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

[woodlandfarms]

That said you are building a 6000 sq ft house money must be less of an issue so there is definitely something to say for turning a hot water valve on and there being hot water.

 

[3Ts]

I had a recirculating pump on my previous house with insulated pipes. Over 100' from water heater to master bath, it took a looonnng time to get hot water without the recirc pump. That house definitely needed the pump, but its performance was disappointing. Part of the problem was the 100', even though the pipes were insulated, they still lost a lot of heat. Best to put the heater close to the end use location.

The house I have now has all hot water usage (except the kitchen) really close to the heater and I get hot water in seconds. The kitchen has a supplemental 2.5 gallon, 120v water heater under the sink so the DW and sink have immediate hot water and the intake comes from the main hot water heater in case I need more than the 2.5 gallon unit can handle.

- I will not use a recirc pump again. The amount of electricity they use IS noticeable although not prohibitive. I even put mine on a timer which worked out fairly well. For that matter, how often do you use hot water? Do you need the hot water flowing past that point constantly? Even at night? If you do put it on a timer, what about needing hot water when the timer shuts the pump off? How fast does the water cool off?

- The "on-demand" hot water heaters will not give you instant hot water any sooner than a tank type, because the piping from the heater to the end use still holds "cold" water. The only benefit I see is if you are filling a 100 gallon spa. The 40 or 80 gallon tank will run out of hot water before the spa is filled.

 

[gsganzer]

The recirc pump is typically on a timer. It only runs during your "use" time. Like first thing in the morning and then in the evening. It doesn't run all the time.

 

[RayfromTX]

Wow, there are a lot of informed people on this thread. I am a home builder with a lot of experience and testing of the various ways to implement these systems. Here is what I've learned.

Tankless water heaters are quite beneficial in a number of ways. For large demand situations, they never run out of water and don't require a large footprint. Homes are expensive and hanging a tankless outside or in the garage is an easy way to get 10 square feet of space back in the interior of your house. For homes with infrequent use they save a little on standby losses. If you only do the math on standby loss recovery then they won't pencil out but that is only a small part of the equation.

Tankless heaters and piping systems will not last as long if you use a recirculation pump. Timers are problematic.

I plumbed one house with a 6 gallon electric water heater to maintain the heat in the loop so the water didn't circulate through the tankless. That is one of their recommended methods. It is a bad idea. I insulated the loop line which was a total of nearly 200 feet with one inch insulation glued and taped and mitered. I measured the electrical use for a month while the client was absent. The pump used 5$ in electricity. The electric water heater used 50$. Lots of energy is wasted by circ systems but only a small part is used by the pump itself.

If you plumb a house with a loop you are stuck with the loop from then on. I regret having done it in the past. I have a loop at my house but I have reasons and I have it on a central switch and when we want hot water we flip the switch and flip it off when we are done with it. I could never ask my client to tolerate that. In my house I would nave needed three water heaters to keep them close to points of use.

Here is what I do on my multi-million dollar houses now. I put one water heater at each end. I run a separate water line of just large enough size to each fixture. A 3/8" line will serve a powder bath or a vanity or a dishwasher. Any low flow fixture will get a small line so as to reduce the volume and therefore the time delay to deliver hot water to the fixture. Where a typical master bath would get a 3/4" line to the bath and then split a 1/2" line to the vanity, I don't. I need a 3/4" line for the big tub filler but if the vanity is supplied by that line I may have a 60 second wait at the vanity. I run a 3/8" line directly to the master vanity and the wait time may be only 12 seconds. Calculations are important because with hot water you should not exceed 5 feet per second of velocity to reduce erosion potential. There are online calculators for these things. irrigation.wsu.edu has one. By plumbing the house this way I have given the client a system that is passively efficient, simple and effective for the life of the home. One bit of pushback I get from plumbers is because they have seen main lines get lined with scale that will eventually reduce flow. This happens on lines that see lots of water. A vanity faucet that uses 1.1 gpm will see maybe a few thousand gallons in the life of the house. It is nothing to be worried about. It costs more to run multiple parallel lines but not more than setting up a circulation loop.

Tankless heaters make more sense as part of the initial design. Only rarely do they make financial sense on a retrofit. They do sometimes but seldom.

 

[plowhog]

Thank you all for the responses. I am going to discuss with the plumber two changes. First is to relocate the on-demand from the laundry room to the outside of the house near the master suite and bedroom wing bathrooms. (Moving it closer to point of use, instead of farther.) As framing has begun and walls are going up, I noticed a "no man's land" in one obscure area where a small closet for an on-demand system could be put on an outside wall without affecting the aesthetics. This was the on-demand he wanted to move in the opposite direction to the garage. Due to close proximity I believe this system would work just fine with no recirc pump.

The other change I am going to discuss is adding a 2.5 gal electric water heater under the kitchen sink. We had one of these in a former residence and it solves the instant-hot dilemma. Yes you are paying $$ for electricity to keep a tank of water hot, but it only 2.5 gals you are heating, not 50. With that addition I think I am comfortable with no recirc pump on that system in the garage either.

Last comment: whether you build large or small, I think everyone wants to be reasonably efficient and not spend $$ that is not necessary. Thanks again for the responses as I have gotten some helpful ideas to achieve that.

 

[DuckSlayer1017]

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.

I just had the same experience with two different plumbers. We have one at our cottage that works great but that’s only 1 bath, and a kitchen sink.

 

[ning]

I have a recirc (no on-demand heater... yet) but it's on a smart outlet, so it's timed so I can adapt it to my wife's work schedule. It's on for 2 to 3 hours in the morning, and 3 in the evening, so about 25% of the time. Of course all the hot water pipes in the house are insulated; there is some loss due to the system (the pump draws current, and you lose some heat) but I rarely hear the water heater kick on (it's got a powered vent, so I can hear it from my office next to the utility room) during those times.

I'm not sure how the recirc system plays with on-demand heaters, because you'll presumably be cycling fairly hot water into the heater.

 

[plowhog]

I'm not sure how the recirc system plays with on-demand heaters, because you'll presumably be cycling fairly hot water into the heater.

My concern is that, as someone said above, the pipe you are circulating through sort of becomes the "tank" of water needing to be kept hot. But unlike a typical water heater with a large tank, and with excellent insulation and exterior jackets available, these are long skinny plastic pipes with many possible heat loss points. Yes, they are insulated, but as they encounter pipe hangers etc. I believe there will be some areas exposed. I just don't know. But what I don't want to hear is the whine of an electric pump while also hearing the on-demand burner fire up, stop, fire up again, etc to keep everything hot in the pipe. That is my concern with what has been proposed.

 

[ning]

Another consideration is that in some places, water is getting scarce and expensive. If you dump cold water down the drain waiting for hot, you're wasting water. If you recirculate, you waste energy. I'd imagine that the wasted water doesn't cost nearly as much as the wasted energy does... yet. I wouldn't be surprised if in the California Bay Area that water rates especially during droughts will start to make up the difference. Also, waste water itself costs money.

[n.b.: I'm on a well, so my water costs electricity to use, plus wear&tear on the well & pump. I'm on septic, so I'm not charged directly for waste water, and my waste water probably helps the septic operatate, to a point ... and the wasted water goes back into the ground, I'll drink it again some years from now ]

 

[John Fitzgerald]

Recirc pumps are a waste of energy. Put the water heaters where the hot water is needed. It's a new house. There is no reason not to put them where needed. The plumber just wants easier work. You could even put small six gallon ones in the bathrooms to get hot water instantly. If using more than that, the water from the main heaters would be there long before six gallons runs out.