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# best way to find GPM of existing system

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Member Posts: 12
What is the best way to find GPM flow of an existing system, excluding cutting in a flow meter.

I'm trying to find out if there is an accurate way of determining the output of a section of baseboard. With the delta T and GPM you should have the BTUH output.

I've been looking at calculating head pressure and using pump curves but it seems that there would be a lot of room for error with this method.

Looking at a pump with a flat curve, if you miss the total head calculation by just a little the GPM could be way off.

Is there another way?

• Member Posts: 3,086
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Heat Loss

Do a heat loss break it out by zone. 1gpm on a 20 degree delta-t gives 10,000 btu's

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• Member Posts: 12
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?

Unless I misunderstand you Chris, that not what I'm after.

If there is an existing system and I what to find what the btu output of a zone or section of baseboard is. To use the standard hydronic formula I need deltaT and GPM to get BTUH. Unless there is another way?
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The Heal Loss

Doing a heat loss is the best way. Do the loss and calculate gpm based on the loss. Unless you can calculate every inch of pipe, fitting, valve, head through the boiler etc your pump head isn't going to be accurate so you wouldn't be able to really size the pump 100 percent accurate.

When the flowrate is unknown use the mfgs 1gpm rating on the board which is generally 550 btus ft. Simple solution would be to go with an Grund Alpha or similar pump it will tell you whats going on.

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• Member Posts: 2,398
edited October 2010
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Try not to

over-think things. When you are just talking one section of baseboard, say ten feet of 3/4" Type M or equivalent copper, if 1.0 gpm, that is 0.36 feet of head or barely 0.16 of a psi per 100 FEET of pipe. At 4.0 gpm that goes to 4.12 feet per 100 feet.

Divide that by ten (ten feet out of a 100 foot rating standard),  and you need a Bupkes Meter, not available in stores.

If you do have that kind of instrumentation, you of course need a Pete's Plug or similar P/T port to measure pressure inside the pipe.

So, I am not clear why this information would be helpful to you, how you are applying this very small increment. As the others have noted, temperature in, temperature out and a change in space temperature over an hour are essential. Not all of the variables are in this because your outdoor temperature will vary, the sun hits the wall, but it will get you an idea. You can also go to a rating chart and get another value at certain water temperatures.

Take this to heart if you are equating flow rate to heat output:

Start by looking at a typical fin tube capacity selection chart. They usually rate them at 500 lbs. per hour water flow through-put and 2000 lbs. same. This is the same as 1.0 and 4.0 gpm for our purposes.  A given selection might have 580 BTUH per foot with 1.0 gpm and 610 BTUH with 4.0 gpm flow rates, (this from a Slant-Fin catalog, actually).

You quadruple the flow and get a measly 30 BTUH per LF benefit for over 11 times the pressure drop.

The short answer is, you can be off by a factor of two and not be able to measure the difference.

Here is another thing to consider regarding system balance:

In a closed system, all branches have the same pressure drop under a given operating condition.

No balancing valves, no particular throttling, all circuits will have equal pressure drops.

These may not be at the FLOW you want in each, but the pressure drops circuit to circuit will be equal.  But can the flow in your worst-case run be that much less than your shortest apparent run? Maybe by half or twice, but note what that does to output. Not much.

This is analogous to the MIT Rule of Arts and Sciences: "Under the most controlled conditions of temperature, pressure, humidity and other variables, the subject will do as it damned well pleases."
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

• Member Posts: 5,853
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Is it hot, or is it not....

Using a thermistor type probe, touch the pipe going into the convector, record that reading, then move it to the outlet and do the same. If it is hot (180 or whatever it is) and it loses a few degrees F between there and the end of the convector, it's doing as good as it can.

If the entering water temperature is extremely low, like 160 degrees F, then it is probable that the convector is on the far end of a circuit, and it is not doing as well as it would with a hotter water temperature, but could do better with hotter water.

We have solutions to that scenario, short of adding additional BBR, but lets not go there yet (4 way flow reversing valve with omnidirectional zone valves)

If there is a BIG differential in temperature, your flow rate is low.

If it is hot, but the rooms not, I'd suspect infiltration as a prime suspect.

As Brad has pointed out, (you crack me up Brad...) as long as you HAVE flow, the difference between a 4 guppy per minute flow rate versus 1 guppy per minute doesn't increase the output by 400%. (That IS what the G in GPM stands for, right...)

ME

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• Member Posts: 1,291
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Availibility....

Bupkes meters are sold in the Amish store in Wapakeneta Ohio. ........;)
• Member Posts: 12
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You quadruple the flow and get a measly 30 BTUH per LF benefit for over 11 times the pressure drop

"You quadruple the flow and get a measly 30 BTUH per LF benefit for over 11 times the pressure drop"

That makes sense, so there is really no reason to look for a more precise number.

I don't have an issue now but I have in the past.

I do energy auditing and in order to get a detailed picture of whats going on I like to know what the output of a heating system,zone or section of baseboard is.

It doesn't happen as often but the cause of a cold area may be the lack of proper heating distribution as opposed too excess conductive loss or infiltration.

Thanks again for your input guys
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Bupkes meters are sold in the Amish store

If those Amish would just get a store on Ebay, just think how many Bupkes meters they could sell
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Isn't That

The reason to do the heat loss?

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• Member Posts: 1,291
edited October 2010
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Seriously though

Couldn't a person measure the temp of the pipe at the beginning and end of the system and use avg output of the baseboard........say 500/ft....... and determine at least a rough flow rate from that.

Thinking out loud here..........  10' of BB should be transferring 5000 btu total if a person uses that average output . So........where am I going with this...........???   Assuming the 5,000 is correct, reverse the usual btu formula and arrive at gpm?

5000btu / 10* / 8.33lb / 60 minutes = approx 1gpm

Oooooorr. just say ta heck with it, install a stratos and dial in whatever you need.
• Member Posts: 2,398
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From an energy auditor to an energy auditor

we are on the same page, I think.

My normal approach to a problem area is to calculate a heat loss (absent something obvious such as the Blue Wall of Death with my FLIR camera), or leaks visible by daylight. Turn off the lights in a room and close the drapes and look around for daylight. This is especially effective in basements.

But to determine heat output "potential" from fin-tube, I go to the catalogs if I have a specific model. Otherwise, 550 to 650 BTUH per LF for residential grade BBR at 180F supply water is not far off. Some can push over 900.

But measuring any of that with temperatures on a middling day, the thermal contrast is not there. Too many variables outside of the design day the system was designed for. In the scheme of things, if you have ten feet of this radiation,  it amounts to

maybe 5500 to 6500 BTUH and only on the coldest day. If the space is

cold and it is only in the 40's out, look elsewhere.

Use Ockham's Razor and go to the catalogs or rules of thumb. The fine measurement tells you little and the big picture says more when first going in. After you gather your larger variables and give them values, only then will fine measurements be worth your time.

That is my approach anyway.
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"