Help with reducing hot water wait time

[Jason Ryan]

Hi all,

My kitchen faucet takes 2 mins (3 gallons) to warm up. I have a whole house tankless water heater (7.5gpm), and I'm on a slab with a 2 story home so can't easily run wire for an on-demand nor a water return line for re-circulation system. Also with my garbage disposal and an RO system, I don't have room for a storage tank.

I've been reading and reading for ideas on how to stop wasting water with not much luck until learning about new tankless water systems with built in recirculating pumps.

Would something like this be a possibility for me? Are there options to retrofit older systems? I'm still a little confused on what tells the system to recirculate. If anyone could offer any advice, I'd appreciate it.

Thanks!

 

[Phog]

You don't necessarily need a dedicated return line for recirculation. They make systems that utilize the cold water line as a return. For example Grundfos Comfort System. It has a thermostat sensor valve that allows recirculation from the hot into the cold, then shuts off the recirc as soon as it detects hot water passing through. You get a small amount of warm water into the cold line this way but it's not very noticeable.

You can typically retrofit an old tankless system with an external recirc pump however you need to pay attention to the minimum flowrate for firing the burner on your tankless model. Size the pump appropriately, it needs to pump at least enough flow to kick on the burner.

Recirc pumps most often run on a timer, every xx minutes they turn on (you have to determine the correct timer setting for yourself by trial and error).

 

[Sponsor]

 

[Jadnashua]

Some tankless systems can work with recirculation, some can't. You'd have to read the manual for yours to see what it allows.

There are lots of recirculation systems out there. Most are designed to work with a tank.

A tankless with recirculation tends to be somewhat of a hybrid...a small tank built in, but that would potentially allow an external pump to circulate that hot water from the tank, and only turn the tankless on when it cools off. How often that happens would depend on the length, route, and amount of insulation on the water lines (including the return, which could be the cold line).

In theory, you could cobble up your own by installing a small tank somewhere, and then run a pump with an aquastat, but it's easier if you can get an engineered system.

 

[Jason Ryan]

I bought one of the recirculating sensors and a pump online. I've also got a programmable plug ready to go.

The problem I'm worried about is dead heading the pump. It appears the recirculating sensor that mounts under the sink seems to have a thermostat in it and it will close as the water heats up.

If this happens while the pump is running to push cold water into the tankless unit, it seems like it would no longer be able to flow and I'd guess the pump would cavitate. I've seen that one company appears to put a current sensor inline with the pump presumably to shut it off if it dead heads.

I'm not really sure what to do. I emailed support for one of the companies that makes complete systems to ask some basic questions, not didn't hear back. Any help would be appreciated.

Thanks!

 

[Jadnashua]

Depending on the head of the pump, it may not matter.

 

[Jason Ryan]

Oh, I didn't know that. Is there a way for me to check? I'll take a look at the pump specs. Thanks!

 

[Phog]

Did you get the pump that matches the recirculation valve? (In my example above, Grundfos Comfort System, there is a matched kit with pump & valve or you can buy each separately). The cold-as-recirculation line system manufacturer will have considered the pump dead head condition when designing the system & made sure that this particular pump is compatible.

 

[Jadnashua]

One mistake people often make is to put too large of a pump in. Also, keep in mind that the pump must be either bronze or stainless steel when used on potable water or it will self-destruct fairly quickly. Potable water has dissolved oxygen in it, and a pump designed for hydronic heating is generally made up of iron components, and they rust with the continual introduction of oxygen.

 

[Jason Ryan]

Did you get the pump that matches the recirculation valve? (In my example above, Grundfos Comfort System, there is a matched kit with pump & valve or you can buy each separately). The cold-as-recirculation line system manufacturer will have considered the pump dead head condition when designing the system & made sure that this particular pump is compatible.

I'd checked out their product as well as the Watts system, but neither stated they work with a tankless system so I didn't go that direction. There is surprisingly very little info on the web about tankless recirculation so I was thinking I'd diy it. I really didn't want a timer on the pump since I've got a home automation system that will let control it more creatively like with motion sensors.

If either system can be programmed to run continuously, maybe I can still use my automation system though. Do you know if there's much advantage of the Grundfos over the very similar Watts system? Anyone have verifiable success with tankless and without adding storage tanks?

Thanks again!

 

[Phog]

The Grundfos system UP15-10SU7P/TLC advertises a flowrate range of 0-6.3gpm in their brochure. The lower end of the range is zero, which means the pump can be safely dead headed. The upper end of that range is probably ideal unrestricted flow (into open air without piping). What flow you see in an actual home plumbing installation will be a bit lower due to pressure drop through the pipes. But it still gives an idea of the general order of flowrate you might see out.

Tankless water heaters have different minimum firing thresholds based on the manufacturer, size of the unit, and age (more recent models often have improved, ie. lower minimum flow required, as compared to ones from 10+ years ago). You would have to find the specifications in the manual for your particular model. But it will probably require somewhere in the 0.3-1.0gpm range as its minimum for turning on the burner.

So, worst case the Grundfos pump only needs to hit about 1/6 of its rated 6.3gpm flow capacity, when pumping through whatever restrictions you have in your plumbing, to trigger the tankless. This seems like a pretty safe bet. You can follow a similar line of reasoning for other pump hardware, including whatever you have already bought.

By the way, for the record I don't have direct experience with Grundfos (no recirculation in my house) and I'm not trying to push them, I'm just using as an example that others have spoken of on these forums. I think you're on the right track, just need to make sure what you've got is all compatible with each other. Good luck.

 

[Jadnashua]

REad your tankless' manual. Some of them will void the warranty if you use recirculation, some allow it, but only if done in a certain way.

FWIW, the Copper Institute recommends NGT 5fps flow rate with hot water. On a 1/2" copper pipe, that's only 4gpm; on 3/4", it's double that. Recirculating water too fast can literally wear holes in the piping. It also starts to cause water noises in the pipes when it starts to go too fast.

Unless there's a buffer tank, your best bet might be to only run the pump on demand, not relying on a timer. The safest way would be to have a small tank, and run the recirculation through that...then, the tankless system wouldn't need to come on every time for small draws. That would also eliminate any cold water sandwich.

 

[Jason Ryan]

Phog, thank you, you've been a wealth of information.

My unit is a Jacuzzi, J-SN180F2, but the side says Rinnai. I got it from Lowe's. The min for rate says 0.26gpm.

I had planned to go ahead and install the pump and sensor to see how it would work (see info on my setup below), but ultimately I halted the experiment when I discovered that the pump appeared to require reducing hardware to take the line set from 3/4" to 1/2". I was worried this might restrict my water flow since my tankless unit is a whole house solution.

Aside from this there was still a concern that the pump would over heat if deadheaded. Also recently I read some reviews that said the pump might not be able to push the water through the tankless unit due to head pressure.

It just seems that tankless recirculation still has too many unanswered questions. Pressure drop, over heating impellers, with and without storage tank, hot (and cold) water sandwiches, etc...

We might soon be upgrading our sink and countertops and will move to a single large basin. This will reduce the plumbing underneath and should open up enough space to add a 4 gallon heater under the sink. I think this might be the most efficient route all things considered.


FWIW here's what I had bought:
Water pump
Trupow 3/4" NPT 110V Hot Water 3-Speed Circulation Pump Circulator Pump

Sensor
Watts Premier 0955801 Sensor Valve Kit for Watts Hot Water Recirculating Pump (0955800)

 

[Jadnashua]

Potable water REQUIRES either a SS or bronze bodied pump. That one is designed for hydronic heating, and the oxygen in the potable water will quickly cause it to destroy itself by rusting away the iron body of the pump. It can work in a closed hydronic heating environment because, after initial filling, all of that oxygen gets used up making rust, but then, there's nothing left to react with anything. In potable water, you're constantly adding fresh water going by that has some dissolved gasses in it.