Outdoor reset vs programmable tstat

Wayco Wayne_2

the Homeowner starts the thread discussing his programmable thermostat got me thinking. It's quite common for a person to focus on his thermostat and I commonly get calls where the customer starts by saying, "there's something wrong with my thermostat." I recently installed a mod con boiler in a house where the homeowner is Scottish. My ancestry is Scottish too and I enjoy the perception that we have long pockets and short arms, much like Disneys Scrooge Macduck. I also think that sometimes there can be a grain of truth in this particular nugget. This customer has a honeywell chronotherm that he sets way back to 50 during the set back modes to get the most savings out of his system. However he doesnt have the enormous oversized cast iron boiler set at 180 that he once had. The first cold weekend I got a call complaining that his house was cold, not recovering, and this boiler must be too small. (It is a lot smaller than the old one. It hangs on the wall too.) I had seen his program on his thermostat and knew that it was not going to recover as fast. For a short term fix I went over and turned the reset curve up higher, but when the house is finished undergoing improvements/additon, I would like to educate him that he can perhaps save more and keep better comfort by finding a good reset curve and turning down the water temp rather than the air temp. What input would you guys give? WW

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kevin coppinger_4

Wayne..

You are correct..education is the best best. It is like radiant floor where people feel they still want to do the set back thing...I tell them to set it and forget it. In fact I would dump that old Honeywell for something more fitting to the boiler....kpc

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J.C.A._3

Wayne,

I think both CAN work together harmoniously, however....I also think that the setback periods will need to be adjusted as well as the heating curve for total comfort.(This poor little boiler WANTS to do it's job...but IT HAS TO BE TOLD how and when!)

Setting back high mass radiant isn't as advisable as say, baseboard or radiators, because of the recovery time. The owner has to understand that too.Two words here brother..Slab Sensor! Find the comfy spot and let it ride the curve.

For radiators, ya just can't beat TRV's. Each its own zone, but tough to control with thermostats. 1-5 seems so much easier.Again, find the comfy spot and let em' ride the outdoor temp. with constant circulation. They are easily adjustable and will let the boiler know if it needs to fire up or down.

The homeowner also has to remember that the old boiler was "dumb" and only knew 1 thing...get to that magic 180°. His new boiler is smart, but needs to be(house) trained. Good luck. Chris

Rob_32

HO opinion

I ditched a chronotherm when I went to outdoor reset. IMHO, it was trying to be too smart and was dueling with the boiler/reset control for cycling, etc. Substituted a more simple-minded programmable stat and have reduced nighttime setbacks to only a couple degrees. Recovery still takes a couple hours since the emitter output is is pretty close to the loss. Fin-tube emitters throughout.

Seems to me that a setback to 50, if the house ever reached it, is not saving any money due to the energy "investment" required to recover. There might be some info at energystar.gov that talks about maximum practical setbacks you could show him. IIRC, it's about 8F?

Good luck on the education part. It probably takes some people a while to understand how all this reset stuff works to save money and increase efficiency. "Slow and steady wins the race," maybe?

Paul Rohrs_5

Setback

I love what we are able to do with the reset curves now, but would only employ setback if the zones could come out of a setback mode with a "Boost" mode like you will see with some equipment. The tekmar TN4 series has done that well along with the Integral controls of the Lochinvar Knight. It has reset temp, programmable setback for 7-days, and then a boost mode to afford quicker recovery. Of course then the dialog moves towards how much net efficiency are we gaining if we are bringing on a higher temp just to satisfy a quicker recovery. That's a different thread entirely.

Honestly, all of my jobs have been high-mass slab-on-grade infloor, and I think I would be doing a big disservice by putting them in setback mode, other than if they were going to be away for lengthy periods of time.

Regards,

Paul

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T Towne

My Lochinvar Knight has a boost function which can be used in concert with the outdoor reset. You can set the amount of temperature increase as well as the ammount of elapsed time between each boost. Very nice function that may be able to help in these set-back conditions.

Ted_9

Reset

It also depends on the reset control. for the Buderus R2107, I don't recommend a programmable tstat. Let the control do its setback.

For other reset systems I usually recommend 4 degree set back for rads and no set back for radiant.

But you know this already. Just help the customer understand why.

Massachusetts

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Uni R_2

Perhaps a net gain?

> Of course then

> the dialog moves towards how much net efficiency

> are we gaining if we are bringing on a higher

> temp just to satisfy a quicker recovery. That's

> a different thread entirely.

>


If for a fixed return water temp a boiler runs better at, let's say, 72% then the controller should take advantage of that. Then it can adjust from this efficiency curve to the otr curve.

Peter_16

I am confused..

Setting back the thermostat should always result in either equal or less energy use. I am not familar with radiant systems and the outdoor reset control, but from a thermodynamic stand point it makes sense that setbacks should lower overall energy input into the home.

If you turn off your heat, then your fuel cost will be zero. When you turn the heat back on the temperature needs to come back up to normal. This is the same amount of heat that you just lost and did not pay for as it dropped. The net energy output is the same to maintain the temp, or let it drop and then bring it back up.

There are two other factors
1.The rate that heat transfered out of the home is based on the difference between the indoor and outdoor temps. Therefor a lower indoor temp should mean less BTUs lost to the outside and lower fuel bills.

2. When you recover from the lower temp setting the heat flow from your boiler into your house if higher in the begining becasue the temperature difference between your heat median and the room temperature would be greater. This should result in less total heating time than if you totaled all the short cycles that would have been required to maintain the higher room temperature.

The big question is how much energy are you saving by setting back. If a house is well insulated probably not much. If it takes 6 hours for your house to cool and 2 hours to get back up to normal temp, then what is the point of setting back the temp for 8 hours?

Just my thoughts. I am not a HVAC guy, I just took my share of enineering classes. Are there other factors that guys who work on heating systems know about that would make setback result no energy savings or even higher energy bills?

Peter

Leo G_99

Highway/City

I am of the opinion that with high mass radiant systems no setback. The best "educational" tool that I have used is the old car anology. Better mileage on the highway then in the city. Trying to reheat X amount of tons of concrete every morning just does not make sense.

Leo G

Wayco Wayne_2

The system in question

has cast iron convectors. After this week it will have a radiant floor also, but tht will be another zone not of concern to this thermostat. To my way of thinking, if you could find the right reset curve that would be the best way to go. When you set back you are owering the temperature of more than air, you are lowering the thermal mass of every object in the room. That's why setting back takes more energy. You are not just heating up the air. The walls and furniture lag behind creating discomfort to the inhabitants. I like the hydronic systems that have a water temp reset mode. Something that shifts the curve bcak 10 degrees, but does not stop heating the space altogether. I also love outdoor rest controls that have an indoor reset sensor, in case of unforseen influences such as high solar load or a door left open. This customer loves his tstat and is loath to give it up. It has been his loyal comrade in the fight against high fuel bills and he understands it. But, there's more to the story than meets the eye. WW

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Ted_9

degrees

Like I said, a few degree set back shouldn't be a problem here. It just wouldn't be good at 10 degrees.

That's my opinion.

Massachusetts

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jalcoplumb_5

Reset and Setback.

I have had the same problem.

One job I had installed a vision 1 reset kit for a Munchkin. The customer read about reset and wanted to save money.

The call came in that the house was always cold in the morning. It took a long time to get back to temp. They had a setback t-stat and liked to set it back at night. I explained the reset and why it took so long to get back to temp. after setback. They had me remove the reset.

It would be nice to have a reset control that can set back the indoor temperature by changing the reset curve. Then when the morning comes and the temp needs to go up, it will return to the previous reset curve and provide a boost mode to bring the indoor temp back up in a timly fashion.

I have waiting for this option for quite some time.

Dan Foley

Boost

Both Prestige and Knight have a "boost" feature that can be programmed to compensate for setback. Others may have this feature as well. -DF

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Mike T., Swampeast MO

Wayne:

"perhaps save more and keep better comfort by finding a good reset curve and turning down the water temp rather than the air temp"

Such is the "setback" philosophy of the Vitodens. Instead of completely stopping the production of heat until the air temp drops to some point, it reduces the supply temperature curve to a lower maintenance level.

While I don't particularly care for daily setback, this philosophy has a real advantage:

Just like with normal (sun dial) operation, reduced temp (moon dial) operation is weather-responsive. Space temp during reduced mode operation will drop and recover at the same rate regardless of the weather. With a simple digital thermometer with "low-high" temp memory you can very easily determine just how much setback you should be using. If your recorded low temperature is consistently higher than your moon dial setting, then you're using too much setback for your particular structure. This is quite different than typical thermostat setback where space temp falls more rapidly the colder the weather.

The only real difference between "normal" and "reduced" modes with the Vitodens comes when you have a directly-connected TRVd system. Instead of variable speed pumping in "normal" mode, the circulator operates at a fixed (but adjustable) speed. This is because the assumption is made that you really don't want to turn all of the TRVs up and down each day. When the TRVs are no longer maintaining setpoint during "reduced" mode, they open wide and are "starved". For this reason you want a fixed rate of circulation sufficient to achieve reasonably flow balance in the system.

If you have a TRVd, multi-floor converted gravity system directly driven by a Vitodens like I do, the backwards sizing of gravity piping can come back to haunt you with all of the TRVs open wide and the single circulator may very well lack sufficient flow ability to retain reasonable balance in the system.

There are a couple of other small but real problems with this method of setback with the Vitodens. When the boiler leaves "reduced" mode and returns to "normal" it will not boost the supply temperature above the "normal" setpoint. Boiler output will however increase because the air and surfaces in the rooms are now colder. This means the emitters are giving off more heat requiring more energy from the boiler to maintain setpoint.

Higher output from any mod-con does slightly decrease efficiency, so some of what you thought you saved via setback is lost due to the decreased boiler efficiency during recovery. It may also be possible in extremely cold weather for the boiler to lack sufficient output ability to maintain setpoint during recovery--such would likely be most noticeable and potentially objectionable with hydro-air. Also, in poorly insulated/weatherized structures where space temperature tends to drop rapidly the recovery period may become objectionably long if larger amounts of setback are used.

Mike T., Swampeast MO

Peter:

From an engineering standpoint you're essentially correct. Condensing/modulating boiler do have a little quirk that causes a bit of a problem however. It takes more boiler output to raise space temperature than it takes to maintain space temperature. As the output level of a condensing/modulating increases, its efficiency drops slightly. So you have additional wasted energy on your way out of setback. This problem is increased if you have completely shut down heat production until room temp drops to some point before heat resumes.

Aside from the engineering perspective, there is also the issue of comfort. If significant (say > 5°F) room temp setback is used or worse if multiple setback periods are used each day, my experience is that people will tend use a "high" setting a number of degrees higher than they would were a lower temperature (or less or less frequent) setback used.

Unknown

Several tekmar controls are capable of this.

rob_26

If your recovery is say 30 min for each degree, would that be acceptable

Mike T., Swampeast MO

Again, just that form of reset is built into the Vitodens. While it doesn't "boost" the supply temperature during recovery, it will boost output to maintain a steady supply temp to the emitters which are under heavier load during recovery.

MikeL_2

My solution to this has been to use hydro air, a buderus GA124/17, a Tekmar 262 with outdoor reset and indoor temp control and a Honeywell t-stat that is set to not cycle. Night time setback 18C, morning 21C, daytime setback 18C, retturn home 21C.

Works like this. At about 4:45 am the Tekmar moves to the daytime reset curve and the boost function provides a temp boost of about 10C. With the AH running at it's low speed of 345 cfm, it takes hours for the house to go back up 3C. However, the AH blower is hooked up to the Honeywell which controls it's high speed but shuts off at 21C. So the blower speeds up at 4:45 am as well and goes back to regular low speed in the hour to hour and a half that the recovery takes to 21C. The Tekmar was none the wiser.

What would be nice is if the Tekmar boost function also provided a signal to speed up the fan. Could eliminate the Honeywell t-stat (not really, it's need for A/C), but at least I wouldn't have to match program times on the Tekmar and Honeywell.

Unknown

Setback and modcons

Peter, what you are saying is essentially correct. The difference with heating with a modcon is that its efficiency decreases as you increase its heat output. I have attached a graph of a condensing boiler efficiency vs. percentage of full heat output.

If you are coming out of setback you will probably need full boiler output to raise the temp of the house. At full output you will be running a higher water temp, and may be getting an efficiency only in the mid 80% range. If you are just maintaining temp to meet heat loss, your output and water temp will be much less and the boiler will be operating on the highest efficiency condensing part( 97%) of the curve.

So from an efficiency point of view, setback with a modcon probably doesnt make much sense.

Dave_4

Setback...........

Peter,

I think I am begining to understand the science here. In a reset system the key to better efficiency is lowering the water temperature. In a setback mode (with reset) you have to raise the water temps to recover. Thus any savings you gained by lowering the air temps (smaller delta tee over night) are negated by the higher water temps in recovery mode.

In a conventional system the water temperature is always 180 even in recovery mode so you do save money in that mode on setback.

With this logic it makes sense not to use setback when using reset.

JR

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Mike T., Swampeast MO

Yes!

And you can't neglect the type and size of emitters either! High mid- to high-mass emitters like nicely conductive radiant floors or old cast iron rads can typically absorb the full output of the boiler even at quite low temperatures--especially during recovery when their relative output is higher because room temperature is lower.

Depending on the control logic used this could easily find the boiler output at or nearly at full output regardless of the weather. With really high-mass like tube-in-slab the efficiency hit wouldn't be too great. With mid-mass the efficiency hit could be significant. With low-mass (like fin baseboard) the hit could be substantial--especially if "automatic boost" control logic is involved.

If the boiler completely stops firing until the reduced space temperature is achieved, you'll find that the efficiency "hit" from recovery increases with load (colder weather). If on the other hand you achieve setback by allowing the boiler to fire to a reduced reset curve even as the space temperature drops you'll find that the efficiency "hit" tends to stay fairly constant regardless of the weather. This is the Vitodens method of setback--to be most effective however you need true constant circulation and probably TRVs/FHVs as well.

Even if "automatic boost" control logic isn't used, there's still an important element of control, "How fast do I want to get the system up to temperature?" The faster the boiler gets the system up to temp, the greater the required output and the greater the tendency to over-shoot. Overshoot too much and you have to reduce the firing level. Thus, there's a good chance that the boiler will be frequently and significantly adjusting the modulation level during recovery. Frequent, significant adjustments of modulation level reduce efficiency.

Again, I'm NOT making a Viessmann commercial, but I can relate my observations on how the Vitodens [appears] to deal with setback. With only it's single sensor ("boiler temperature"), the Vitodens effectively learns the characteristics of the emitters to which it's attached. As it learns and fills its operational data store it gets a better and better idea of the nature of the emitters. If you use daily setback of supply temperature it will get a better and better idea of how to make a one-time "jump" in output necessary to have the normal (sun dial) target temperature just maintained at the time recovery begins. (Of course changes in outside temperature will change modulation rate, but such generally occurs relatively gradually and over a relatively long period of time.)

Judging by that graph, my system operates at 96.5%+ efficiency in all but the most severe weather unless I monkey with boiler or TRV settings.

MW_2

set-back

Excessive set back can definitely cost bookoo bucks. I have been on several service calls during cold Montana winters which involved one or two zones not working. After determining that zone valves are indeed opening or pumps are indeed running, I check out the programming on the thermostat. Some veteran Montanans actually program set back temps as low as 50 or 55 degrees, even when its 25 below outside, burr. This lack of circulation and heat in the zone piping often leads to frozen and/or broken pipes, and this, my fellow heating professionals, can lead to astronomical repair costs.

BillW@honeywell

Here's the view from my side of the fence..

Programmable stats, especially the current generation of our VisionPro line are +- 1 degree accurate. They have very sophisticated recovery systems, allowing them to "ramp up" the temperature gradually from setback to comfort level, so you don't get out of bed into a cold house, or come home to a hot home in the summer. Setback works fine on scorched air, and with fin-tube type baseboard. I do not recommend a deep setback with hydronics using cast iron emitters, or with steam.

Our new Aquatrol will provide outdoor air-based temp reset for just about anything else, radiant, panelrads included. High mass shouldn't be set back, and I agree that TRV's are the best for panelrads, teamed with outdoor temp-based reset & constant circulation. The FocusPro non-programmables work very well with zone valves or zoning panels where TRV's aren't an option.