankless heaters, not really sure why.  The important information is minimum incoming water temperature, volume of water consumption, and the temperature rise the heater can provide at that flow rate.  For example, our minimum incoming water temperature is around 43 F, our heater provides a 60F temperature increase at 2.5 gpm - so we can easily get 103F water (which will feel hot, anything higher than body temp will feel warm to hot) when running two 1 GPM fixtures.  Typical 1 GPM fixtures include a shower head, so bottom line is we can have two hot showers going at the same time.

Redo your math starting with the definition of a BTU and the mass of water.  40k BTUH won't come close to matching your claims even at 100% efficiency.

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Yes, they burn more fuel than a tank type water heater.  Tank type = 40k BTUs.  Tankless = 180k BTUs.  No, the room cannot provide combustion air.

Also, the flue piping will likely be 6" or 8" and the combustion air piping will likely be 6".  You're going to have to cut a larger hole.  They have concentric vent kits for these, also.

Also....it is probable that the current gas pipe size is undersized.  This could be remedied possibly by a couple of ways- increase the supply pressure at the meter, which will require new regulators at each appliance along with regulator venting (usually 3/8" soft copper to the outside) or have a new line run to the water heater that is medium pressure from the meter.

It sounds as if you need a fairly experienced contractor to install this.

The largest residential tankless use 3" intake and exhaust.  

Use the tables in the appliance install manual to determine vent size based on equivalent lengths and code acceptable termination location.