105 replies to this topic

[BrewerGeorge]

I would buy a 120v hybrid gas water heater if I could, but fully 240V electric isn't going to cut it.  Water and gas are on the inside wall of the garage, but 240V is on the outside wall with the panel.  Getting 240V across the garage would be a big deal to the point where digging a trench across the garage floor is probably the easiest way.

[positiveContact]

what's the right metric for comparing operating costs of different kind of hot water heaters?

 

I believe I pay $0.70/therm for natural gas

 

My electricity is about $0.24/kwh but I think it is about to drop to about $0.21/kwh.

 

 

So how do I compare a conventional gas hot water heater with one of these heat pump systems in terms of operating cost?  I could look at the summer months to get a pretty good idea about how much gas I use just for hot water.  In the winter I also use gas to heat the house so that would muddy things up. 

 

like is there a way to equate one therm into a gas hot water heater with one kwh into on a heat pump water heater?

Edited by postSingularityHumanoid, 26 January 2023 - 07:31 AM.

[SnowMan]

like is there a way to equate one therm into a gas hot water heater with one kwh into on a heat pump water heater?

 

 

Yes.  BTUs

 

1 therm of gas is  equal to 100,000 BTU of energy.

 

How many BTU you get from 1 KWH is going to depend on the heat pump's performance, efficiency, etc.  It's probably in the specs somewhere. 

[positiveContact]

Yes.  BTUs

 

1 therm of gas is  equal to 100,000 BTU of energy.

 

How many BTU you get from 1 KWH is going to depend on the heat pump's performance, efficiency, etc.  It's probably in the specs somewhere. 

 

is that conversion for gas pretty consistent across the board?  I guess there isn't a way to "get more out of it" and it's so simple everyone does it about equally well?

[SnowMan]

is that conversion for gas pretty consistent across the board?  I guess there isn't a way to "get more out of it" and it's so simple everyone does it about equally well?

 

 

Well... given that the unit is defined that way... https://en.wikipedia.org/wiki/Therm

 

 

I guess the volume of gas that holds those 100,000 btu could vary ever so slightly with pressure and temperature, but not enough to require vendors to change orifices or regulators, so I can't see how it would effect your calculation to any meaningful level.  

[positiveContact]

Well... given that the unit is defined that way... https://en.wikipedia.org/wiki/Therm

 

 

I guess the volume of gas that holds those 100,000 btu could vary ever so slightly with pressure and temperature, but not enough to require vendors to change orifices or regulators, so I can't see how it would effect your calculation to any meaningful level.  

 

just to make my point you could design something really stupid that dumps heat not into the water and instead into the exterior of the tank which then just heats air around the tank or you could not insulate the tank very well.  those kinds of things would cause it to appear that X therms in do not always equal Y BTUs into the water.

Edited by postSingularityHumanoid, 26 January 2023 - 08:45 AM.

[dondewey]

Yep. That's the efficiency value snowman mentioned

[positiveContact]

Yep. That's the efficiency value snowman mentioned

 

He did?  He mentioned it with regards to the heat pump type system but not for a gas system.  and I'm not sure how to get that number for a gas system or if it really matters that much in practice.

[SnowMan]

He did?  He mentioned it with regards to the heat pump type system but not for a gas system.  and I'm not sure how to get that number for a gas system or if it really matters that much in practice.

 

For gas to water, it's closed combustion and thus very efficient.  I mean some heat goes out with the exhaust... but not a lot. For the level of comparison you're making, I don't think it matters much. 

[positiveContact]

not sure how accurate this is but...

 

https://www.danville...sense-questions

 

[SnowMan]

not sure how accurate this is but...

 

https://www.danville...sense-questions

 

 

 

Interesting.  I expected it to be much higher efficiency. Industrial boiler systems are often around 90% and condensing boilers more like 95%.  I honestly would have expected residential water heating to be in that range.  It's not a condensing application but certainly lower exhaust temperatures than a traditional boiler application. 

[positiveContact]

Interesting.  I expected it to be much higher efficiency. Industrial boiler systems are often around 90% and condensing boilers more like 95%.  I honestly would have expected residential water heating to be in that range.  It's not a condensing application but certainly lower exhaust temperatures than a traditional boiler application. 

 

yeah, I would have expected higher than that as well.  I can't vouch for this source or anything.

[Kremer]

Search for energy star rated gas heaters if you want efficiency to compare to. The standard ones are not very good.

[positiveContact]

Search for energy star rated gas heaters if you want efficiency to compare to. The standard ones are not very good.

 

it seems like the energy star ones are still <0.70

[porter]

I drained and flushed my gas water heater last week. I was impressed with how hot the flue got when heating the refilled tank vs the flue on the furnace running a fairly high duty cycle. My thought was the water heater is not very efficient (and we have a 3 year old "high efficiency" model).

[Julius H Gardetto]

I drained and flushed my gas water heater last week. I was impressed with how hot the flue got when heating the refilled tank vs the flue on the furnace running a fairly high duty cycle. My thought was the water heater is not very efficient (and we have a 3 year old "high efficiency" model).

Could be my head cold, but I’m not tracking your take away here.

[positiveContact]

Could be my head cold, but I’m not tracking your take away here.

 

the exhaust gases were pretty hot which means that heat is getting wasted.  in other words, low efficiency.

[SnowMan]

the exhaust gases were pretty hot which means that heat is getting wasted.  in other words, low efficiency.

 

 

Correct.  You want to be as cool as possible, without condensing.  That means as much of the energy as possible is going into heating the water and not out the stack

 

We don't want condensing in a residential application for a bunch of reasons... the biggest being cost of construction.  Condensed combustion gases are typically pretty acidic.  You have to build stainless stacks and a stainless drain system to handle it.  $$$$

[porter]

the exhaust gases were pretty hot which means that heat is getting wasted. in other words, low efficiency.

Yes, and the BTU output of the furnace is probably double the water heater. The furnace is 20+ years old and probably 80% efficiency.

[BrewerGeorge]

Were you judging the temp with water in it or empty?