propane on-demand dwh

[chuck172]

anyone here using an on-demand propane water heater?
I wonder if that might be a good option vs. running the tarm every 3 or 4 days during the summer.

 

[jimbom]

I examined one closely. I recommend you read the online installation manual and operation instructions for the model you are considering. The input water temperature may be an issue. Some do not work if the incoming water is above a certain temperature. I did not buy since I was considering it to feed my radiant setup and the return water would have been above allowable temperature.

I did use these things when I lived overseas. Typically, one would be on the wall in the shower and one on the kitchen sink. Most were propane, but one was electric. They worked, but really reduce flow. I guess that was good from the energy savings point of view. But after being on the construction site all day, I really wanted a rip roaring shower, not a strong dribble. The modern models probably do not have the flow reduction problem.

 

[DaveBP]

The most widely available models (like you see displayed at Low's Depot) are designed to heat cold water. That is their shortcoming for hydronic heating. The return water is not cold. They need cold water to absorb the heat from that big flame through a not-so-big heat exchanger.

For heating a radiant slab it might work fairly well. But with typical baseboard it can be disappointing. The return water can be hotter than the unit is designed to supply.

For more money, the units that can modulate their flame to match the load can work better. They will often mention non-DHW heating applications in their advertising. I know Takagi makes models intended for dual use and I think Rinnai does also. I'm sure there are others but the Takagi TK-3 is the one I use. But mine isn't connected as a backup heater yet, so I can't give any testimonials about how well it works. That will be next winter.

Either way, the lower the temperature of your return water to the unit, the closer it will come to the advertised efficiency.

 

[chuck172]

I'm interested only in DHW to use in the summer as opposed to firing up the tarm every 3 or 4 days, or burning oil to heat my superstor water heater. I wonder if the units are as cost saving as I heard. Propane is expensive I know.

 

[DaveBP]

I’m interested only in DHW to use in the summer as opposed to firing up the tarm every 3 or 4 days, or burning oil to heat my superstor water heater.

That makes it a lot simpler. I went with the propane unit because we occasionally need lots of hot water (washing wool) that a standing tank style water heater couldn't keep up with. And because of that it became my backup heat plan as well.

If you don't have need for endless hot water (like not having a bunch of teenagers taking showers for hours every morning) I think a good large electric tank-style water heater would be much cheaper to buy and install from scratch but you say you have an indirect already.

Price out a Bosch (Aquastar) propane tankless unit and its gas supply piping and venting and shutoffs, etc. and a circulator and control thermostat (maybe use what is in the indirect already?) and see if you want to spend that much money. Use it to heat the indirect whenever the thermostat calls for heat. It would be very convenient, but it's going to be expensive.

 

[jimbom]

I’m interested only in DHW to use in the summer as opposed to firing up the tarm every 3 or 4 days, or burning oil to heat my superstor water heater.

...

Price out a Bosch (Aquastar) propane tankless unit and its gas supply piping and venting and shutoffs, etc. and a circulator and control thermostat (maybe use what is in the indirect already?) and see if you want to spend that much money. Use it to heat the indirect whenever the thermostat calls for heat. It would be very convenient, but it's going to be expensive.

That is what I found. Once the entire Aquastar installation was priced, I recall it was very expensive for my situation.

 

[Rick Stanley]

Oh no. Now you got me wondering. I just ordered an electric 40 gal water heater intending to pre-heat up to 100 or some degrees with wood and let the electric heat it the rest of the way up to 120 or whatever. Now I'm reading here about heaters being made to heat "cold" water. Is my plan not going to work??
Thanks!!

 

[ewdudley]

Oh no. Now you got me wondering. I just ordered an electric 40 gal water heater intending to pre-heat up to 100 or some degrees with wood and let the electric heat it the rest of the way up to 120 or whatever. Now I'm reading here about heaters being made to heat "cold" water. Is my plan not going to work??
Thanks!!

A tank electric wouldn't have the same problem as an on-demand NG/LP unit with preheated input, but there still could be a problem. Read the installation/operating documentation carefully to see what they have to say about it.

You wouldn't think there could be a problem, but with mine there were two prohibitions, one against feeding too-hot water in from the top and another against feeding not-so-hot water in to the bottom. As I recall 100 degF into the bottom wouldn't be a problem for mine.

I believe the too-hot-top might have to do with setting off the safeties and they claimed that too-hot-bottom input would confuse the hell out of the new super-duper micro-controller green-living we-know-when-you-need-hot-water-before-you-do control unit.

--ewd

 

[Hunderliggur]

I have a Rinnai 94 and have been using it for radiant heat and DHW for 3 years. The first year was 100% Rinnai. The second year I added the boiler but no HX (the 1000 gallon storage was a big radiator in the basement). This year I had the HX from storage but still piped with the Rinnai, no problems, it has worked pretty well. The Rinnai warranty is only one year when used for heating (versus 3 years for DHW). I have not had any problem with warm (100F) input to the boiler. The Rinnai modulates the flame based on the target output temperature and will reduce flow if it cannot meet the temp. The only issue I had was in the fall with a dead cricket in the blower cage.

 

[Gary_602z]

Oh no. Now you got me wondering. I just ordered an electric 40 gal water heater intending to pre-heat up to 100 or some degrees with wood and let the electric heat it the rest of the way up to 120 or whatever. Now I'm reading here about heaters being made to heat "cold" water. Is my plan not going to work??
Thanks!!

I have electric tank style that I am preheating the input with a 30 plate water to water hx works great.

Gary

 

[Rick Stanley]

Oh no. Now you got me wondering. I just ordered an electric 40 gal water heater intending to pre-heat up to 100 or some degrees with wood and let the electric heat it the rest of the way up to 120 or whatever. Now I'm reading here about heaters being made to heat "cold" water. Is my plan not going to work??
Thanks!!

I have electric tank style that I am preheating the input with a 30 plate water to water hx works great.

Gary

Thanks Gary,
I needed to hear that. Picking the heater up today.

 

[in hot water]

most of the tankless water heater brands have models that can be used with pre-heated water. Tankless style are commonly used with solar pre-heated water. Here is some info from one of the online tankless sites. www.abouttanklesswaterheaters.com

hr

 

[willworkforwood]

Oh no. Now you got me wondering. I just ordered an electric 40 gal water heater intending to pre-heat up to 100 or some degrees with wood and let the electric heat it the rest of the way up to 120 or whatever. Now I'm reading here about heaters being made to heat "cold" water. Is my plan not going to work??
Thanks!!

A tank electric wouldn't have the same problem as an on-demand NG/LP unit with preheated input, but there still could be a problem. Read the installation/operating documentation carefully to see what they have to say about it.
You wouldn't think there could be a problem, but with mine there were two prohibitions, one against feeding too-hot water in from the top and another against feeding not-so-hot water in to the bottom. As I recall 100 degF into the bottom wouldn't be a problem for mine.
I believe the too-hot-top might have to do with setting off the safeties and they claimed that too-hot-bottom input would confuse the hell out of the new super-duper micro-controller green-living we-know-when-you-need-hot-water-before-you-do control unit.
--ewd

I have a 40 gal Superstor indirect, and really appreciate the "free" DHW during the entire heating season. It's too bad that you have to use electric at all, when your boiler is capable of easily doing the entire job. So, the controller can't be overridden (or reprogrammed) to go into a quiet mode and function as an indirect when the water in from the boiler is above a setpoint, and then revert to standard operation when input is below setpoint-diff? And, assuming no, then how about an external temp-driven switch for cutting power to the unit?

 

[ewdudley]

A tank electric wouldn't have the same problem as an on-demand NG/LP unit with preheated input, but there still could be a problem. Read the installation/operating documentation carefully to see what they have to say about it.
You wouldn't think there could be a problem, but with mine there were two prohibitions, one against feeding too-hot water in from the top and another against feeding not-so-hot water in to the bottom. As I recall 100 degF into the bottom wouldn't be a problem for mine.
I believe the too-hot-top might have to do with setting off the safeties and they claimed that too-hot-bottom input would confuse the hell out of the new super-duper micro-controller green-living we-know-when-you-need-hot-water-before-you-do control unit.
--ewd

I have a 40 gal Superstor indirect, and really appreciate the "free" DHW during the entire heating season. It's too bad that you have to use electric at all, when your boiler is capable of easily doing the entire job. So, the controller can't be overridden (or reprogrammed) to go into a quiet mode and function as an indirect when the water in from the boiler is above a setpoint, and then revert to standard operation when input is below setpoint-diff? And, assuming no, then how about an external temp-driven switch for cutting power to the unit?

I guess we're drifting off-topic, but anyways, I invested in an extra Taco 500 mixing valve. One is used to temper DHW supply to the house and the other is set up in diverting configuration such that if the water is above 125 degF or so it bypasses the electric tank goes directly to the house, otherwise it goes to the electric tank inlet and the electric heater brings it up to temperature from there. I don't have the operating manual in front of me but I believe 125 degF is low enough not to violate the manufacturer's inlet limit, or at least it doesn't violate it by very much.

During the heating season the electric heater is turned off. During the summer the electric heater will be turned back on again, but it will only be used to make up the difference when I let the storage temp fall below DHW setpoint.

 

[whippingwater]

anyone here using an on-demand propane water heater?
I wonder if that might be a good option vs. running the tarm every 3 or 4 days during the summer.

Chuck 172, I was thinking the same route as you, but I guess we will have to get the answers ourselves. I'm researching a solar water heater or propane tankless for summer.