What is acceptable delta T

Mike T., Swampeast MO

Then you shouldn't have a problem adjusting as was suggested to you. Just still expect boiler delta-t to be significantly lower when only a single zone calls.

Dave_61

questions on delta T for boiler

What is an acceptable delta T for a boiler (mine is copper tube)? I know there tends to be a design standard of 20 degrees. What is the reasoning behind the 20 degree value? At the minimum flow of 12 gpm that my boiler can possibly have (per manufacturer), the delta T is 40 degrees.
Is 30 degrees acceptable? What would be the drawback?

Mike T., Swampeast MO

Many consider the 20° delta-t nothing more than mathematical convenience because 10,000 btu = 1 gpm. In reality and in most systems, a reasonably steady 20° delta-t is the far exception rather than the rule.

All else equal, the only difference between 20° and a higher delta-t is lower flow; difference between 20° and a lower delta-t is higher flow.

When planning do not however forget that average emitter temp is the mean of supply and return temps. Use a higher delta-t and you need a higher supply temp to retain the same average emitter temperature; lower delta-t requires a lower supply temp.

When it comes to copper tube boilers, do pay careful attention to minimum flow requirements. Probably not a bad idea to err towards higher than the minimum requirement...

Dave_61

So Mike,

as long as I am significantly above the minimum gpm (12 gpm), we should be OK? With the pump I was going to use, we'd probably have a 25 delta T. Currentlt, with our oversized pump running and our furthest air handler calling for heat, our delta T is about 1. Which reminds me, to set the flow, I was told that we should have all zones calling for heat and then throttle back the globe valve at the boiler pump outlet to get about 25 delta tee. Is that correct?

pitman9_2

If you're throttling the circ back

it's too big and using more energy than needed. Why not go to a smaller circ if possible?

Mike T., Swampeast MO

Am presuming that you have a multi-zone system with a single circulator and the hydro-air zones controlled by zone valves. Correct? How many air handlers and what size each?

to set the flow, I was told that we should have all zones calling for heat and then throttle back the globe valve at the boiler pump outlet to get about 25 delta tee.

Sounds reasonable with the provision that you MUST ensure minimum flow through your boiler at ALL TIMES--including the smallest zone calling for heat by itself. This is where multi-stage copper fin boilers can come in very handy as you can "lock out" stages without a "big" heat call thus keeping the boiler "happy" with both flow and delta-t. It's quite possible that a VERY low delta-t when only one air handler is calling will be the normal situation has flow through the boiler must still be maintained.

hr

The delta t is ever changing

and as you noticed with only one zone calling it can be fairly tight even with the correct circ.

It's the multi loading and sometimes small zones that are hard for a low mass boiler to get a handle on.

Two stage gas valves can help, but expect to see changing delta t with changing loads.

A modulating condensing boiler would be better yet

If the boiler was sized exactly and all loads were calling on a design day you may see the delta t you are after. aswell as a good long, hot burn cycle. possibly non stop.

Primary secondary is wise on a boiler like that. It assures the boiler always has the proper flow regardless of how many zones are calling. Use of the same circ for the boiler and distribution gets to be a challange.

A PAV might be a good idea also.

hot rod

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Dave_61

Mike,

we have a primary loop for the boiler and off that loop, we have a supply header with all separate circs for zones and a return header with all heating returns.