Calculating pump head on a multi zone hot water baseboard system

carlg

Hi,
I have a multi zone hot water baseboard hydronic heating system. I want to have 3 zones, but I want to calculate the system head to make sure that the pump can handle the load of all 3 zones.

I have asked this same question on other forums before and I've read the instructions from the Taco pumps. For some reason I seem to be getting conflicting answers, so I thought I would try here to see what you guys think.

When calculating the head for the size of the pump I should get in a multi zone system,
do I

sum up the total head of all 3 zones and use this number?

or

do I just use the head calculation from the greatest zone?

This is the number that I will use on the Taco selector chart.

What do you think?

Thanks!

Gordy

Use the head of the largest head zone. If the pump can overcome that head at the proper gpm it will run all three zones, or any smaller zone.

This is where delta t, and p pumps can shine. Since if you are using a fixed speed pump that accommodates the largest zone it will be over pumping the smaller zones. Not a huge deal other than a little higher electrical consumption.

With a delta ecm circulator the pumps speed can be adjusted automatically to slow down, or speed up depending on how many zones are calling. This will equate to less electrical usage. With a delta p circ it would eliminate the need for a pressure differential bypass in the system that a fixed speed circ would require.

carlg

Thanks!! Good to know!!

Ironman

Don't forget to add in the zone valve. It's Cv rating is the gpm that can pass through it with a 1 psi drop.

newagedawn

how many btu's is your boiler?

Rich_49

Highest head zone and all associated piping and the total flow of all 3 zones . Pressure differential bypass valve can be eliminated using either a Delta T or P circ . VT 2218 ( T ) or VR1816 ( P ) or 007E
( ecm ), one of these will probably suit your needs .

carlg

These are the same answers that the Taco instructions give.

For some reason I feel like you should be going total and not the highest calculated zone. I'm having problems understanding that. For example, what if I had 50 zones?? Not that anyone would ever have that many zones, but for example purposes. Then does the "Take the highest zone" rule still apply???

Canucker

Yes, you're calculating the zone with the highest resistance to flow(head). Then you calculate your flow requirements of all your zones and pick the appropriate pump based on head and flow. If you've done the calculation properly and picked the appropriate pump, it doesn't matter how many zones there are.

Jamie Hall

You're having trouble distinguishing between head and flow. Head is the resistance to flow, and it is determined, as has been said, from the loop in your system which has the highest resistance. Flow, on the other hand, is found by looking at all the loops in the system (or at least all that can be open at once) and finding out how much water will go through them and adding that up.

It's a little more complicated than that in real life, but just don't confuse the two.

Rich_49

carlg said:

These are the same answers that the Taco instructions give.

For some reason I feel like you should be going total and not the highest calculated zone. I'm having problems understanding that. For example, what if I had 50 zones?? Not that anyone would ever have that many zones, but for example purposes. Then does the "Take the highest zone" rule still apply???

See the attached report for a real job . It has 2 manifolds , each manifold has several zones . Each has a single circulator .

When the highest head zone in either if these manifolds closes or is satisfied the head for that entire assembly goes down , the cir then changes speed . If you used a PSC circ or another fixed curve circ when the head lowers the flow increases until your system curve (actual conditions) and the feet of head intersect on the pump performance curve . Less head higher flow , more head lower flow .

These reports show what the head and flow would be when all zones are calling , to see how much head and / or flow would exist at other times just subtract or add accordingly . Hope this helps , visual stuff is sometimes all it takes .

Dave H_2

@carlg ,

Whenever sizing a circ, the two pieces of info needed is total flow and highest head.
Your circ is running three zones in parallel, think of it this way;
If you provide enough head to go the longest loop in the system, wouldn't the circ be "strong" enough to handle a loop that is shorter and then the third which is shorter still?

Gotta have enough flow to handle when all three call at the same time
Flow is cumulative, head is not; head is the highest.

Enjoy
Dave H

Gordy

So back to flow. To be accurate you would need to do a heat loss of each zone.

Then apply the hydronic formula to determine gpm needed to move the btus required at a 20* delta. I think you have baseboard I believe?

Example:

Heat loss of zone 1 10k.
Heat loss of zone 2 10k.
Heat loss of zone 3 10k.

Total heat loss 30k

Using the hydronic formula Gpm= btuh/500xdelta t

500x20=10000 btuh
Total heat loss = 30000 btuh

30000 btuh/10000 btuh = 3 gpm


Each zone would require 1 gpm at a 20 degree delta. However all three zones running at the same time would require 3 gpm. So the pump selection would have to intersect the calculated head of the highest head loop at the desired gpm. That should intersect in the middle of the curve. That would be for a fixed speed pump

Now in doing that the other zones will most likely be over pumped when one or two zones are calling.

This is where a delta t, or p pump manages things to keep the pump running at a speed needed for one two, or three zones calling.

Use the attatched thermal hydronic formula at the bottom of the page.

carlg

Ok, think I got it thanks!!
Taco wants 2 variables to tell me which pump to select, head and flow rate (GPM).

For the head variable, I use only the highest of the 3 zones.
For the flow rate, I sum all of the 3 zones.

So If I had 3 zones at the following (These are fictional numbers)
#1 Head = 8, GPM = 4
#2 Head = 10, GPM = 5
#3 Head = 7, GPM = 3

I would select the pump for head = 10 and GPM = 12.

Am I correct??

Gordy

Yes