Does a recirculating pump defeat the purpose of on-demand water heating?

[plowhog]

I'm building a new residence, about 5,600 sq ft total. The garage is on one side, great room/laundry room about the center, and bedrooms and master on the other side.

My architect planned an on-demand water heater in the garage, mainly servicing the kitchen and a bath in that area. And a second on-demand in the laundry, to service the bedroom wing and master suite.

My plumber wants to relocate the laundry on-demand heater to the garage, so the two on-demand heaters are together. Then plumb them as a master/slave unit, where only one fires if needed, or both fire if there is a lot of demand. I think he also wants to do this when a lot of temperature rise is needed, such as when the incoming cold water is ... cold ... during the winter. I am on a well.

I pointed to the plans which would not be a looooong run from the on-demand heater to the bedroom wing and master bathroom. Almost as far away as you could possibly get. He said no problem: add a recirculating pump. This would constantly recirculate the water through the piping under the house, keeping everything hot while waiting to be used. But also burning electricity and $$$ even while we are sleeping or are away from the house. The underside plumbing is in a crawl space, is PEX, and is insulated.

I asked if constantly running a recirculating pump defeats the energy-saving approach for on demand units? And there are apparently some on-demand units that are not compatible with such a pump?

My plumber says this is the best way to go. I'm not so sure? Do you have any experience or ideas that could help? I'm waiting to get the model numbers of the on-demand heaters so I can examine them in more detail.

 

[TractorNH]

The original design makes the most sense. Locating them side by side is easier for him but not the best solution. The recirc loop will essentially be a storage tank when no water is being drawn and will be less efficient both through heat loss in the pipe and also electricity to run the pump. It can be put on a timer so it isn't circulating all the time but why add that level of complexity.

If it were my house I would want the heater as close as possible to the users which is how your architect originally planned it. In the long run you'll be happier with that setup. I have 20 years experience in HVAC and plumbing so I've seen a few things....

Also, the incoming water temp will not vary that much winter vs summer. Ground temperature is fairly uniform once you get about 6' below the surface and your water comes from deeper than that.

 

[Dadnatron]

I agree... completely defeats the purpose and costs you $

 

[4570Man]

The original design makes the most sense. Locating them side by side is easier for him but not the best solution. The recirc loop will essentially be a storage tank when no water is being drawn and will be less efficient both through heat loss in the pipe and also electricity to run the pump. It can be put on a timer so it isn't circulating all the time but why add that level of complexity.

If it were my house I would want the heater as close as possible to the users which is how your architect originally planned it. In the long run you'll be happier with that setup. I have 20 years experience in HVAC and plumbing so I've seen a few things....

Also, the incoming water temp will not vary that much winter vs summer. Ground temperature is fairly uniform once you get about 6' below the surface and your water comes from deeper than that.

Not around here. Maybe 2’ for the main lines.

 

[RNeumann]

I get the master/slave approach with a 5600sf residence. It’s more of a “light commercial/multiunit” type setup. but at 5600sf you are in the category.
Running a circ pump on demand water heater isn’t ideal at all. It’s hard on the demand heaters or it adds a circ tank to keep the demand heaters from cycling.
If it was me I’d add a little 10gal water heater on that far end of the house and skip the recirc all together. The 10 gal heater (or so depending on volume in the pipe from the slave/master) would be plumbed into the slave/master setup so all it’s doing is providing near on demand hot water until the hot water from the slave/master gets there.
I also get the architect’s idea. But without seeing the plans it’s hard to see if that layout works, as far as load, between the two water heaters.

 

[radios1]

I agree... completely defeats the purpose and costs you $

I agree, but, it might not matter too much for someone that owns a mansion in California, comfort may matter more.. save petty change, or have instant hot water..

 

[KennyG]

I agree the original design sounds best. Is this building entirely one level? No basement? If it is multi-level you could add thermo-siphon loops and not require a recirc pump. I have this arrangement on a large house with the master bath at the opposite end from the water heater and it works great.

 

[ericm979]

We have a traditional water heater and a recirc pump to the master bath which is on the other side of the house. It works ok but it's using electricity and wasting heat. I'd prefer an on-demand heater in the master bath, set up as a slave so it just heats the water in the pipe and then doesn't need to do much once the hot water from the tank arrives.

The thermosiphon sounds neat. Would not work for us as the master bath is on the same level as the water heater.

 

[woodlandfarms]

we to have a normal recirc with a normal gas hot water heater. I am sure our gas bill is a bit higher, an we have about 50ft at least from the hot water heater to the kitchen. It works Ok but not great.

The only thing I would say is we just had our hot water heater fail after 20 years of neglect. When replacing, we asked 3 plumbers about On Demand. All three of them have stopped installing, not sure if it was the luck of the draw but all 3 said it was way too expensive for what you got, and the reliablility is very suspect. You will never save the money from the gas bill to cover the additional cost and replacement.

 

[MossRoad]

we to have a normal recirc with a normal gas hot water heater. I am sure our gas bill is a bit higher, an we have about 50ft at least from the hot water heater to the kitchen. It works Ok but not great.

The only thing I would say is we just had our hot water heater fail after 20 years of neglect. When replacing, we asked 3 plumbers about On Demand. All three of them have stopped installing, not sure if it was the luck of the draw but all 3 said it was way too expensive for what you got, and the reliablility is very suspect. You will never save the money from the gas bill to cover the additional cost and replacement.

Amen to that.... the energy savings payback can be 20-40 years.
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