Delta T

28W

Happy Holidays, everybody!



This is my first heating season with a mod-con (Vitodens 100).  The system is piped primary/secondary with a low loss header. The circulators are Grundfos 3-speed units.   I'm trying to figure out which flow rate on the secondary loop will give the greatest delta T (i.e., coolest return water).



I attached a strap-on pipe thermometer (calibrated) to the boiler return, so I can compare the return water temperature to the temperature of the water as it leaves the boiler (as shown on the boiler's display). I ran the pump on speed 2 for several days, and took multiple readings each day, always while the boiler was firing.  Then I switched the circulator to speed 1, and took readings over several days, again always while the boiler was firing. 



In both cases, I got a 15-degree delta T.  Am I correct in assuming that on the slower circulator speed, there is a greater delta T across the radiators, but also that the secondary loop flow rate may not be sufficient to "capture" all of the boiler output so that some of that hot water is returning directly to the boiler? 

If both circulator speeds yield the same delta-t, should I use the slower speed, to save some electricity?

SWEI

what were the temps?

coolest returning water temp will produce the most condensate (highest efficiency) from the boiler.  Generally that means the lowest speed.  What's the ∆T and return temp like on your primary loop?

28W

Primary loop delta t

I think that is actually what I've been measuring. The pipe thermometer is on the boiler's primary loop return, right after it exits the low loss header.  So I've been comparing the boiler water temperature in the heat exchanger (that's what the boiler display is showing, right?) with the return water temp on the primary loop side.  Should I be doing something else instead?

So, for example, this morning when the boiler completed its firing cycle, the display read 140, and the primary return temp was 125.

SWEI

primary loop

Sorry - mis-read your original post.  Again, you're aiming for the lowest returning water temp.  Have you measured at high fire under load?

28W

Not sure . . .

I've just been measuring whenever the boiler has been firing for heat (not DHW),   It never high fires - always seems to spool down to low fire within a few seconds after start-up, and fire for about 10 minutes each time.

SWEI

circulator speed

Should be set so you have maximum ∆T (40F IIRC) at high fire under load.  I usually create load by dropping the indoor temp during a cold day or letting the DHW tank go cold.  I'm not familiar enough with Viessmann controls to advise specifics but there will be some method of forcing the burner into high fire for combustion and load testing.

28W

Are you talking about primary circulator?

I assume that you mean the primary loop circulator speed.

I'll do a quick check next time the boiler high fires for DHW

SWEI

Yes, primary circ

Secondary or zone circ(s) will have much lower ∆T -- typically 10-20F unless the emitter system has been sized for something higher.  Running those at a lower ∆T burns a little bit more electricity, but will give a higher average loop temp.  Returning water temp to the boiler drives fuel efficiency.

28W

Got it . . .

and the strange thing is that the secondary circ speed seems to have no effect on returning water temp to boiler.  Secondary speed 1 or secondary speed 2 yield the same return temp, for any given temp at the hx.

28W

looks like about 35 degrees

At high fire (DHW) the prmary loop has a 35 degree delta t.

SWEI

35F ∆T

can't do much better than that.  All good.