Domestic Hot Water Circulation Pump Flow Rate

[EnOm]

Hi
I am wondering why the flowrate for a domestic hot water circulation pump is based on heat losses through the pipes and not the expected hot water demand (Similar to the sizing of a pressure booster pump for example).

For a set of calculations for a large residence I did, I got heat losses of approx 3000 Btuh. This seems quite low and the reason is that I'm using insulated PPR pipes. Based on this using a quick rule of thumb (Divide by 10,000) I got a flow rate of 0.15 GPM. This seems low for such a large residence, at least from my limited experience. So it lead me to think what would happen if the hot water demand exceeded this? And why isnt the flowrate based on the demand?

The heaters are on the rooftop (21 ft above the ground) so will the force of gravity be enough satisfy the demand?
Thoughts?

Regards

 

[LittleInch]

Why on earth do you think that the flow rate is based this way?? Who told you?? It defies logic that it is calcualted that way - what you need is the heat load being transferred from the boiler(furnace) to the radiators or heat load.

Heaters on the roof top??

My thoughts are that you have no real idea of your system or the design. Sorry.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DRWeig]

LittleInch, I think you missed the "domestic water" part of the post.

The purpose of the recirculating flow in domestic hot water is to keep hot water at or near the point of use at all[tt][/tt] times. That way, you don't have to wait a long time to get hot water from the spigot when you turn it on.

When you turn on the spigot, the supply water pressure brings the full flow to the tap, not the pump.

Calculating flow based on heat loss from the pipes while circulating is correct. All the pump does is flow enought water to offset the cooling of the pipes.

It is a tiny amount when it's a residence.

Best to you,

Goober Dave

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[LittleInch]

Fair enough. You must live in a much bigger house than me. I just turn the tap on and hot water appears after a few seconds.... I thought it was a heating system. Also sounds like a highly inefficient system designed to loose heat on an constant basis. I still don't understand the bit about the heaters on the rooftop though....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[EnOm]

Thanks for the replies.

LittleInch: As DrWeig said, it's a domestic hot water supply system, and not a heating system.
As for the "heaters on the rooftop" part, the residence has a central water heater, that is placed on the roof of the building.

DrWeig: So ultimately the water heater's operating pressure is what usually determines how much flowrate and pressure reach a certain outlet, with a fixed pipe size that is. Is this correct?

Regards

 

[DRWeig]

EnOm: Yes. And the pressure in the water heater is the same as the pressure in the cold water pipe that feeds it. So when you turn on the hot spigot in the sink, the same thing happens as when you turn on the cold spigot. City water pressure is the driving force. The hot water just flows through the heater on its way there.

LittleInch: It is wasteful. However, in my house (pretty small) the kitchen is located maybe 40 feet horizontally and 10 feet vertically from the water heater. The flow is restricted by the water-saving faucet to 0.5 GPM. So 50 feet of 1/2" pipe with 0.5 GPM flow and 49 feet per minute velocity gives 60 seconds for water to get from the tank to the sink. I wish I had a recirc pump when I do the dishes. One minute seems like eternity.

Note to both: I'm not an ME but I have simulated these recirc things over the years with energy analysis programs, so I had to understand them. It's unusual to see a question I can answer in the pump forum!



Best to you,

Goober Dave

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[LittleInch]

So it's plumbing we're talking about here. Flowrate also needs to take into consideration time taken for hot water to return back to the heater before it looses too much temp. Also if the heater is a demand type then flowrate may not be enough to turn it on or lead to multi short cycle. I thought these systems run using a stored hot water system. Pump will need a bypass non return valve to allow the normal flow to bypass the pump.

Drw, your water saving feature sounds like it sucks to me!

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[ccfowler]

Is this a solar water heater system with a rooftop reservoir above the rooftop solar coils so that there is natural convection circulation through the rooftop portion of the system?

Is it safe to presume that this little circulator operates with a small return line transporting the slightly cooled water from a distant point in the hot water distribution piping of the house back to the rooftop reservoir?

If these presumptions are correct, then a very small pump will serve adequately, but it's materials of construction must be suitable to avoid corrosion or contamination problems. Little TACO pumps of something like 1/40 HP are sufficient for a similar duty serving a hotel or multi-dozen unit apartment buildings with none-too-impressively insulated piping systems.

I doubt that you will find an economically practical alternative to whatever commercially available similar small pump you may choose to use. I wouldn't care to bother spending much effort on fancy heat loss analyses, flow studies, etc. on such a system. Any suitable commercially available pump will probably prove to be more than adequate to the needs of this system. The normal domestic demand flow never passes through this little pump, it only sees the modest circulating flow.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[DRWeig]

LittleInch. I agree on the water-saving thing. Anybody know a source for bootleg Canadian toilets?

Best to you,

Goober Dave

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[hydtools]

Just pick something from TACO

http://www.taco-hvac.com/uploads/FileLibrary/100-41.pdf

Ted

 

[LittleInch]

I've also see electric trace heating used for this purpose, especially self limiting tapes for insualted pipes. Saves on the pump and return line, but puts up your electricity bill a bit...

Still sounds like a fantastic waste of energy to me though....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DRWeig]

It is SUCH a waste, LittleInch. But it can have a pretty short payback in terms of dollars.

Per Forbes, John H Hammergren (CEO, McKesson) makes $131.19 million per year. For a 2080-hour work year, that's $1051.20 per minute. If each day we can eliminate one minute of nonproductive time waiting on the sink water to get warm, he will perform $250,000+ of additional work at McKesson's cost. Straight to the bottom line, yessir!

If we circulate 1 GPM with a 20°F delta-T, it'll take about 3 kW to replace the lost heat. I think we'd better leave it on 24/7 in case Mr. Hammergren works on flex time or has rotating shifts. That's over 26,000 kWh in one year. At $0.10 per kWh, I would normally think that we have thrown $2600 out the window. But in Mr. Hammergren's case, we've paid for our energy within the first three days.

Sorry, all. I know this belongs in a venting forum or the pub, but I just happened to read about this guy.

Best to you,

Goober Dave

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[EnOm]

Hi
I apologize dearly for my late response. I was very busy with work lately and it kind of slipped off my mind (like morning to night time busy). Anyhow, thank you all for the information.


DrWieg:
Over here there is no city water pressure. How it works over here is: City water pressure is pretty low, it only fills main storage tanks on the ground. The water is then pumped from the ground storage tanks to secondary storage tanks on the roof. The water is then pumped down using a pressure pump to the water outlets on the different floors of the building. Storage water heaters (usually electric) are placed on the roof after the pressure pump. Sometimes a circulation pump and a return line are used to maintain hot water in the pipes constantly.
"And the pressure in the water heater is the same as the pressure in the cold water pipe that feeds it" Does this include storage heaters connected to a pressure pump as well? You lost me a bit with "The hot water just flows through the heater on its way there" because the water does not exactly flow through, it is stored.


ccfowler.
No. There is nothing solar in it. Its a trio of electric water heaters placed on the roof of a 3 storey villa. They are connected after the pressure booster pump from one side, and to the house's piping network on the other.
As for your second question, yes what you mentioned is correct. It is a trio of circulators connected on the return line of each of the three water heaters. The capacity is split over 3 heaters for the sake of redundancy.

Again thank you all, this was very helpful.

 

[DRWeig]

Hi again EnOm,

I would have to see a diagram of that system, I am not good at visualizing such.

However, if both the hot water tank and the cold water supply to the sinks are on the same side of the pressure pump, then they are at the same pressure. They are both connected to the same pipe; cold water to the heater tank, and cold water to the points of use.

When you open a hot water spigot in a sink, water flows through the hot water system and comes out. It's flowing through that storage tank too. Cold water goes in, hot water comes out.

In my example above, substitute "the pressure made by my pressure pump" for "city water pressure."

Best to you,

Goober Dave

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[EnOm]

DRWeig,
I'll post an example diagram the system later on. But for now, thanks for clearing that up.

Regards