Delta T

Greg_T

I have been able to get 20 degree temperature rise from my radiant system and mod con boiler but a repair guy that was recently here mentioned that I should be shooting for a 30 degree rise on the heating side and 40 degrees on the dhw loop.  My question today is are those suggested goals reasonable or do they sound high?  My design data and notes are packed away in some unknown box so I have not yet looked to see what was used for design. 

He suggested that without digging into it deeper one way of improving the rise is to partially close isolation valves to slow down the flow of water and allow more heat transfer.  Is this commonly done to help increase rise?

I have more questions once I know what you think about these.

Thank you for your help.



Greg

Tom Blackwell_2

Heat transfer

I am sure to get slammed here, but this needs to be discussed. Heat transfer does not go down as velocity goes up-quite the opposite; all the way to over 25 feet per second. It is true that slowing the water down will result in a higher delta-t through that circuit, but in doing so the amount of heat transferred goes down. Think of a radiant circuit as a series of one foot long pieces all in series. The first foot sees supply water temperature and the last foot sees return temperature, with the pieces in between spread out between these two temperatures. Say you are heating a slab to 95 degrees with 120 degree water. First foot has a difference of 25 degrees across the tube wall, but the last foot would only have 5 degrees difference for a 20 degree delta-t. It is the difference between water temperature and slab temperature that transfers btu's into the slab. All of this averages out with the layout of individual tubes to give a reasonably homogenous temperature in the slab. By slowing the water down, there is a pumping savings, but the water will have to be hotter to transfer the same amount of heat into the slab. 20 degree delta-t is standard because it is easy to calculate-there is nothing magic about this number. Each designer has different ideas and most are right, just not the same as the next guy.

CMadatMe

Radiant Flow Rate

We like to run a 10 degree delta-t across the radiant zone. Keeps the floors evenly heated. If you run a larger delta-t the floors run uneven. For example if I ran 120 degree water and was returning at 90 then my average water temp through the floor could be 105. 105 may not be enough to heat on your design day. Also, keep in mind that your flow rates change significantly.. Let's say you need 10,000 btu's and you run a 30 that means you would have a flow rate of less than 1gpm. To accomplish this you would most likely have to use a pump like a Grundfos Alpha. If you mod/con is your mixing then why worry about the delta-t? It's condensing from the get go already. You should be focused on your comfort.

NRT_Rob

wider dt

can result in lower returns and better efficiency on a mod/con boiler.



also it lowers pumping requirements. it is through the magic of 20 dt distribution that you can push very large houses on very small pumps. and I've never heard anyone notice. (ask anyone with a climate panel job.. they are ALL designed for 20 dt).



however, at least one mod/con manufacturer requires a 30 dt OR LESS on their heat exchanger. be careful with wide dt on mod/con boilers and check with the MFG before restricting flow.



I will back up the other poster though and note that anyone saying slowing flow "helps heat transfer" does not understand how this works, and unfortunately that is a very, very common misunderstanding.