Outdoor reset = sliced bread?

DaveGateway

Hereabouts, outdoor reset is talked-up like its the "best thing since sliced bread" I need help understanding this!

Thanksgiving, it was cold (10*F) but sunny. With all the cooking, kids, & some solar gain, we had to open a window. My hydronic heating system didn't come on for hours!
That night a warm Chinook wind (35*F) started blowing VERY HARD, with infiltration & something called "Wind Chill Factor" my heating system started running alot, like it was 0* outside! This sort of divergence, between outdoor temp. & actual heating requirements, happens alot!

Can you guys explain how an outdoor sensor can be used to predict what temp. my system should run at? Seems like I would get a similar effect by moving my thermostats outside?

Thanks BP

Mike T., Swampeast MO

Moving Thermostat Outside

That's essentially what outdoor reset does.

It changes the supply temperature in response to an ANTICIPATED change in demand indoors. I say "anticipated" because your house shell and insulation introduce a lag in demand.

If your system is digitally controlled it will try to keep calls for heat as long and as infrequent as possible regardless of the temp outside. The more closely your reset curve matches the actual loss, the longer and less frequent the calls for heat.

If your system is proportionally controlled it will try to keep flow through the system as consistent as possible by ensuring that the temperature of the circulating water closely matches the heat being lost by the structure.

Dave H_2

Weather responsive

Outdoor reset, so many different names doing the same thing.

Outdoor Reset = matches the heat that is lost at that particular moment in time. A home with outdoor temp of 0 has a higher heat loss than the same home at 40 degrees outside.

Theoretically, you don’t need a single thermostat in a home with outdoor reset. The controller should match the heating load as it changes. However that is nearly impossible considering the way we all like to have thermostats controlling our comfort.

Best analogy, cruise control in your car. You set it going 60 mph along a flat road, you hear the fuel injectors working at the same pace. Now take that same car and go up a hill set at 60 mph. You will heat the fuel injectors working harder however you still maintain 60 mph (colder outside). Now go down the hill and you hear the injectors backing off while still maintaining 60 mph (warming up outside).


Dave Holdorf

bill clinton_3

reset to what??

Say you got an upperstory "sunny" room that has a good deal of glass facing East and South; has high heat loss at night because of said glass; catches morning and mid-day sun through said glass. Add to that a West facing lower story room that catches afternoon sun. Then there is the North facing suite of bedroom, bath, office that gets no use during the day so the thermostat is turned off. Yet when the owner comes home at 5 pm, he/she wants to get the heat crankin' pert darn quick. Now, short of "Yes, Virginia, this is rocket science." how you gonna apply outdoor reset in a sensible manner??

Bill

eleft_4

responsive

Only in theory, the indoor thermostat is a safe guard for interior changes such as cooking, sun coming in at different season changes, more occupants etc.
You can tweak the curve and other activities inside will throw your tweaking a curve.

al

Mike T., Swampeast MO

Deep setbacks don't necessarily save energy, but if desired in that situation (and you're not using proportional control) oversize the radiation in that space relative to the rest of the structure (obviously it must be on its own zone).

Even if your reset curve is tightly matched to the REST of the house, you will have greatly enhanced recovery from setback ability in that zone.

Heat Boy

re: Outdoor reset

Outdoor reset ONLY works if the heat inside your house is properly sized.If a proper heat loss is done and the water temp at zero degrees ( or what ever your area calls for)is enough to heat the home comfortably then you will see savings. If it isn't then you have problems.

hr

Outdoor reset really needs

the indoor feedback RTU's also. This way the control has more data to respond to. Just because the outside temperature plunges doesn't mean the inside has!

Internal gains need to be accounted for as well.

With indoor feedback the control can decide how much to shift. tekmar info at their website has a better explanation of this.

hot rod

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Roger Litman

A voice against outdoor reset

I think if you are able to put radiation in a home and use 300 BTU per foot instead of the usual 600BTU, then combined with the boiler need for 140°(viesmann and Buderus not included) the use of outdoor reset may not give significant savings. If you are using Hydroair where you need about 130° water to avoid cold air blowing effect there is another situation where outside reset give marginal savings.If you want to warm the floor all year round with radiant( think bathroom)- where does outdoor reset do for you?

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Mike T., Swampeast MO

"If you want to warm the floor all year round with radiant( think bathroom)- where does outdoor reset do for you?"

While I don't do it year round, during the heating season my hydronic bathroom floors have a supply temperature varying from 95° to about 138° via outdoor reset. Constant circulation with no thermostat or adjustable flow control. Surface temp of the floor only varies from about 77° to 83°. "Neutral" when warmer outside, slightly warm when moderately cold, invitingly warm when bitter cold. Extremely variable climate.

Without the outdoor reset the floors would either be too warm or too cool most of the time.

When you couple indoor feedback with outdoor reset your control versatility increases.

hr

I agree, Roger

there are some limitations to reset. Try waking up a slab that has been off for WWSD quickly on a mild day. And I think year around bath floor warming deserves a simple 3 way thermostatic, else you have cold floors in the summer.

All in all reset does bring a lot more pluses when installed and dialed in properly. But it's not always the perfect control for every application.

hot rod

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Duncan_13

Seems like an odd question.

Not sure what your point is, Bill.

If you go back and re-read your post, you could ask the exact same questions about ANY hydronic system. For instance, a system with one zone per floor in a three story house.

If the solution to tighter temperature control of a traditional system is more zones and more zone valves...... The outdoor reset solution would be thermostatic radiator valves to control temperature overshoot in the warmer rooms.

Clearly, just as a traditional system's temperature must satisfy the coldest room's heating demand, the outdoor reset temperature curve should also be set to satisfy the coldest room in the house. And with Heatboy's balanced system design concept of matching radiation to load, the curve is very close for all rooms.

There are some real savings -double digit % savings- from reset in colder climates. And in milder climates, superior comfort, not necessarily savings might be the goal.

Why would you want nightime setback if you had outdoor reset? You would, in effect, be piling on two setback strategies at once. You see, outdoor reset IS setback. Maybe it would make more sense to call it outdoor temperature setback.

Realistically, without using PID logic controls like Tekmar's, which I've found works fine, I think I'd forget nighttime setback in combination with outdoor reset (which already IS the "setback").

If I had thermostatic radiator valves for overshoot control with outdoor reset, and I really wanted nightime setback... The rocket science I'd use for quick ramp-up after nightime setback would be a two stage thermostat.

It could be resonably argued that nightime setback on top of outdoor reset is too much trouble and expense for a realistic return on investment. However, I'd argue the lion's share of REAL savings and comfort are found in lower system temperatures for the entire system, not temporary, few degree setback of one or two rooms.

With sophisticated house controls with PID logic and Room Temperature sensing Units like hot rod mentioned, and balanced design that Heatboy mentioned, it's easy to have both outdoor and nightime setback in one unit.

I'm guessing your point is that the mild California climate doesn't see SAVINGS benefits of setback? Or that it's too complicated for the benefits? Maybe so. I finally visited California for the first time a month ago, and loved the mild climate! I couldn't believe how GREEN things are in February in the Bay area! Besides visiting relatives, I really only needed two things to make my trip complete... To see palm trees... (check!). And for someone to call me "dude"... got that, too. The beautiful countryside, friendly people, great seafood, sea otters, redwoods, wine country, and everything else was a bonus.

S Davis

Boost

Some of the Tekmar reset controls have a boost feature that helps with this.

S Davis