Delta T on Lochinvar Knight. What's the final verdict?

tuffcalc

Hi - read through so much info on delta t. What's the final verdict on a proper delta T for a primary loop?

My setup

Lochinvar WHB085, 0-10v controlled boiler pump (controlled by the WHB085), feeding into a secondary loop zones with actuators and fed by a Magna3 pump on autoadapt.

Lochinvar has a chart showing a 20, 25 and 35 degree delta T in the manual but no recommendation on which to choose.

Is there a general consensus on what's best?

EBEBRATT-Ed

The higher the delta T the more chance that the boiler can run in the condensing mode=better efficiency.

But, the wider the delta the lower your average water temp.
For instance with a high limit of 170 and a 20 deg delta return water would be 150 average water temp 160

with a 35 deg delta and a high limit of 170 return would be 135 average water temp 152.

Average water temp determines out put of radiation

Also, the system affects the delta so pump flow and radiation also control the TD so boiler set up and system set up should balance in the real world

tuffcalc

Thanks - helpful.

So since my lochinvar will control the delta-t by controlling the speed of the boiler pump with 0-10v control + modulate it's heat, it makes the most sense to dial in a delta T of 35degrees to get the highest chance of condensing, correct?

Leon82

It depends on the radiation attached. I have seen 6 to 8 on fin tube at 115 degree starting temp on one zone

On extra cold days it goes up to maybe 15 with both zones

hot_rod

Make a new plan, Stan
You don't need to be coy, Roy
Just listen to me
Hop on the bus, Gus
You don't need to discuss much
Just drop off the key , Lee
And set your ∆T free,

Delta t is an indiction of energy being transferred
It will vary depending on load conditions
Allowing the boiler ∆ to vary will assure you are operating at or near thermal equilibrium..
Imposing a constrained ∆T limits the ability to achieve your best operating condition, basically putting a false limit on the flow rate.
Certainly conventional boilers need to be protected from low temperature operating conditions a properly piped and applied ∆T circulator has value in that application.
Some condensing boilers have a manufacturers suggested maximum ∆T allowed to prevent stressing the HX, that needs to be accounted for.
If you design and flow the system properly a variable ∆ is not a problem.

ODR and forced or constrained ∆T don't get along well.

Viessmann has been varying the boiler pump speed for years, tied to the firing rate, which makes the most sense.

In a multi zoned, low mass distribution with fixed SWT, as zone valves close the ∆T will decrease, the ∆T circ could see that and respond by reduce speed to re-establish the ∆ for remaining zones, this will result in reduced electrical consumption, and prevent a single speed boiler from excessive cycling. Still I'd prefer to lower SWT by modulation and ODR if possible, gets you close to constant circulation which many consider optimum hydronic operating condition.

The ∆T at which a hydronic circuit operates is always determined at any time, by the circuits ability to release heat.
The primary loop is just an extension of the distribution system, in my mind.