Outdoor reset mod cons with nighttime heating curve setback

roundrightfarm

I'm looking for a boiler that has this feature. We are getting ready to put in a mod con boiler with a home run distribution system having a radiator in each room with a TRV. If the main outdoor reset curve keeps the rooms at 70F during the day, I'd like the curve to automatically shift cooler by a programmable amount and at a preset time (and/or day) so that the night house temps are, say, only 60 degrees. I'm assuming this will make the TRVs open fully and stay that way while the radiators circulate lower temperature water until the curve automatically shifts back at, say, 6am.

Which mod con boilers offer this programming feature?

Gordy

Is the dwelling even capable of losing 10 degrees over night?

Ironman

Are you planning this for comfort or energy savings?

If energy savings, I'd forget it. The energy saved during setback will be lost bringing the house back up in the morning. And it will be a slow recovery: hydronics systems that are properly sized don't give rapid recovery.

NY_Rob

The Lochinvar Knight Boilers (KBN series) have that feature. Whether or not you use it is up to you.
A 10 deg. setback seems very servere.

http://s3.supplyhouse.com/product_files/KBN106-Brochure.pdf

Gordy

I can see a setback of that magnitude for long unoccupied periods. Daily as ironman says is counter productive.

If you can lose 10 degrees in a night you need serious envelope upgrades. Maybe if it were -10 and 20 knot wind.

Ironman

How often will we see a 70* delta T? 3 - 4 nights a year?

Gordy

I disagree. Maintaining setpoint is just a series of bumps to hold it. Letting it fall then returning to setpoint at the very least requires the same energy to get back to the same set point.

20% less over what time frame? 12 hours? If setback starts at 7pm and comes out at 7 am surely if you could lose 10 degrees it would be of a very short duration at 60. If a learning thermostat is used it would be coming out of setback before 7 am.


What's lost is the MRT in letting setpoint fall. Then bringing MRT back.

Then there is comfort of the lowered MRT until it stabilizes.

Now if you were going to use the 60 verses 70 over a longer period than overnight then yes there is savings with the lowered delta.

We had a 36 degree delta last night. I still held 70 with out heat on.

roundrightfarm

Thanks everybody for the great responses.

The amount of heat lost overnight is highly related to the size and shape of the building. A Square cube is better at holding heat than a long skinny rectangular cube. Think ranch style house vs a Victorian.

Even more important is the size of the structure. This has a great effect on the ratio of interior volume (held heat) to external surface area (where heat is lost). Double the length, width and height of the house and you will get 8 times the interior volume.

For the sake of this discussion, could we assume that the boiler wouldn't fire all through the night, due to the automatic setback?

So then we have the question of whether the heat lost during the night is the same amount it will take to replace it in the morning. Or put another way, would the boiler fire for a shorter amount of time in the morning than the sum of the evening cycles without temperature setback?

And how do low thermal mass systems, (like modern radiators with TRVs), which are supposed to respond more quickly to desired changes in room temperature factor into the discussion?

It would be great to hear more thoughts on this.

Brewbeer

@roundrightfarm, in general terms, the maximum achievable efficiency is obtained by matching the boiler firing rate to the rate the building is loosing heat, and this is generally achieved with outdoor reset.

If it takes 10 hours to loose 10 degrees during a night time setback, and if your boiler's outdoor reset is adjusted so the boiler firing rate matches the building heat loss rate (at 70 degrees), in theory, it will take on the order of 10 hours to recover that 10 degree setback.

If a quicker recovery from setback is desired, e.g., you want to recovery from setback in 1 hour, then higher water temperatures will be needed to achieve that goal. Depending on how rapid of a setback recovery is desired, and the size of the emitters, the higher water temperatures required to achieve the setback recovery may be well above the condensing range. Higher boiler temperatures means lower boiler efficiency. See: https://www.csemag.com/uploads/RTEmagicC_CSE1206_FHVAC_1.jpg.jpg

For example, let's say your building needs 20,000 BTUs per hour to maintain 70 degrees when it's 20 degrees outside. If you turn the boiler off by using a deep setback, and the temp drops 10 degrees in 10 hours, your building has lost a net of 200,000 BTUs [(20,000 BTUs/hr) x (10 hrs)]. If you are going to recover from that 10 degree setback in one hour (the 11th hour), you will need your boiler and emitters to deliver 220,000 BTUs in the eleventh hour. To make this recovery possible, you need to oversize your boiler and massively oversize you emitters. Or, accept a slower recovery, or a smaller setback.

While the above example simplifies the math, it is a good rough approximation for illustration purposes. Precisely quantifying the variables is challenging since the variables (indoor temp, outdoor temp, boiler temp, boiler efficiency, wind, sun, emitters, etc.) vary, which makes the math more intensive. However, the general consensus is that daily deep setbacks with subsequent setback recoveries don't save enough (or any) fuel or money to be worth it.

Jason_13

If you don't use boost to raise the water temperature the thermostat will start the boiler much earlier to bring the building temp up to the desired temp and desired time. Therefor not a true setback for 8 hours or whatever time period. Looking at the boiler side if you do use boost you will raise the water temperature in the system and decrease the run time to recover the building temp and lower the operating efficiency on the boiler. I would forget about night setback. The tighter the structure the less effective night setback is.

Brewbeer

@Hatterasguy since you are correct about the error in my approximation, let me do the full calculation on the above scenario (20,000 BTUs/hour at 20 F):

Hour 1: 20,000
Hour 2: 19,600
Hour 3: 19,200
Hour 4: 18,800
Hour 5: 18,400
Hour 6: 18,000
Hour 7: 17,600
Hour 8: 17,200
Hour 9: 16,800
Hour 10: 16,400

10 Hour total: 182,000

So you are correct Hatterasguy, it's not 200,000 BTUs lost in 10 hours, its 182,000 BTUs. But that does not change the fact that we are ASSUMING none of the other variables are changing, which we know in the real world, is incorrect.

What is needed now is for someone ( @Hatterasguy ) to run the calculations on the OP's system to see what water temp is required to achieve the setback recovery as per the example above.

The best way to make your case Hatterasguy is to show the math. @roundrightfarm your detailed heat loss analysis and emitter survey, as well as your boiler information, are needed to make those calculations. Without the specifics, we are arm waving.

Brewbeer

@Hatterasguy in your post above you stated "The board constantly makes the error in the calculation when doing setback to bolster the argument that setback is not energy efficient when the opposite is the actual situation."

In order to show that "setback in not energy efficient when the opposite is the actual situation" you need to show how much energy is consumed when recovering from the setback, and compare that to the amount of energy used in a non-recovery situation. Without making that direct comparison, you are arm waving, just like everyone else in here.

Which is fine, but it doesn't answer the OP's question about setbacks: "... whether the heat lost during the night is the same amount it will take to replace it in the morning."

Jason_13

I am trying to understand your philosophies here. Wouldn't the heat loss per hour be reduced as there is less to loose as the inside building and contents cools?
Also would it not take less btu's to replace the heat lost as your heat added to the building is quicker than the heat lost while recovering.

roundrightfarm

Thanks again for the continued discussion. If it helps, the building heat loss was calculated for 70F inside at -10F outside. The radiators were sized to achieve this with 140F supply and 120F return. Please let me know if there are other details I could provide to help inform the discussion.

Brewbeer

But those fewer BTUs are being replaced at a lower boiler efficiency, since higher SWTs are required to make the rapid recovery.

Brewbeer

roundrightfarm said:

Thanks again for the continued discussion. If it helps, the building heat loss was calculated for 70F inside at -10F outside. The radiators were sized to achieve this with 140F supply and 120F return. Please let me know if there are other details I could provide to help inform the discussion.

A few things we will need to know to help make the calculations:

1) the total building heat loss at -10F
2) the output characteristics of your emitters at various temps
3) the maximum setback you would use
4) the maximum amount of time you want the setback recovered.

Gordy

Last years Ng prices you saved .10 cents..... 3 bucks a month. Comfort and peace is worth more than that.........

Gordy

If you gain nothing by setting back then why bother other than sleeping preference if so desired.

roundrightfarm

1) total building heat loss 70F to -10F is 121,000 BTU
2) Buderus radiators- output characteristics found here https://www.yumpu.com/en/document/view/40839771/intergral-panel-radspdf-heating-help/17. We will mostly have type 33
3) let's stick with 10 degree maximum setback
4) with a two hour morning recovery

Gordy said:

Last years Ng prices you saved .10 cents..... 3 bucks a month. Comfort and peace is worth more than that.........

I agree, and it seems the consensus is that there are not much if any energy savings to be gained when morning recovery is considered. The NTI boiler I was considering does not have this feature and i didn't know if the lack of it made it worth considering other boilers, so thanks for helping me clear that up.

The NTI boilers do have a nighttime setback for indirect DHW tanks. Is the philosophy different with DHW? Recovery is quicker, but are there any energy savings?

NY_Rob

roundrightfarm said:

The NTI boilers do have a nighttime setback for indirect DHW tanks. Is the philosophy different with DHW? Recovery is quicker, but are there any energy savings?

Probably not necessary with modern indirect tanks.
We recently went out of town- no one was going to be home, so no need for space heating or DHW. I turned the boiler power off at 10am as we walked out the front door. I have the DHW Differential Set Point on my SuperStore Ultra indirect tank set at 10deg. We came home the next morning- I switched on the boiler power at 10:45 am- it had been off for 24hr 45min at that point. I was expecting it to fire up immediately on a call from the indirect tank... but it didn't. My wife ran some hot water in the sink... it still didn't fire up. The boiler only fired up once one of the kids started taking a shower. So in 24hrs of sitting- it lost less than 10f, for the 6-8hrs or so it would sit overnight- losses would be minimum.

Ironman

Newer indirect tanks lose less than 1/2* per hour with 125* water.

Ironman

Any interesting note on the use of setback with mod/cons: most of the European ones do NOT come with setback or boost. They know there's no real savings in doing it and their engineers think we're foolish for wanting it.

If you understand their philosophy of a mod/con is that it should run at the lowest firing rate possible, for the longest time , in order to achieve the highest efficiency, then you see their wisdom. After all, they invented mod/cons and have been doing it for decades. Setback is outdated technology that was designed to work with bang bang equipment and leaky structures.

Brewbeer

@roundrightfarm the literature you provided for the radiators doesn't seem to provide output at varying water temps, which is what is needed to run the calcs.

Also wondering about your estimated heatloss at 121,000 BTUs. Seems too high for an average house unless its drafty, old and quite large.

I did a quickie analysis of my system using a 10 degree setback, 20,000 BTU per hour heatloss, and recovery over a two hour period. Ignoring the fact that my 55,000 BTU (gross) boiler isn't large enough to recover 182,000 (net) BTUs in two hours (it would take at least 4), and factoring in the decrease in efficiency (94% at 100F SWT for steady state, 86% at 180F SWT for setback recovery), I calculate 216,000 BTUs of fuel needed to maintain steady state , and 211,000 BTUs needed to recover from setback, a 2.5% difference in favor of the setback, if all we are doing is looking at the 200,000 vs. 182,000 numbers.

So for my house, I would need a boiler twice the size as I have now to recover over two hours, for a gain of about 2.5%. What this doesn't account for is the reduced efficiency of the extra cycling that will occur due to having an oversized boiler. Does a boiler that is 100% oversized result in a 2.5% loss in efficiency?

Paul Pollets

If the radiators are sized for 140 supply water on design day, than I've found the mod cons work best. TRV's should be used on each radiator. I'd be wary of a 10 degree setback, 6-8 degrees works better. I'd give the radiators 1 hr to warm up after setback, which should be 1 hr earlier than when the household gets up in the AM.
I use Viessmann boilers which have a self-learning feature that is adjustable within the Vitodens control. By looking back 16-48 hrs in past water temp requirements, the computer has more data points to calculate the required output temperature. If the radiator design uses smaller radiators, or rads that are designed for 170-180 degree water, than a mod con has limited advantages and a cast iron or non-condensing boiler should be considered.

Paul Pollets

The timer allows 4 programmable setbacks per day. The boiler firing does not use a "boost" per se, but modulates accordingly to demand load. It will automatically ramp up to high fire when there is a call for DHW recovery. You can set the parameters for DHW tank recovery by specifying Delta T and max coil supply temp, or setpoint via high fire.

Zman

roundrightfarm said:

Thanks again for the continued discussion. If it helps, the building heat loss was calculated for 70F inside at -10F outside. The radiators were sized to achieve this with 140F supply and 120F return. Please let me know if there are other details I could provide to help inform the discussion.

This is a good a discussion.

It should also be considered that the optimal outdoor reset curve is set up to maintain a fixed temp. In other words, replace the btus being lost one for one.

In order to recover from a set back you need to gain btus therefore run higher temps at the boiler. Additionally mod cons are more efficient under low load conditions. In recovering from a setback, the boiler is likely to be running at high fire.

These factors should be considered in the discussion as well.

Harvey Ramer

Setback is advantageous in a forced air system but it also comes at a comfort sacrifice.

What is very advantageous, is overnight setback with an air-source heatpump. It is typically colder overnight and that greatly effects the efficiency of the unit. What is even better than that, is a heat pump coupled with thermal storage that allows it to glean energy at the warmest part of the day and remain off during the coldest parts.

Since a modcon's combustion efficiency is not affected by outdoor temp, "for all practical purposes", it is generally not advantageous to do much of a setback, particularly in a system that is designed to modulate the heat output to the buildings heatload.

One thing that is forever true though. A boiler that is not running is using the least amount of fuel.

Ironman

Harvey Ramer said:

One thing that is forever true though. A boiler that is not running is using the least amount of fuel.

But only when it's not running. Sooner or later, it will be.

Gordy

What's even better is an envelope that loses very little energy. The one time gift that keeps on giving.

Gordy

Hatterasguy said:

Gordy said:

If you gain nothing by setting back then why bother other than sleeping preference if so desired.

Let's agree that there is a gain when doing setbacks.

The amount of the gain is small.

The value of the gain is typically not worth the effort.

That's palettable in certain scenarios. The envelopes make up has a huge factor.

Harvey Ramer

@Ironman
Agreed.

Brewbeer

Hatterasguy said:

I understand your intense desire to believe that setbacks produce nothing as a return but don't manipulate the data with erroneous information to get you there.

The example calculations provided, while obviously hypothetical, demonstrate that the example 10 degree setback recovery in a two hour time frame has a very small (2.5%) increase in efficiency, while requiring the boiler to be massively oversized relative to the design heatloss. Since oversizing a mod con boiler is highly undesirable due to the problem with excessive cycling in shoulders seasons and the resulting efficiency loss from such excessive cycling, the example demonstrates large setbacks are incompatible with mod con boilers.

Of course, if the OP wanted to include the large buffer tank needed to compensate for the oversized boiler, that would help with the short cycling.

hot_rod

Ironman said:

Newer indirect tanks lose less than 1/2* per hour with 125* water.

Maybe some of the newer Euro tanks with 3" or more of insulation? Closer look at the test parameters indicates high ambient around the tank, and low tank operating temperature to make those claims.

With my well insulated tank, heat trap nipples I hear the boiler trip on twice during the night to maintain DHW. If the aqua stat has a 5- 8 differential, it must be more than a 1/2 per hour loss?

One manufacturer tested 6 or 8 indirects in their lab and they could not match that claim under "typical" operating conditions.

hot_rod

1-1.5 per hour IF it is not allowing any energy to migrate into the piping.

But still far better than a tank with a burner and flue up the center.

Ironman

Hatterasguy said:

hot rod said:

Ironman said:

Newer indirect tanks lose less than 1/2* per hour with 125* water.

Maybe some of the newer Euro tanks with 3" or more of insulation? Closer look at the test parameters indicates high ambient around the tank, and low tank operating temperature to make those claims.

With my well insulated tank, heat trap nipples I hear the boiler trip on twice during the night to maintain DHW. If the aqua stat has a 5- 8 differential, it must be more than a 1/2 per hour loss?

One manufacturer tested 6 or 8 indirects in their lab and they could not match that claim under "typical" operating conditions.

Definitely.

Folks love to quote that "1/2 degree per hour" value garnered from some sales literature.

I bet that none of them are better than 1 degree per hour in the real world.

Have any doubts?

Use a 140F tank temperature and set the tank differential to 20F and see if the unit will sit there for 40 hours without a call. If they can do 20 hours, they'd be doing very, very well.

Everybody lies.

Obviously, it depends upon the delta T between the water in the tank and the ambient air. And, they probably are using about a 40* delta. So, with 140* in the tank, and 60* ambient, you would surely be loosing at least 1* per hour.

Like the old timers used to say: "figures don't lie, but liars figure".

Brewbeer

Hatterasguy said:

Brewbeer said:

Hatterasguy said:

I understand your intense desire to believe that setbacks produce nothing as a return but don't manipulate the data with erroneous information to get you there.

The example calculations provided, while obviously hypothetical, demonstrate that the example 10 degree setback recovery in a two hour time frame has a very small (2.5%) increase in efficiency, while requiring the boiler to be massively oversized relative to the design heatloss. Since oversizing a mod con boiler is highly undesirable due to the problem with excessive cycling in shoulders seasons and the resulting efficiency loss from such excessive cycling, the example demonstrates large setbacks are incompatible with mod con boilers.

Of course, if the OP wanted to include the large buffer tank needed to compensate for the oversized boiler, that would help with the short cycling.

The example calculations were fundamentally in error because they assumed a far worse efficiency on the setback model to bolster your opinion.

Your own calculations show an energy savings of nearly 10% based upon heatloss.

Even if you give up 2% during recovery, you're still looking at 8%.

And, no, you absolutely do not need an oversized boiler to do a recovery. What you need is a clock thermostat that starts the recovery three hours before you desire the higher indoor temperature.

The only reason large setbacks are "incompatible with mod-con boilers" is the difficulty in implementing them with the available software. The stepped boost function leaves a lot to be desired. A proper control would include a time schedule that matches the thermostat so the boiler knows that high fire is required throughout that entire period and doesn't need to guess at it using steps.

The efficiency numbers I used came from the efficiency vs. SWT that posted above in my link. If you believe them to be inaccurate, you should post information showing so.

If you set the thermostat to come out of setback three hours earlier, then we don't have a 10 hour / 10 degree setback, which changes the numbers and is different from the conditions asked about by the OP.

What doesn't change, however, is the need to massively oversize the boiler from the heatloss, in order to achieve a recovery from a deep setback.

nicholas bonham-carter

I believe that comfort is improved with a lower constant temperature setting, with no setback.--NBC

Chester

"What doesn't change, however, is the need to massively oversize the boiler from the heatloss, in order to achieve a recovery from a deep setback."

A daily ten degree setback is an extreme challenge for a high-efficiency heating system. There's a tradeoff between comfort and efficiency in your scenario, as Ironman said in the very first response on this thread.

If you want a mod-con that can recover from a 10-degree setback you're going to have to get a much bigger boiler than you need or want. A perfectly sized boiler would run continuously at max output just to maintain 70 degrees on your design day and would have a very hard time recovering from that big of a setback during cold weather conditions. And remember that you're not just reheating the air in the living space; you have to bring the the thermal mass of the structure up to temp. You'll never get the efficiency you want out of the mod-con so you might as well save some money up front and get a cast iron boiler.

My personal experience as a homeowner with a correctly-sized mod-con and cast iron radiators is that even a 2-degree setback takes awhile to recover from. I've found it's best to 'set it and forget it' with respect to finding the right balance between comfort and efficiency. You can keep the bedrooms cooler with TRVs. Or open the widows a crack :-)

To answer your specific question, the Bosch Greenstar series is programmable via the FW200 outdoor reset controller or via the CT100 thermostat, which is essentially indoor reset control.

Gordy

Well said Chester. MRT is really the whole comfort enchilada.

Gordy

There are some that would complain of not being able to do that in 1 hour.....

Jason_13

Don't forget you will also need much more radiation. You can double the size of the boiler but the btu's transferred is limited by the amount of radiation. you have to keep in mind it is not only make up btu's but also the building still has heat loss going on during recovery.
I would rather avoid night setback, dial down the ODR curve to try to match the heat loss, keep boiler operating at a higher efficiency and increased comfort level. All items in home maintain the same temperature.