Water flow detection but not for leaks

[Ira]

You might want to look at the flow switches offered by Dwyer Instruments... 
 
http://www.dwyer-inst.com/Product/Flow/FlowSwitches
 
I've never used them, but looks like they have some good stuff (that's not terribly expensive) for monitoring flow, pressure, usage, etc.

 

[picta]

3/4" pipe contains 0.023 gallons per foot of pipe.  So 150 feet holds about 3.5 gallons.  It doesn't sound like 30 seconds is unreasonable to move that amount of water around the loop, depending on the size of the circulator pump.  7 minutes seems very long, but I'll take your word that's what you see.

 
The water is hot and has to be pushed back into the tank under pressure. It takes my pump about 90 sec to the closest fixture and up to 5 minutes to the farthest. The times vary with other variables, such as temperature in the tank, outside temp, if other fixtures are open etc.  I think that while certainly possible, the OP's idea of controlling the pump via water flow detection will be much more difficult to implement than using simple controls for water "on demand" near each fixture. Also, how would he request the water for a dishwasher or a washer? We have control buttons near each fixture and also use simple automation rules for events like coming home or morning alarm or motion detected in a bathroom. I have temperature sensors attached to the pipe and shut off the pump when the hot water reaches the farthest point in the loop. For the far fixtures we even have HAI make announcement when the water is ready, so that you can do other things while waiting for the water to reach it. Works well.

 

[Automate]

The water is hot and has to be pushed back into the tank under pressure.

The static pressure in the pipes is irrelevant.  The pump sees the same pressure on both the inlet and outlet side.  It does not need to fight against a static pressure differential.
 
There are different types of hot water re-circulation pumps.  Some are low flow pumps, they usually run on a timer so the low flow does not matter..  You just schedule them such as to come and circulate for a while before your first use in the morning.
 
Other pump have higher flow rates and only need to run a short period of time to heat things up before they can shut off.  These are better for an on demand system.

 

[wkearney99]

It takes my pump about 90 sec to the closest fixture and up to 5 minutes to the farthest.

 
FIVE minutes?  Just what's wrong with that plumbing setup?  I mean, how far is the run?  Does the hot water line run through unconditioned space or something?  

 

[picta]

FIVE minutes?  Just what's wrong with that plumbing setup?  I mean, how far is the run?  Does the hot water line run through unconditioned space or something?  

 
the run is ~100' with elbows, pipes are insulated but in non-conditioned attic. the pump was probably intended for constant circulation. in the old house we had "on-demand" system installed after the house was built, and it took about 3 min to reach a shorter distance.

 

[picta]

The static pressure in the pipes is irrelevant.  The pump sees the same pressure on both the inlet and outlet side.  It does not need to fight against a static pressure differential.

it is irrelevant for the pump, but shooting cold water into the tank at a high speed is probably not a good idea?

 

[wkearney99]

it is irrelevant for the pump, but shooting cold water into the tank at a high speed is probably not a good idea?

 
What?  
 
What do you think is coming into the tank when hot water gets used from a tap?  COLD water.  How would that be any different than water being recirculated?  If anything the in-wall water would likely be a bit warmer than what's coming in from underground.  So, no, there's no more risk of thermal shock than present during normal operation.  
 
That and when circulating there's no changes in pressure involved.  The water is already in the system and it's all operating at the same pressure.  As it pumps water out, it's pulling water in.  The same pressure out, the same pressure in.   Water recirculating systems like this have been around for decades.

 

[picta]

I have described the system that  I have, it was installed by professional HVAC people. I think the pump may have been intended for constant circulation. It seems that others have similar flow rate in their systems. It would be interesting to find out what requirements currently exist for limits on the flow rate. Perhaps only in California where the local codes require low flow faucets, for example.

 

[fcwilt]

The characteristics of the pump (flow rate, etc) have nothing to do with building codes requiring "low flow faucets".
 
The pump used is based on the nature of the system.
 
If the goal of the system is to always have hot water available at the faucets the flow rate need be no more then is necessary to compensate for heat loss as the water circulates.
 
On the other hand a demand based system which is using a circulation loop to prevent wasting cold water down the drain is going to want a pump that can get the hot water to the faucet as quickly as possible in keeping with other system considerations such as budget, noise levels, etc.
 
Converting a "constant flow" system to a "demand system" without changing out the pump is likely to be less then satisfactory.

 

[dpilati]

I am curious about pumps and I do realize that I am hijacking a thread...
 
I was told by my plumber that the flow was not pump limited - and I believed it. Really stupid now that I think about it. I've known the 3.5 gallons in the loop (or so) and of course it doesn't take 7 minutes for the last bathtub - a high flow fixture. It should take a minute assuming 3.5 gallon flow at the fixture. 
 
It would really help my situation to have a faster pump - anyone know of a good one? The one I use is 1/33 hp - Grundfos

 

[fcwilt]

I am curious about pumps and I do realize that I am hijacking a thread...
 
I was told by my plumber that the flow was not pump limited - and I believed it. Really stupid now that I think about it. I've known the 3.5 gallons in the loop (or so) and of course it doesn't take 7 minutes for the last bathtub - a high flow fixture. It should take a minute assuming 3.5 gallon flow at the fixture. 
 
It would really help my situation to have a faster pump - anyone know of a good one? The one I use is 1/33 hp - Grundfos

 
That's what a good plumber would help you do - upgrade your system with the right equipment. However as you have found not all plumbers are good plumbers - of the "not good" ones some are ignorant of "state of the art" plumbing practices, others just don't care.
 
The Grundfos stuff is good equipment - when properly installed - what is the issue you are having?

 

[wkearney99]

However as you have found not all plumbers are good plumbers - of the "not good" ones some are ignorant of "state of the art" plumbing practices, others just don't care.

 
Amen to that!  Lately I'm finding the only way to get to ones savvy about SOTA practices is to make a phone call to the vendor.  Even their websites point to generic or higher-volume suppliers, not to ones known for their smarts or quality of work.  Even when you find a decent plumbing company you're still at the mercy of whatever random idiot they send out to do a given job.  By the time you learn enough to be able to tell just what they're eff'ing up you're practically able to do the job yourself.  But I shouldn't hijack the thread to rant about that...

 

[dpilati]

I don't have issues as a continuous system but I want like to be in a quasidemand setup. If my wife leaves the house, I'd like it to shut down but she would not do well with the 7 minutes to restart when she comes home. If it tripled in speed, I might have a fighting shot to have the pump turn on when the garage door is activated.
 
I could also have it turn on when she wakes in the morning if it started up faster. 
 
I have solar hot water and insulated pipes all in conditioned space so it isn't a horrible waste but if I could cut the amount of time it is running, I could get to near zero waste. My backup doesn't run for more than 10 kwh a month in the summer despite running the pump 16 hours a day. And in the winter, it does help heat the house. And I heat my water 90% off peak when my electric rate is $.05 per kwh so there is no way a $400 pump could ever pay for itself but I still obsess...

 

[fcwilt]

I don't have issues as a continuous system but I want like to be in a quasidemand setup. If my wife leaves the house, I'd like it to shut down but she would not do well with the 7 minutes to restart when she comes home. If it tripled in speed, I might have a fighting shot to have the pump turn on when the garage door is activated.
 
I could also have it turn on when she wakes in the morning if it started up faster. 
 
I have solar hot water and insulated pipes all in conditioned space so it isn't a horrible waste but if I could cut the amount of time it is running, I could get to near zero waste. My backup doesn't run for more than 10 kwh a month in the summer despite running the pump 16 hours a day. And in the winter, it does help heat the house. And I heat my water 90% off peak when my electric rate is $.05 per kwh so there is no way a $400 pump could ever pay for itself but I still obsess...

 
Doesn't you unit have a timer on it?

 

[shielde7]

Four years ago I had an issue in the house I still live in where the water would take 5 mins to get hot water to the main bath tub (it's less than 30 feet from the hot water heater) so I looked into fixing the situation.  At first I was thinking about doing the hot water recirculation but ended up scrapping that idea after I replaced all the pipes in the house and now it takes about 15 seconds to get to the farthest fixture from the hot water heater.  
 
I installed a viega manifold system (I was lazy and didn't want to build one myself) and ran all the hot water and cold water in pex pipes from it to all the fixtures in the house.  The only down side I had from doing this is I had to then replace the drains to all the fixtures (the drain lines were at least 30 years old and when I cut them out they were all packed with nastiness) due to the fact that the water pressure had more than doubled (in some of the fixtures 3 or 4 times) and all the drains were backing up.