Recirculating pump for tankless water heater

Submitted by GeekX2 t3_120u4ub in DIY

Today I saw a post that mentioned a recirculating pump for hot water that made it sound like a (fairly) simple install. So I've been looking into it and have a couple of questions.

First, my situation. We completely re-plumbed our house about 12 years ago. All of the plumbing we did then is accessible in our basement/crawlspace. A couple of years later we added an extension with a guest bath on a slab. We don't use this bath very often so it's not as big a concern. I mention this because my biggest concern is the kitchen sink which is farthest from the water heater. However, the guest bath is pretty far in another direction so I'm not sure how much a recirculation scheme that helps the kitchen would help with the bath. (I may be overthinking this...)

Question 1: Recirculate through the cold pipes or dedicated loop? The pipes are accessible, new PVC and the expense of a ~50 foot run doesn't seem like an issue. But is there enough advantage to a dedicated loop to justify the extra work? Are there cons to a dedicated loop? Is returning through the cold pipes just as good?

Question 2: Recommendations for kits/manufacturers?

Question 3: Things to look out for that could sneak up on me in my decision making or installation?

(Full disclosure: I'm going to post this on the plumbing sub, too. I hope I don't get flamed for it.)

1

Comments

You must log in or register to comment.

[deleted] t1_jdj98ka wrote

Wouldn't a point of use heater be a much more efficient (and easier) choice?

7

GeekX2 OP t1_jdjdbk3 wrote

The ones I've looked at require 240V which would be a bigger deal. And would only fix the problem in one fixture.

0

fredsam25 t1_jdjosot wrote

If you get one with a small tank for the kitchen sink, it can run off 120v. It'll be a heck of a lot more efficient than constantly recirculating.

For the bath as well, if you add a ~2.5 gallon 120v water heater in line with with hot water line, it'll give you hot water long enough for the water from your main water heater to arrive.

3

[deleted] t1_jdjh53t wrote

Personally, I would rather run the electric (assuming there's space in the breaker box) then deal with new plumbing.

I did misunderstand your post though. I thought that the kitchen was your main focus. Even if you are concerned about more than one fixture I would still strongly consider the point of use heaters. Or get rid of the tankless heater.

I probably wouldn't end up doing any of this. None of these options make good financial sense...but not everything we do in life needs to be the sensible thing.

0

GeekX2 OP t1_jdjhv2g wrote

Keep in mind that I would have the same problem with a tank water heater. That is, it takes a long time to get water at the fixture.

Space in the box is a concern.

1

[deleted] t1_jdjihse wrote

Right, but tankless+recirculation is a nonstarter in my mind. Your options are tankless and point of use on key fixtures or tank and recirc.

3

noeljb t1_jdjah1g wrote

Recirculating pump and you have a tankless water heater? .. .. .. Yea, NO.

You don't have hot water to recirculate. If you recirculate enough water to turn the heater on then you are heating water 24 hours a day in pipes that are not even insulated.

Smaller point use heaters are your only real option. I guess you could put small (tanked) water heater in bath and kitchen, but that kinda defeats the hole thing.

7

lollroller t1_jdjhus8 wrote

I can’t believe this wasn’t said sooner. A recirculating pump makes NO sense whatsoever with a tankless heater.

EDIT: I should have looked into this before that comment. It looks that a recirculating pump and water return circuit can be used to return hot water into either an insulated storage tank, small tank water heater, or even another tankless; which in turn feeds the warm/re-heated water in series back into the hot water supply.

This would reduce the time needed to get newly heated water from the primary tankless to the fixtures, not a bad idea

3

Nearby_Maize_913 t1_jdk1jus wrote

I was sort of thinking the same thing. Though a recirculating pump isn't all the efficient on a tank, but probbably way more efficient than tankless

1

noeljb t1_jdkr7cx wrote

But wouldn't you just be running the heating elements constantly?

1

lollroller t1_jdkv75i wrote

The examples I saw the tankless would only be heating water when a faucet was running

1

noeljb t1_jdkwwsy wrote

So how does that get you instant hot water at the far bath?

1

lollroller t1_jdn11bw wrote

By locating the storage tank just ahead of the far bath.

Any time anybody runs hot water in the house, the recirculating loop warms up and fill the storage tank with hot water

1

GrimResistance t1_jdlytr2 wrote

Some tankless water heaters have built in recirc pumps or have an option for an external pump. You do need insulated piping though.

2

Diligent_Nature t1_jdj5eb7 wrote

It can be done, but the advantage of a tankless heater is that you don't have to store heated water and lose energy due to inefficient insulation. A recirculating pump will decrease efficiency by storing hot water in the even more inefficient pipes instead of in a tank. Returning through the cold pipe means having to waste hot water whenever you use cold or mixed water. If you don't care about efficiency then it could work for you.

5

GeekX2 OP t1_jdj85cf wrote

Good points. The selling point on tankless for us was the unlimited supply of hot water more than the idea of not reheating water we weren't using.

I see your point about wasting hot water when we want cold. It's kind of the opposite of what we have now--wasting cold to get hot. Maybe that's a vote for dedicated loop if I decide to recirculate.

Thanks.

2

Remanage t1_jdjqidg wrote

If you want to get some of the benefits of the recirculation system, put the pump on a switch (or even better, a timer switch) near the point of use. Flip it on a minute before you use the water, then turn it off and enjoy your hot water. You're not wasting water or energy then.

3

Diligent_Nature t1_jdjford wrote

A limited hot water supply is not a bad thing. It encourages conservation.

2

youngrichyoung t1_jdjds5s wrote

Some tankless water heaters have language in the warranty fine print that voids or shortens the warranty if they're used in recirculating systems. They're not designed for that kind of constant duty.

You should consider point of use heaters (as other users have suggested).

5

GeekX2 OP t1_jdje626 wrote

I'm thinking more along the lines of a thermostatically controlled pump, perhaps in conjunction with a timer so that it only circulates at certain times of day. These are available.

0

CyphirX t1_jdjcpro wrote

I’ve looked into one as well and the common use I’ve seen is putting the recirculating pump on a timer and set that for common use times like morning and evening showers and off at all other times. Depending on the pump, you can have multiple valves since each can run independently. Common complaints I’ve read about the cold line usage is it takes some time to get the cold water cold.

2

Carpenterdon t1_jdjsn44 wrote

Unless your house is 30,000 square feet and the furthest faucet is 100 yards away from the water heater....you're talking 10-15 seconds at the absolute "worst plumbing job ever" case for hot water. Just turn the water on and wait... Recirculating a on demand water heater is about the dumbest thing I see on Reddit from time to time....

If you really see the need then put in as efficient a standing tank water heater as you can then recirc that. It'll be much better for the world and for your gas bill since that on demand uses a hell of a lot more gas when its running all day every couple seconds than a standing tank already insulated and hot. Not to mention you are going to chew thru that "MUch" more expensive on demand heater running it near continuous... to save you waiting a few seconds to get hot water at a sink...

2

Worldly_Unit_9031 t1_jdmncsp wrote

If you do this, make sure you insulate both the delivery and return pipes to reduce the amount of heat loss on water returning to the water heater to be reheated. Also install a pump with a adjustable timer so that water is not being recirculated at unnecessary times

2

jackfish72 t1_jdjifc9 wrote

I have a tankless hot water heater, and recirculating pump. But to work, it requires a small electric hot water tank. So… we get the best and worst of both worlds. Unlimited hot water and quick to the tap. But not energy efficient, and very costly. I’d rather just have hybrid hw tank

1

Wellcraft19 t1_jdkdvq4 wrote

Yup, tankless are promoted for [energy] savings that really never materialize due to the extra cost compared to a ‘tanked’ gas heater. With unlimited supply, saving are literally down the drain.

Hybrid/heat pump heater makes a lot of sense if natural gas isn’t available. Otherwise it’s more of making a point.

1

jackfish72 t1_jdknkqa wrote

When green electricity is available, I think hybrid electric tanks win. ?

1

psykh85 t1_jdjtd3y wrote

Hey, I did recirculating pump through cold water lines in an upstairs bathroom. Basically the pump would run for one minute and it was on a Motion sensor. It wouldn’t run again for 30 minutes after initial run. Made it nice for instant hot water in the morning for a shower or washing your hands or whatever.

1

l397flake t1_jdjzmz4 wrote

Some recirculating pumps have timers, so you set them to go when you most need it. Typically you run the circulating line ( hot of course) to the furthest fixture.

1