Hot Water Circulation Pipe Sizing

[EnOm]

Hi,

I have a question regarding domestic circulation pipe sizing. I have already calculated the flow required from the circulation pump. I sized the main return pipe to keep velocity under 4 ft/s, but what about the pipes branching out to the separate floors and the different utilities on them (bathrooms, pantry..etc). Its a 3 storey house by the way.

I am thinking maybe I need to estimate how much of the circulation flow each will be getting and then base the pipe size on that flow using the velocity limit but so far I have no leads.

Regards.

 

[PEDARRIN2]

In general you want to route the circulating piping so that the circulated main is within 10-20 feet of any faucet. However for a home, that rule might be a bit conservative. If you are interested in conserving water and getting hot water to your faucet quickly, the main would be closer. If not, then you have to decide.

I have never sized a circulating pump based on velocity, mainly because they never approach the upper limit.

A rule of thumb is to use the gpm which will only give a heat loss of ~9 btuh for each foot of pipe with a 5-10 F loss. That would give you 0.002 gpm/ft of pipe. This flow overcomes the heat loss a stagnant pipe would have, which will maintain the temperature in the main.

 

[cdxx139]

Most systems I design are 1/2", larger ones are 3/4". You have a recirc pump that turns on at 100F and shuts off at 120F. Cant imagine a lot of flow is needed to do that.

If my line is not a basic loop and resembles an eight legged spider instead, I place balance valves on each leg so I have a way to guarantee flow

knowledge is power

 

[EnOm]

PEDARRIN2,
Thank you for your input. However I am a bit confused by your statement "I have never sized a circulating pump based on velocity, mainly because they never approach the upper limit". Did you mean to say you had never sized the pipes based on velocity?
Furthermore, it does make sense to base the flow through the pipe branches based on the pipe length going to a specific floor, kitchen..etc. However I am not using a rule of thumb, I am doing the actual calculations taking into account pipe surface area, material U-value..etc. Sounds a bit tedious, but I have already made an Excel sheet so it shouldn't take too much effort.

Regards

cdxx139,
Thank you for your response. Placing balancing valves makes perfect sense, however looking at consultant drawings I never saw any indication of the gpms going into the pipe branch-outs (spider legs). Only the pipe size is indicated. I assume that is left up to the contractor to balance upon execution?

Regards

 

[PEDARRIN2]

I should clarify my statement. I size based upon counteracting any anticipated heat loss through the piping. As a rule of thumb, I never use 1/2" on the recirc portion unless it is a very simple system. I try to minimize pressure loss in the pipe to minimize the pump HP, so I am, in a way, sizing for velocity. But, most of the time, I never get to any limiting velocity.

For example, limiting 3/4" pipe to 8 ft/s would give a flow rate of ~12 gpm which is a fairly high flow, which should only occur near the pump, which would likely be 1" pipe.

Not indicating the flow for each balancing valve on the drawings and leaving it to the contractor is an open invitation to owner/client dissatisfaction at not having hot water in a timely manner. This will likely result in the contractor fixing the problem and will likely be charged as an E&O to the engineer. At the least, it will be a RFI which the engineer will have to provide the information that should have already been on the drawings.

I always include the flows at the balancing valves, which allows me (and the internal QC) to verify the recirc pump size.

 

[cdxx139]

Agreed, never leave things for the contractor to figure out. Spread the flow accross the balancing valves and proportion it to the quantity of fixtures it is serving.



knowledge is power

 

[EnOm]

PEDARRIN2,
Thank you for your elaborate response. Noted.

cdxx139,
Thank you for your response.