Equivalent length for pipes in parallel?

sunlight33

Say I want to calculate head loss for three-zone system in parallel:
zone 1: 100 feet equiv.
zone 2: 75 feet equiv.
zone 3: 50 feet equiv.
What should be the overall equivalent length for the system?

Jamie Hall

If you're thinking about head loss for sizing a pump, the only one you need to worry about is zone 1. That's the one which will set the maximum head you need from the pump for any given flow. If you have more than one zone calling, then the flow will greater -- split between the zones calling (much more in the shorter zones if you don't use balancing valves) and the pressure drop less.

sunlight33

I need to see what the flow rate is on the secondary pump (system side) so it will work with the primary pump. Right now the primary pump is doing 5 GPM, and system flow (w/ zone 1 only) is about 5.5-6 GPM. With all three zones valves open, the system flow should increase because pressure has dropped more?

Jamie Hall

yes. By far the easiest way to do this is to pick a reasonable head loss and calculate the flow in each of the pipes for that head loss Add the flows. Then solve for a pipe of some length and diameter as convenient which will have the same flow as the total at the same head loss.

sunlight33

For electrical resistance at 100, 75 and 50 ohms, the parallel resistance is 23 ohms, does the same analogy apply here as in 23 feet?

Jamie Hall

No. Electrical current is predicted by the familiar E = I*R. For water, the head loss is proportional to the flow raised to the 1.83 power. So the familiar equation for parallel circuits from electricity does not apply.

sunlight33

Thanks for clarifying!

Zman

sunlight33 said:

I need to see what the flow rate is on the secondary pump (system side) so it will work with the primary pump. Right now the primary pump is doing 5 GPM, and system flow (w/ zone 1 only) is about 5.5-6 GPM. With all three zones valves open, the system flow should increase because pressure has dropped more?

I am not sure I am following you. Are you suggesting that the primary and secondary flow rates need to be equal?

The flow rates through your zones should be align with the emitter BTU rating and desired delta T for the zone.

sunlight33

Relatively speaking the secondary should be a tad above the primary so to prevent some supply from going straight back into the return at low loss header, to keep the return temp lower longer. Of course the priority here is the design of the zones like you mentioned.

Retrofit a mod-con w/ODR in a house built in the 80s with baseboard designed for hotter water is never going to be a very good solution. I did however enjoy the savings on heat bill each year and the house was pretty comfy at 68-70F regardless of the outside temp. But I wonder what can be made possibly better:
1. Replace with baseboard designed for cooler water
2. Replace some baseboard with modern radiators that have more mass.
3. Add a buffer tank

Option 3 was out due to limited space in the boiler room, and it would involve too much work. Is option 1 better than 2 in terms of cost and performance?

Brewbeer

Option one: replace fin tube baseboard with cast iron baseboard. This also kind of gets you option 3. Cast iron baseboard holds about 3 gallons of water for every 10 feet, so it's like a buffer tank built into your radiators.

sunlight33

I looked up the cost of replacing existing baseboard with cast iron, total is about 6k with shipping fee of 1k. That's roughly the cost of 7 buffer tanks, ouch!

Zman

The emitter flow rates needs to match the needs of the emitters.
The boiler flow rates need to meet this requirements for the boiler. Copper fin boilers require fairly high and consistent flows, cast iron boilers are not very particular. With condensing boilers, you do increase efficiency by widening the delta(as long as you are condensing) , but still need to meet the minimum required flows. The new Lochinvars with VS pump controls do very well with this.

Take a look at the Runtal panel radiators for low temps. They are low profile and work well at low temp.

sunlight33

Thanks for all the suggestions, I look at the room again and option 3 is possible with a smaller buffer tank. I'd like to know if there is any significant difference for the two layouts below:
1. Replace the existing LLH (Viessmann model 80/60) with a tank like the Boiler Buddy.
2. Leave the LLH in place, feed the secondary return first to an electric water heater, then out to LLH return.

Option 2 is cheaper but will there be any performance hit?
What do you think?