It is my understanding that the manual mixing valve remains partially open to allow boiler supply to mix with radiant return to lower the radiant temp to a fixed temperature (lets say 120*). This valve always remains in this partially open state. I assume, cooler <120* returning water is always entering the furnace return loop, thus cooling the furnace temp faster? Does a motorized mixing valve open to lower the water temp and then close once the set temp is reached, unless the water cools in the system requiring an increased temp (then reopen)? If so does this allow for the furnace to maintain temp longer thereby reducing cycling?

the return water blends with outgoing water to modulate the temp leaving the hot circuit. it will cool down the boiler SLOWER, actually, since it slows the rate of heat removal, essentially.

motorized valves will open or close to maintain whatever target leaving temperature they are shooting for.

cycling will typically increase with mixing valves. however, if you have an intelligent control with the mixing valve, IT can reduce boiler cycling by firing the boiler across a wide temperature range and then "sipping" the heat out of it over a longer period of time.

There are "manual" mixing valves that are "fixed" but you are probably referring to a thermal bulb style with a "fixed" temperature setting, in which case...Rob is right!

It is a TermoMix 4 way valve by Mr. Pex... see http://radiantandmore.com/mrpextmb425.html
Is this a thermal bulb style?  It has a dial to dial in temperature.  There is no actually temp setting, just based on thermometer on supply line.

Thanks

no. that is a valve body that is fully manual, it will never adjust and your water temp coming out of it will vary if temperature on either side varies. good for proportional mixing but not fixed temp. Or you can fit the actuator that runs it along with a controller and use it as a motorized mixing valve.

IMO, if you want to reduce cycling, add a tank.

Tell me more about this tank...   I assume it would be a hot water storage type tank plumbed in the supply line.  I would have to remove current mixing valve, plumb tank then set the outgoing water temp to whatever I need.  I assume these can be controlled with an outdoor reset.  Boiler keeps tank at a temp, then that water feeds the radiant system.  The tank, which is better insulated, doesn't cool as fast thereby reducing cycling.  HOWEVER, if the boiler has to keep a larger volume of water hot, would I use the same amount of oil over the long run.

What kind of boiler is this, a condensing or non-condensing one?

Oh, just read the last line....oil....so non condensing. The 4 way is necessary to protect the boiler but how big is the low temp loop? Is it just a small add on or the whole house?

It covers three zones, which heats the whole house. two zones are engineered hardwood (5/8ths thick solid strips)/tile with thermofin plates (1st floor), 2nd floor is in joists staple up (I believe from the poor install on the first floor that had to be fixed).

Oil boilers generally need to maintain at least 120F return to the boiler which means 140F output which is not desirable for floor heating so the 4 way valve is necessary. It also means that the boiler often is putting out a lot more heat than needed. This type of valve can have an motor attached which can be driven by an outdoor control and this is your best bet for control over the water temp over a wider range of temps.

I realize that the outdoor reset control will help change the temp running through the system to accomodate to outdoor temps, but really how does this effect furnace function if I don't have a cold start boiler? My understanding is that the boiler will still have to heat to 180* whether the outdoor sensor says run 110 or 120* temp through my system.

it won't, if the boiler is not cold start. if the boiler can be converted to cold start you'd be better off with a proper control though.

Posted By MikeSolar on 05 Dec 2012 07:05 AM
Oil boilers generally need to maintain at least 120F return to the boiler which means 140F output which is not desirable for floor heating so the 4 way valve is necessary. It also means that the boiler often is putting out a lot more heat than needed. This type of valve can have an motor attached which can be driven by an outdoor control and this is your best bet for control over the water temp over a wider range of temps.

Many (or most) older oil boilers demand at least 140F return temps, and would self-destruct with 120F return water.  Anything below 140F return water also requires acid-tolerant flue liners and flue condensate management.  It's important to know the spec on the boiler you have, as well as inspecting the type of flue before cranking the return temps south of 140F.

Boiler bypass/system bypass and other near-boiler piping configurations can allow the radiation to run at low temp without damaging the boiler or the flue. It's not rocket science, but not all DIYers are willing to do the "hydronics 101" design steps necessary to make it all work.

(Almost) all oil boilers in residential applications are putting out a lot more heat than needed, 100% of the time they're firing- they don't modulate with load.  But it's the firing rate (which is fixed), not the output temperature that determines that heat output.  The boiler will always duty-cycle, and it's up to the heating system designer to manage it to keep minimum burn times long enough to avoid negatively impacting efficiency or boiler longevity.

[edited to add]

If there isn't sufficient radiation in the zones to keep the boiler from short cycling on zone calls when running at low temp, one can either add thermal mass (to the radiation, or to the boiler loop) or use a heat-purging economizer control on the boiler to maximize the hysteresis on the boiler's own thermal mass to lengthen burn.  Few things will short-cycle a fixed output boiler more reliably than outdoor reset control on a multi-zoned/micro-zoned system with low-mass radiation.

Consider replacing the boiler with a more efficient condensing one:

http://www.peerlessboilers.com/Products/ResidentialBoilers/PeerlessPinnacleOil/tabid/115/Default.aspx