Outdoor reset mod cons with nighttime heating curve setback

Gordy

Hatterasguy said:

Gordy said:

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

Well, in that case, those that suggest a giant boiler is required are absolutely correct.

And that is where the fixation comes from to accomplish that silliness. Most installations of old can accomplish this task because they were oversized. when sizing is done correctly there are trade offs.

Brewbeer

Hatterasguy said:

Your stated requirement for setback and recovery is a bit ridiculous.

Hat, you might want to go back and re-read what has been posted in this thread. The 10 degree setback / 2 hour recovery requirement was stated by the original poster, not me.

Also, the graph I linked earlier in the thread shows an efficiency of 94% at SWT of 100F, and 86% at SWT of 180, substantially more than the 2% you posted. If you have a source which show these efficiencies to be in error, link us up.

I never claimed the boiler could do the job, hence the usage of the word "hypothetical". It was merely a set of calculations designed to show that the setback and recovery requirements specified by the OP were not realistic.

Paul48

Thinking out loud............How much energy does it take to warm everything within the home 10 degrees? Every piece of furniture. Every interior wall. Every floor or ceiling that's not included in a heat loss? Don't those things figure into recovery?

Gordy

Paul48 said:

Thinking out loud............How much energy does it take to warm everything within the home 10 degrees? Every piece of furniture. Every interior wall. Every floor or ceiling that's not included in a heat loss? Don't those things figure into recovery?

Harvey alluded to the fact forced air can accomplish setbacks even deep ones. This is because forced air is heating the air. Mass will lag if the mass is the same temp as the setback that was achieved.

So F/A heats the "air" quickly satisfying the tstat for recovery. However the mass won't be to the same temp as quick, which would lead to discomfort. Think cold 70 because the cooler mass of the the objects you mentioned is drawing heat off the human body until the MRT is stabilized.

Which brings a point that even F/A may cycle more during a setback recovery because the colder MRT will cool the air until the MRT is stabilized.

Gordy

Convection verses FA. Obviously quite different air velocities circulating the warm air

SWEI

Most mod/cons have the ability to define more than one rest curve. Use a programmable thermostat to switch between them.

Brewbeer

Hatterasguy said:

That was your stated requirement for recovery from a deep setback and it is ridiculous. You do not need to massively oversize the boiler. As I previously mentioned, most mod-cons are oversized by 50% on the design day in any case for DHW needs. There is plenty of reserve for recovery without any real difficulty. No, you can't do it in one or two hours but for those that have a clock thermostat and properly use it, it really isn't a problem.

Again, I did not state it, the Original Poster set forth the 10 degrees set back with recovery in two hours in his question to this board:

roundrightfarm said:

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

Hatterasguy said:

This is now the third time I have explained to you that the boiler can never achieve a SWT above 150F. Apparently, you simply cannot understand that. Your cited efficiency at 180F is irrelevant. The boiler isn't going to lose more than 2% in efficiency if the SWT is 150F and the RWT is 130F.

And this is the third time I explicitly stated that the example calculations I provided were hypothetical, only designed to show what is involved in recovering the heat lost during setback that roundrightfarm asked about. We aren't talking about my boiler, we are helping @roundrightfarm make a decision about HIS boiler. If he wants to recover 10 degrees in 2 hours, like he stated, he may in fact need 180 degree water to do so.

In your own words:

Hatterasguy said:

Apparently, you simply cannot understand that.

Harvey Ramer

@roundrightfarm
Setback theory aside, I don't know of any boiler that has the programming features you are looking for.

A tekmar boiler control, will however have everything you need. You would need the control, tekmar timer, outdoor sensor, and indoor sensor. The timer allows you to program a schedule between occupied and unoccupied and switches the control between 2 room temps that you can set in the controller. The control modulates the boiler via 2 wire communication of multiple different configurations. The tekmar will have the odr curve programmed in it. It uses the indoor sensor to adjust the odr curve to exactly what the house needs. This enables a rapid recovery from Setback at the lowest possible water temp. You don't need a thermostat with this control. The tekmar uses the indoor sensor to determine room temp.

Chester

Hatterasguy said:

Chester said:

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.

This is agreed.

How many folks have a "perfectly sized mod-con"?

The general approach is to size a mod-con a bit larger for DHW use, and, therefore, the H/O might have 50% reserve on the design day for recovery.

It's a rare machine that is sized within 10% of the heatloss.

As an example, I installed a UFT-80 in a house with a 42K heatloss. Think that house would have any trouble doing a recovery? The UFT-80 is the smallest boiler in the line and the use of it allows me to utilize a 35 gallon indirect without any cause for concern in a household with six members and three bathrooms.

I didn't mean to imply that boiler output exactly equal to heat loss is the right approach. I'd think there's just too much uncertainty related to heat loss. And a certain amount of headroom isn't going to hurt overall efficiency, especially with a good turn-down ratio. My 57,000 btu boiler is the smallest in its range and is still more than double my heat loss, and still takes quite awhile to fully recover from a setback. And I have about 5 times as much radiation as my heat loss, so that's not the problem.

Chester

And I'll add that I used to use a 5-degree setback that took about a half-hour to recover with my previous boiler at 140,000.

SWEI

If you setback the water temp and not the air temp, you might have a chance. If the boiler supports three reset curves you can use a two stage stat, but that doesn't really account for potential overshoot on the high side.

Harvey Ramer

This one would do the job.
http://s3.supplyhouse.com/product_files/263-install.pdf

Jean-David Beyer

I have a radiant slab in my biggest (of two) zones; the only one where doing set back might make sense. But it takes well over 24 hours for that slab to recover from a setback because the ODR curve is set to be just barely enough to maintain temperature. After the system was off for 6 1/2 days after storm sandy, I deliberately changed the ODR curve for the slab to put much hotter water (120F instead of 95F) into the floor, so it was pretty well recovered by the nest day, but it did overshoot because the slab was too hot by then. So it was not really recovered until the second day.

It is too bad, in a way, since I have a fancy Honeywell thermostat that allows me four different temperatures for each of the seven days of a week. So I could set it to 28 different temperatures each week. But as a practical matter, it would make little difference.

Erin Holohan Haskell

Interesting discussion. Thanks to all involved.

SWEI

As usual, it depends.

In a tight, well-insulated building (think Europe) that is regularly occupied setbacks make almost no sense. In a high mass building that is even moderately well insulated they make little sense. In intermittently occupied buildings (churches, schools, factories not running three shifts, etc.) they can work quite well.

Accomplishing them in a radiant heating system is no easy task. We've developed DDC algorithms which work well even in uninsulated very high mass churches, but the work effort was significant and we are still fine-tuning after three winters of operation.

smithfan

Just to add my 2 cents to the discussion. It seems like a balance between efficiency and comfort. If the OP intends on using a 10 degree setback for the sake of comfort maybe a mod/con is not the way to go. A traditional boiler will also last a long time, and will still allow for cost savings with these setbacks. Yes, a high efficiency boiler is "efficient", but that's secondary to comfort in my opinion.

Paul48

@SWEI touched on it............ If you are simply turning the thermostat down, you aren't setting back efficiently. If you are running a single curve,, when there is a call for heat in setback, you are then sending too many btus to the emitters.
How many contractors in this country are capable of developing algorithms to make their systems efficient? Europe mandated the use of mod/cons because a poorly installed mod/con can still operate at 80+% efficiency. A poorly installed conventional boiler can be much worse. Sound familiar? "I've been doing this for 30 years"......."There's no need to do a heat-loss".
This is not meant to condemn the many fine contractors out there, but, unfortunately they are outnumbered by a huge margin.
There is a "learning curve" associated with mod/con heat. You can no longer warm your arse in front of a piping hot radiator. The whole house, and everything in it becomes your "radiators". The house/building has to be over-radiated for the system to work at it's claimed efficiency, period. Every other way that is being offered, to do so, is "smoke and mirrors".

InterpreDemon

FWIW, I am a homeowner/tinkerer who has dealt with the comfort/efficiency balance for years. I run a conventional Smith/F5 with four baseboard zones, fired to design of 0F, meaning continuous operation holds 65F indoor at that OAT. For the rest of the heating season my WR1050 bulb outdoor reset controller modulates between the boiler low of 150 and high limit of 220 which, with four zones (single pump, four valves) results in virtually continuous circulation and, for this old house, no creaking radiators. A fifth zone is a heat exchanger with a bypass thermo-valve that supplies a 100F glycol loop that goes out to a huge circular unit heater in the ceiling of the detached garage as well as a small unit heater in an attached shed.

But back to comfort vs efficiency. The only issue with ODR which, when the curve is tweaked in properly makes a baseboard system virtually inaudible and unnoticed in one's life, was re-heat after night or vacation setback, which could take hours. So for many years I used a "morning boost" timer that would bypass the reset controller for two hours in the morning allowing the boiler to run up to high limit on sustained demand, and if we went away for a few days or more I would just manually trip that timer "on" when we got home. It worked fine, but toward the end of the "boost" period the more efficient or smaller zones would overshoot or short cycle.

But the best solution was to install a H6350 2-stage thermostat on my main living area zone which also happens to be the weakest emitter/Ft2 zone, with the second stage kicking a relay to do what the timer used to do. Now, as the main zone approaches set point it allows the boiler to go back to ODR and things simmer down perfectly. I also do not have to go down to the basement to trip a switch when we get home from vacation.

Of course none of this is an issue in the coldest weather because even with night setback the ODR keeps things essentially running at a slightly lower duty cycle and in unusually cold events I do not set back in the first place, preferring maximum flywheel time if something fails. As to data and calculations, I have Hobbs meters that track the elapsed time of the burner as well as each of the zones, have logged those times weekly as well as all deliveries and service performed for the past 25 years, can tell you the burn rate to two decimal places, usually can predict deliveries to my twin 330's to +/- 10 gallons and other information few in the business would ever know or care about. For example, I replaced the failing blower motor bearings after 11 years and 15,526 hours of operation... I don't know how much I save compared to my neighbors, certainly not anything worth the time and thought I expend, but I have a lot more fun.

BobC

Sometimes it;s the trip not the destination that matters.

Bob