[Greenbuilding] Max Temp Storage Water Heaters+TemperatureSensors

[elitalking]

Thanks for the responses for Max Temp Storage Water Heaters+TemperatureSensors

 

Steve Satow writes:

Eli, this begs the question: if you are already replacing the water heater, why not get an on-demand electric tankless system that supplies water to the bathroom only when you want it?  More compact than any tank so you can put it somewhere convenient and it would eliminate virtually all the unavoidable standby losses associated with a tank?

 

My response

 

I reviewed your suggestion for online electric on demand tankless systems.  We are on the same concept of heating the water when it is needed instead of storing it with standby heat losses.  Instantaneous heater with a low flow shower is likely the most efficient way to go.  However, I am not ready to give up my hot bath.  I found a range of amp size water heaters for Rheme as a representative of the performance I can expect.  http://globalimageserver.com/fetchDocument.aspx?id=b125d219-d5f8-46c3-ab02-5934e3043c7d

The limitation is how much the water temperature will be raised above the starting temperature at various flow rates.  If starting at a ground temperature of 55F it would take a rise of 55F to achieve 110F bath.  2.1gpm for RTE Model 13 will raise 50F.  The unit requires a 240v at 60amp breaker.  This could work if I was building a new house.  However, I have recently removed a heat pump, making two wires already wired to breaker box available for new service.  Installing a new line to breaker box is a significant increase of effort.  I would like to use a 30amp (old compressor) or 50amp circuit (old air handler with heat element backup).  The preference is for use of 30amp wire that can reach the new location next to tub.  I want to use 50amp circuit for new breaker box for potential expansion.  However, with a junction box, I could use that wire.  

My intention is to heat the water when it is needed, and not store hot water that has to be reheated.  Instantaneous heaters fit this concept.  

 

Perhaps water could be stored in a un-insulated tank that would be warmed from 55F to room temperature of 70F.  That would then only require a boost of 40F to achieve 110F.  RTE9 (220v x 40amp) can achieve this with 2.2gpm flow.  In this case the tank would not be insulated at all to use the room to preheat the tank.  Make no mistake, this will take heat away from the room that must be made up by the air heating system.  If heating with a heat pump, this has COP of that system.  Currently we are primarily using wood heat.  I am uncertain of the recovery rate would be for this preheat tank.  

 

On this list I here that heat losses from tank or other appliances are captured as a benefit for heating the house in the winter.  This is only a COP of 1kwh heat for 1kwh electric consumption.  This list knows that electricity generally takes about 3kwh of fuel to deliver one kwh.  This limited benefit for heat loss only applies in the winter.  In the summer the hot water heat loss is a double liability.  If the house is cooled in the summer, the cost of removing that heat loss is more than a kwh/kwh heat loss.  In my case, since I removed the heat pump, it is not energy consumption, it is comfort.  If I am replacing  room temperature water 70F with ground temperature water of 55F, it will have a slight cooling benefit for comfort in the summer.  

 

However, the main concept is to heat water up when needed and to not store it.  In our current hot water tank location, we are serving the kitchen and the bathroom that are separated by about 30'.  The tank is immediately over the kitchen sink.  We would sometimes allow the water to reheat while we took our bath.  As is pointed out, the savings from the gradual lowering of temperature may be negligible.  However, if that stored water is consumed in the kitchen, then the heat loss is completely halted.  In fact when ground temp water replaces the consumed hot water, the heat flow reverses direction.  This is a preheat before switching heater back on.  This allowed us some water for kitchen consumption.  

 

Our current approach is to separate the kitchen and bath to produce hot water in those locations only when needed.  I do not consider a slight delay at time of use to be an extreme hardship.  We will heat up water for tea or coffee at the time of use instead of storing continuously reheated water.  I think the instantaneous heater would work well in the kitchen.  We currently use a basin instead of the full kitchen sink which reduces the total water needed.  I am considering plumbing a really small un-insulated tank (1/2gal) with no intentions of storage to be heated by a induction stove top.  In this way, I can within a few minutes have adequate warm water for sink basin for dish washing water.  While washing the dishes in basin, the heater can be quickly be re heating up the next batch for rinsing.  In this way, we are continually resupplied with hot water until we are done.  Then the induction stove top is switched off and the small tank returns to room temperature waiting for its next use with no standby heat losses.  Until I can get something like that together, I am not averse to setting the basin of water on our current induction stove top to heat the water.  I have this list to thank for making me aware of this great fast heating, efficient and very responsive technology. 

 

The instantaneous heaters can do this also.  However to be instantaneous requires a larger wire.  Using an induction stove or possibly a microwave (if someone can work through those issues) to heat water on demand but not instantaneously, allows for a simple 120v receptacle to meet the need.  This greatly increases the flexibility to heat water at many locations.  This also avoids the heat losses from long pipes that connect one central heater serving the whole house.  

 

For us, the bathtub is the most demanding hot water use.  Perhaps the hot water of a 30amp 220 volt instantaneous heater for hot bath could be boosted to desired temperature by a circulating water heater such as a whirlpool.  Is there such a product for heating water in an open pool such as a tub?  This would allow for mixing of creek water to supplement cistern water during draughts.  Could this possibly be an alternative to pressurized water heater for bathing?

 

Long story short, heat only the quantity water needed at the location when it is needed.  



Again, thanks for input.  I hope for more feedback.  

 

Eli  
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