Questions on return thermostatic mixing valves...

I have a Maxim pellet boiler connected to a forced-air furnace and a sidearm and no storage (I figure I don't need it for a pellet boiler).

The boiler is easily able to keep up with the load presented by the water to air hx and the sidearm - I have it set to 180f, and the lowest I've seen it go is about 165f coming off a long idle (fire almost out). Once it clicks to the highest burn mode, it is able to gain temperature even under full load.

My question is this: Using my infrared thermometer on the pex (not the best method ... but it's what I have), I believe that my return water temperature may be a bit below the 140f that I've read is a 'safe' temp. Do I need to be concerned, even though the temperature readout on the boiler shows that water temp in the boiler is never actually dipping that low? Will the low temperature water returning to the boiler cause problems before it has a chance to mix with the hot water in the boiler's tank?

I could put a bigger circ pump in, but I've got a taco 007-f5 pushing water through about ~100ft equivalent length of 1" pex (not including the hx, for which I have not calculated the equivalent length), and from the information in Taco's sizing guide, I believe that I should be OK.

Hollow Pine said:

What size Maxim do you have? I am running a Maxim M250 into my existing oil (standby) boiler and I have tuned the Firestar controller to not let the water temp drop much more that 4 degrees.

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It's a Maxim 175. I changed the control variables so that it goes to 'normal' at 2 degrees down and 'high' at 8. Using this setup, I typically never see temps below 170 when it's set to 180 (it only goes lower if there is a big heat call - say going from 65f in the house to 74f due to thermostat change) and the fire is almost out from sitting idle for a long time.

My concern is that even though the boiler temp rarely goes below 170 or so (and never goes below 165), I'm pretty sure that when under load the return water temp can get well below 140, and I'm wondering if that's a problem even though the boiler temp never gets that low.

Taylor Sutherland said:

I'd worry more about the long idle time tarring up the boiler than a return temp of 140F

TS

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Hollow Pine said:

Several thoughts on this. Your combustion air fan speeds and fuel feed rates are adjustable independently for the three firing rates on the boiler. By tweaking these a little, you can significantly affect how close your actual water temp stays to setpoint. The other thing that you can do is to try raising your temp setpoint to 185. This is where I run mine as soon as it get s cold (December). One other thing that you can experiment with is is a three speed circulating pump. The Grundfos and perhaps other pumps allow you have three different pump rates at the flip of a lever. My boiler and heating system work great, but even after two full winters under my belt I am still learning. Everything from fuel quality to stack height to everytihng else in between comes in to play.

Good luck!

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Taylor - Since I changed the control variables and it's gotten colder outside, the long idle times and tar problems have gone away (it still idles for a few hours at night when the thermostat goes from 74 to 67, but there's nothing I can do about that, and it's offset by a good hot burn in the morning anyway). Spends much time in 'normal' burn mode now, and I fully expect that once it gets a little colder it will spend nearly 100% of its time in 'normal' burn mode. I still haven't figured out how to fully remove the thin layer from when it was warmer, though :/.

Hollow Pine - I've got it set up right now so that my setpoint is 180, and it spends 99% of the time between 170 and 180. My concern is that the return water temp is low - the boiler temp itself never goes low enough that it is a problem.