Circulator, flow, and load

Jason_13

I have a job where I am sizing circulators for zones. All zones has excessive radiation. We will look at just one zone of three zones, although all zones are 3/4" copper.
The old boiler is 120k, the new boiler is sized to the 72,800K heat loss, house has had updates.
This is a retrofit boiler, the heat loss for zone 2 is 21k. I want a 20f delta-T so flow should be 2.1 gpm. So far pretty straight forward.
Now, enter the question. The radiation in this zone is way over-rated as is the other two. If figured at 180f the 58' of baseboard is almost 35k output.
When I size my circulator do I size it for 2 gpm since that is all the heat I need? My concern is water temp will drop too much before the end of the loop.

ScottSecor

2.1 gpm is the flow rate you need. The head needs to be calculated as well.

Based on zone 2, I suspect you will have a tough time with a single speed pump that operates most efficiently at 2.1 gpm or even 3gpm. Might want to consider a single pump with variable speed and use zone valves instead. Something like this https://www.supplyhouse.com/Grundfos-99285998-ALPHA1-Circulator-Pump-1-16-HP-115-Volt.

Jamie Hall

And while you are at it, can that boiler operate at a reduced temperature? Remember that the heat output of your radiation is affected by the water temperature. You figured that one zone at 180, but you want less than two thirds of that output. Can you either run the boiler lower, or mix down to perhaps 140 or so?

leonz

Have you considered doing the simple thing and using orifice plates to just regulate the flow to the zones and not mess with anything else??

Tim_D

The math says that 2.1 GPM in a circuit that can emit 35000 but's will result in a 33 degree delta T. Looking at this a couple of different ways, your average water temperature will be 6.5 degrees lower than 3.5 gpm at a 20 degree delta T but possibly more importantly, your water temperature at the end of the loop will be 13 degrees cooler. I would suspect that this may generate a call back. I would probably leave the flow at 3.5 gpm and back the water temperature down until I hit the 362 btu/per lineal foot that you need.

hot_rod

Can you split the 58 foot loop into 2 loops?

Jason_13

Right now each zone is a series loop. I only discussed the problem with one zone but all three have the same issue.
Way more radiation than required for each zone.
I can increase the system flow and pipe the boiler p/s so as not to get too much flow through the boiler. This should take longer to heat and reduce the outlet temperature due to more flow in the system which will drop my system water temperature and still give me proper flow through the boiler.
If I add flow restrictors I can play with them if needed.

Jamie Hall

Series loops present their own sets of problems. Increasing the flow has the benefit of reducing the delta T through the loop, giving more even heating between the first radiation in line and the last.

hot_rod

500 X flow X delta T

running a 20 delta, 500 X 4 gpm X (180-160) = 40,000 btu/ hr.
2.5 fps velocity in 3/4 type M


500 X 6gpm X 180-160)= 60,000 BTU/ hr
6 gpm would be 3.7 fps in 3/4 type M

Increasing flow requires more pumping power. If you know the piping length and all the fittings you could come up with a head number.

You could run up to 5 fps velocity, above that expect noise

Robert_25

Jason_13 said:

Right now each zone is a series loop. I only discussed the problem with one zone but all three have the same issue.
Way more radiation than required for each zone.
I can increase the system flow and pipe the boiler p/s so as not to get too much flow through the boiler. This should take longer to heat and reduce the outlet temperature due to more flow in the system which will drop my system water temperature and still give me proper flow through the boiler.
If I add flow restrictors I can play with them if needed.

I suggest using an ECM circulator and zone valves, and balancing the flow of each loop. You will need a certain amount of flow to avoid an unacceptable level of temperature drop through the loop. If the house is as over-radiated as you say, I would reduce the operating temperature of the boiler.