Instantaneous water heaters and required service

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Several years ago, I was hired by a tankless water-heater company to wire heaters they sold. They stopped calling me after the first two, because I told them and the homeowners that service upgrades were needed.

The second one was ridiculous. All-electric home, 200a MB panel stuffed full, with two 100a breakers: one to a full basement apartment, the other to a 4-car garage with an apartment above it, both with full kitchens.
 
kwired said:
If you change from gas to electric then yes electric should go up, but at same time gas should go down. How severely can depend on usage habits, rates, etc. In theory the tankless should use less energy overall (gas or electric) because as long as things are functioning properly should nearly eliminate idle time losses. Now if in a location where heating systems run more than cooling systems, heat lost by tank type heater isn't totally lost and does lessen heating load. In location where cooling runs more than heating it is the other way around though and tank losses increase cooling load.
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Before the recent run up in petroleum prices, nat gas was 1/16 the cost of electricity on an energy-equivalent basis for me. I think it’s about 1/5 now.
Point is, it’s likely the electric bill went up by a lot more than the gas bill came down.
 
But in areas, like mine, where nearly everyone is on a TOU billing system, chances are good that you are going to be using hot water at the worst times of day. Around here, peak time (4-9PM) usage rates are now $0.49/kWh! Even the off peak summer rate is $0.34/kWh if you manage to stay below your baseline, $0.43 if you don’t.
 
hbiss said:
With the way things are going gas isn't going to be an option much longer. Already here, no new gas services are allowed to be installed. I suppose you could use propane, but I don't know if it's any savings over electric.

-Hal
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Yeah, the same thing is happening here, but it only applies to new homes being built (after 2025 I think). I have a feeling that my gas hookup is going to become a major asset once that fully kicks in.
 
retirede said:
Before the recent run up in petroleum prices, nat gas was 1/16 the cost of electricity on an energy-equivalent basis for me. I think it’s about 1/5 now.
Point is, it’s likely the electric bill went up by a lot more than the gas bill came down.
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The fees, minimum, and taxes make nat gas cost way more than people think around here, probably still cheaper than electric for heating air or water
 
readydave8 said:
The fees, minimum, and taxes make nat gas cost way more than people think around here, probably still cheaper than electric for heating air or water
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Just swapped out my 40 year old gas water heater, last month with the old WH spent $20 for hot water (kitchen is all electric), this month was about $13, both were 40 gallon nat gas. Two months are not the full picture but gives a good idea. Due to the amount of lime scale in the old one it's quieter too, & had to affect the heat transfer from the flame too.
 
retirede said:
Before the recent run up in petroleum prices, nat gas was 1/16 the cost of electricity on an energy-equivalent basis for me. I think it’s about 1/5 now.
Point is, it’s likely the electric bill went up by a lot more than the gas bill came down.
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Well, since a lot of electric utilities decided to convert coal-fired plants to gas, that's hardly a surprise.
 
The problem with _tankless_ water heaters is the huge instantaneous load they present.

A standard gas service can trivially supply this load, with electric you need a huge service.

A storage (tank) water heater might be less efficient (because of storage losses) but the benefit is much lower peak loading.

A storage heat pump water heater is more efficient then either a standard tank or an instantaneous electric.

A fun exercise is figuring out an instantaneous heat pump water heater. The thing would be huge.

-Jon
 
winnie said:
The problem with _tankless_ water heaters is the huge instantaneous load they present.

A standard gas service can trivially supply this load, with electric you need a huge service.

A storage (tank) water heater might be less efficient (because of storage losses) but the benefit is much lower peak loading.

A storage heat pump water heater is more efficient then either a standard tank or an instantaneous electric.

A fun exercise is figuring out an instantaneous heat pump water heater. The thing would be huge.

-Jon
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If the heat pump type is taking heat from the air inside your home, it does help with your cooling load in summer, but it is taking heat your heating system already placed in the room to use to heat the water so it will increase heating load regardless what type of heating you have.
 
kwired said:
If the heat pump type is taking heat from the air inside your home, it does help with your cooling load in summer, but it is taking heat your heating system already placed in the room to use to heat the water so it will increase heating load regardless what type of heating you have.
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The thought is to have a heat pump capable of heating the entire house, drawing heat in from outside. But when you want hot water the system focuses its output to the water heater.

This might be plausible for the systems that use heat pumps to provide hot water for hydronic radiators. But I don't think the numbers really work.

-Jon
 
winnie said:
The thought is to have a heat pump capable of heating the entire house, drawing heat in from outside. But when you want hot water the system focuses its output to the water heater.

This might be plausible for the systems that use heat pumps to provide hot water for hydronic radiators. But I don't think the numbers really work.

-Jon
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A heating/cooling heat pump where you can draw some heat from it to heat domestic water, yes. That is more common to see with geothermal systems than most typical air to air systems, at least in northern half of the country AFAIK. A stand alone heat pump water heater not so much, unless it either has a remote evap coil or a method of piping heat from whatever source is to the evap coil, like they do with the ground loops in a geothermal system.
 
Electric on demand water heater from everything I've looked at not really practical in the nothern states. It will require nearly as much power as a typical entire service to be able to raise the water temperature to normal hot water levels of about 120 deg from the ambient ground temperature. Gas is the way to go if you need "on demand" hot water in the nothern 3rd of country.
Rheem sizing for the nothern states indicate needing 150A of water heater to get a "normal household" enough hot water, wheras Florida and similar areas you can get by with just marginally more than a standard tank type at about 45A.
So for electric "on demand" is just like real estate it all about "location, location, location".
Another consideration, these type of water heaters don't play well with other efficiency things like the water misers to reduce the water use. The reduced flow has been noted to stop the heater from coming on and heating. Had a customer complain that the water heater intermittently stopped working, but if you open 2 taps it would run. Found they put on the low flow water misers on the kitched and shower head, cutting flow just below the flow threshold of the heater with only one source open.
 
Fred B said:
Electric on demand water heater from everything I've looked at not really practical in the nothern states. It will require nearly as much power as a typical entire service to be able to raise the water temperature to normal hot water levels of about 120 deg from the ambient ground temperature. Gas is the way to go if you need "on demand" hot water in the nothern 3rd of country.
Rheem sizing for the nothern states indicate needing 150A of water heater to get a "normal household" enough hot water, wheras Florida and similar areas you can get by with just marginally more than a standard tank type at about 45A.
So for electric "on demand" is just like real estate it all about "location, location, location".
Another consideration, these type of water heaters don't play well with other efficiency things like the water misers to reduce the water use. The reduced flow has been noted to stop the heater from coming on and heating. Had a customer complain that the water heater intermittently stopped working, but if you open 2 taps it would run. Found they put on the low flow water misers on the kitched and shower head, cutting flow just below the flow threshold of the heater with only one source open.
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I am not sure whether it is actually done in practice, but in theory an electric heating element can be throttled down for low flow rate well beyond what can safely be done with a gas unit.

(In theory there is no difference between theory and practice. In practice, however....)
 
I'm surprised that an electric unit has such a problem with moderate flow. My gas unit easily handles a single hand wash sink (with flow restrictor), and I can dial my shower way back before I hit the low flow limit.

But when I do hit that low flow limit it sucks because there are hysteresis and my shower is freezing for a while.

Jon
 
I've said it before, seems like the best way to use this would at least be to have a storage tank that is either non heated or only heated to about room temperature or not much above room temperature, then run that water through the on demand heater with that higher incoming water temperature. Would lessen kW needed to raise the temp to final use temp.
 
kwired said:
I've said it before, seems like the best way to use this would at least be to have a storage tank that is either non heated or only heated to about room temperature or not much above room temperature, then run that water through the on demand heater with that higher incoming water temperature. Would lessen kW needed to raise the temp to final use temp.
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I think their real utility is in bathrooms for the sink, usually some public restroom where you can have short periods of high demand and long lulls. Think movie theaters and the like. A storage tank large enough would be huge and standby losses might be significant, whereas the demand at each individual sink would be relatively low, but pretty constant when the theater lets out, for 20-30 minutes.
 
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