Why no savings from reduced aquastat setting??

zeke

Date: March 23, 2007 07:13 PM
Author: zeke
Subject: under

quote" Date: March 12, 2007 09:53 PM Author: John Ruhnke Subject: RE:

Zeke,

Short cycling is not a consequence of reduced boiler water temperature. Proper control logic can control the cycling of the boiler. In fact hotter water tends to be more of a problem with cycling then warmer water....... --------------------------------------------------- Hate to dredge this up , but I strongly disagree with you. High boiler temps DECREASE CYCLING, the opposite of which you said; since the boiler has higher capacity, it heats up slower and loses heat slower. I have verified this analytically as well.

Dr. Z

Why no savings from reduced aquastat setting?

I've been reading about all the savings possible from an outdoor reset. Since I don't have one, I simply changed my aquastat setting from 180 to 130. I did this at the end of December.

For January and February I averaged about 17,000 BTU/DEG DAY. This is compared to a historical average of 15,200 BTU/DEG DAY over the previous 3.5 years.

Can anyone suggest why my gas consumption went up? Can anyone tell me what my savings should be with an ODR?

jp_2

what you need

what you really need is an efficiency curve of your boiler camparing efficency to output temp. if it happened to be flat, same efficiency at any temp, i could see where running 24/7 could use more energy.

instead of running an 80% boiler 30 minutes an hour, you are running an 80% boiler 1 hour per hour. thinking outloud here.

ALH_4

Boiler

What kind of boiler is this? How old is it?

Mike T., Swampeast MO

What type of emitters?

If fin tube you may have have greatly increased the frequency of burner cycles during a heat call with such a radical change.

If cast iron radiators, no system bypass and the thermostat simply fires the boiler there's a decent chance that the system never approached 180F in most weather to begin with.

If your weather was anything like what seemed "typical" this season, the "real" winter didn't hit until January/February anyway.

Be VERY wary when using degree days that way, e.g. comparing a month or two to an average from the last 3.5 years. If you check monthly btu/degree day values I think you'll understand why I say this.

Unknown

Andrew

Read this posting, I think it will give you the info your looking for, "Boiler size based on gas consumption". That one is his too.

Dave

ALH_4

Thanks

Dave, Thanks. I wonder if he has two buildings? That one says 27,000 btu/hdd and this one says 17,000?

Mike makes some good points. There are so many variables in each system that it is hard to say what might be going on without observing it in action. Reducing the operating temperature of the boiler should result in some reduction in fuel consumption, but given certain circumstances it may not.

Mike T., Swampeast MO

I was looking for that post to see if it came from the same person.

Especially difficult to compare via degree days that way with a multi-unit apartment building. Tenant habits certainly vary but perhaps you've had the exact same tenants for the entire period...

Uni R_3

Cycle Times

How long are your firing cycles? Is is possible that you are severely short cycling?

Unknown

Good points guys

I guess the next explanation would be the differences between bang-bang, and flame-modulation would be in order.

Dave

realolman

Why

It seems to me he asked a straightforward, legitimate question. One that I'd like to understand better myself. I'm not able to discern a straightforward answer.

Assuming everything else is equal, why would you increase the fuel usage by lowering the boiler temp, as some of you have suggested may be normal under certain circumstances? Why would a boiler go from running part time to running full time by lowering the water temp?

What should you expect from an outdoor reset?

I think discussion of the difference between modulating burner and a "bang bang" would be beside the point.

Unknown

It really isn't off track.

Think about this: What is your stack temperature 40 seconds into the run cycle? If you don't burn long enough to get the fire up to steady state efficiency, you aren't getting the rated efficiency during that cycle. Do that every cycle, and your combustion efficiency takes a huge beating. Do that the entire life of the boiler (XXX,000 cycles), and that life span will be extremely shorter, in years.

Now, why would reducing your water temperature, hence your output per foot, WITH THE SAME FIRING RATE, make your efficiency go UP? How does that compare to a control that MATCHES the water temperatrure to the current load. Matching the current load is where the fuel savings are, not in just guessing.

Mess with a designed system, and it usually won't improve. Mess with a system that was roughly sized to begin with, and who knows what you'll get.

Taking load off an oversized boiler doesn't improve combustion efficiency.

Perhaps if you had ADDED load (baseboard, etc.) so that the size of the load at the reduced water temperature matched the boiler DOE output, your COMBUSTION efficiency would improve, but if the boiler is too big for the building, your system efficiency won't improve.

It'll short cycle. Boilers that are too big do that.

Noel

Dave_4

Your thermostat.........

Dr. Z,

Most thermostats are designed for hot air systems. They go through many cycles. Often they shut off the boiler before it ever gets up to the aquastat setting. Thus you most likely were running low water temps and never new it. The boiler then runs off the cycles set by the thermostat. Now that you lowered the water temp setting in the boiler the aquastat now also adds extra cycles to your system. You now have both the thermostat and the aquastat cycling your boiler. This is way to many cycles. Too many cycles kills the efficiency of your system.

You want to pick a thermostat that is designed for hydronics. Tekmar makes some nice thermostats. Or turn off the cycle ability on your thermostat.

When I was testing Exquisite Controls I encountered the very same problem. I changed the settings on the thermostat and now the thermostat calls all the time and the Exquisite heat control cycles the boiler.

Your boiler is also most likely over sized adding to the short cycling problem too.

Low water temps do save money. But things are more complex then that. Remember just because your aquastat is set for 190 doesn't mean your system is running at that. At my clients house the boiler aquastat was set for 190. I ran a data logger over night on it. It never got above 140. The thermostat was turning off and on the burner constantly.

JR

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realolman

thank you

I think that is very relevent, and is probably the kind of discussion the guy ( and I )is looking for.

The beckett heat manager, I think, relies exclusively on the water temp of the load, and I think the tekmar stuff does, I'm not sure... but do ALL ODR's take into account the water temp of the load?


I am asking from a position of ignorance... I don't know... do they take the load into account, or just adjust the boiler temp based on the outdoor temp?

If they just adjust the boiler temp according to the outdoor temp, I see little difference between the ODR and doing it manually.... although around my place, the time to have done that was before October, not January.

Unknown

You're right

That's where the modulating feature really shines. On oil, that's where the low-hi-low firing burner strategies shine, too.

Modular boilers really make sense, too.

Noel

Mike T., Swampeast MO

Straighforward and legitimate question: Yes.

Straightforward answer (especially via the comparison method and structure from another post by same person presented): No.

Even if the number and habits of the occupants of this four-unit building have not changed in 3 1/2 years, there are MANY other variables and don't forget that a one-time radical change was made in the aquastat setting.

You asked, "Why would a boiler go from running part time to running full time by lowering the water temp?

For this [conventional] boiler that WON'T HAPPEN!!! What will change however is the length of a call for heat. While the system may well have been able to be satisfied with the lower aquastat setting it may have resulted in a nearly constant call for heat. This is a GOOD THING for a modulating boiler, but with a conventional boiler--especially a quite oversized conventional boiler--it could easily find it short-cycling like mad.

Type of emitter (let alone emitter sizing relative to heat loss) is still unknown.

Dave_4

Hi low not good........

Noel,

A modulating burner is great! Fuel mixture is important to efficiency. In a modulating burner, the air volume is adjusted along with the fuel volume. The correct mixture in mantained.

In a hi low atmospheric burner the air volume is always the same. When you go from high fire to low fire you run a leaner mixture. The excess air not used in combustion that is part of the leaner mixture carries excess heat up the chimney thus reducing the efficiency.

JR

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Unknown

Sorry John

The oil burners I'm familiar with are from a few years ago and commercial, I'm not current. They did have motorized air dampers on them, and we tested combustion at both high and low fire.

I have to agree with you, though.

Noel

bill nye_3

Because

Because I can't heat my house with 130°F water on a cold day. I have outdoor reset and I would never expect it to heat a house designed @ 180° water temp. to heat the house on a cold day with 130°water.

I don't know where you live, but my outdoor temp was at zero or below several times in Feb. and March this year. Many days it did not get above 20°.

On a very cold night my boiler temperature got up to 165-170°. I know for an absolute fact my house temperature will not maintain 68° inside on a cold day with 130°supply water. You are trying to defy physics. Try 150° you may be happier, 130°in mild weather. I would be very concerned about your chimney at that temperature.

Christian Egli_2

The Dr. Z. with the accent and the moustache?

EDIT, I would have preferred to post this lower on the thread. It would make more sense, but I goofed up. Sorry.

The very plain straightforward answer to this problem is... Take the boiler plus the pipes plus the radiation, that's the whole system, and you'll find that system efficiency depends all on fluid temperature: SYSTEM EFFICIENCY INCREASES WITH INCREASED FLUID TEMPERATURE.

It's no secret, it's the game played in any power plant and in any car engine. Intuitively, you know this as the fumbling problem posed by trying to heat a boiler with a candle: it raises temperatures, oh yes, but so little that your fluid gives you no usable heat at all. Your candle power went into pure waste, and it would not have mattered even if you did burn the candle 24/7 up until you'd match your normal fuel consumption. There is heat and there is HEAT, if you can't feel and use the energy you paid for, it's all gone in inefficiencies.

The same problem occurs when soldering with low temperature propane. You know exactly how you save so much fuel by just using the high temperature tools - just because of the higher temperature, you heat your parts so much more efficiently - and you use much less fuel. Sometimes, you can't even get the job done with propane... because it fails to be thermally efficient enough.

Of course, standing boiler losses are higher on a hotter fluid, that's always painfully obvious and modern boilers fix that. Likewise, standing losses on the acetylene torch are higher than for the propane version. However, these highly visible losses are not all that big of a deal in the great scheme of things (and as the discussion continues on bang bang, hi-lo and modulation, it's not even certain any boiler savings can necessarily be had)

This makes a good point for buying the new boilers with the better efficiency, no matter what the system.

Next

What Dr. Z did is not outdoor reset (I am merely restating what has been said already in this post) Outdoor reset provides savings, sometimes huge savings, by simply preventing the overheating of the indoor space by anticipating the solar gain on the home. Solar gain is always powerful and usually dwarfs boiler output even in the winter and so, stopping the man-made heat flow just as the solar heat starts pouring in is a great way to save.

Outdoor reset is particularly effective and necessary on hot water heat where you're riding a system with gigantic inertia - really gigantic. This is the big dilemma of hot water systems, you need to trade off system efficiency for manageable inertia, and the shifting point is movable along the season. (Somewhere in another post, I provided computations for bewildering comparisons)

Dr. Z.'s only accomplishment was to reduce system inertia, a sound idea in shoulder season, a bad idea in mid winter.

Playing with the mixing valve is a perfectly valid thing to do. Manually follow the season, week by week. This gives sound control if it is also combined with a timer to plainly shut off the heat in the morning end - this to prevent afternoon overheat.

All this of course, a modern electronic control does and then some.

I think hot water systems get the most benefit from smart controls first, then, secondly, from low loss boilers. One idea is then to retrofit old systems with new controls, or better, to retrofit with a new boiler that comes with new built-in controls. There are many ways to get great hot water heat.

There is a whole Ask Dr Z website, it is amusing the answers would be here at HeatingHelp.

zeke

It is true that short cycling is a consequence of reduced boiler temperature and the losses associated with more frequent starting of the burner may offset some or all of the gains savings associated with lower temps.

However, for the standard boiler the more significant savings would occur if the low limit boiler temp is reduced, thus getting the benefits of lower average temp and reduced cycling.

jp_2

oops

thats right, i was thinking reducing boiler supply temp.

isn't the aquastat the high limit? if you lower the high limit the boiler will constantly bounce off that short cycling like crazy, unless you increased flow rate.

John Ruhnke

RE:

Zeke,

Short cycling is not a consequence of reduced boiler water temperature. Proper control logic can control the cycling of the boiler. In fact hotter water tends to be more of a problem with cycling then warmer water.

Short Cycles and highwater temps will cost you money.

If you have one without the other that is an improvement.

The best though is less and longer cycles with cooler water.

The reason to install a good quality boiler control is that it can lower the water temps and at the same time improve cycling. Also remember to look at how the thermostat reacts with the boiler control. A good thermostat that comunicates well with the boiler control is important too. Then there is combustion efficiency abd tunning the boiler properly. Many factors effect the efficiency of your heating system. Oversized boilers cause cycling problems too.

JR

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John Ruhnke

Aquastat settings

J P,

Another problem with Dr Z's boiler could be the improper setting of the aquastat. Some aquastats are not designed to run at such low temperature and may cause short cycling if not set up properly.

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TK03

Let's keep it simple for what he asked. Maybe he just set the temp down too low on a high limit so the boiler run time increased alot as the outside temp went down. We all know we just came out of a cold spell.

Mad Dog!!!!!!!!

If doing ODR were that simple and cheap.........................

Uhhh, you know what I'm sayin'? Did your boiler have a puddle under it most days? That is the same thing that happens when a steam boiler is undersized for the load (ask me how I know). The boiler will run...and run...and run, and still not properly heat the house. The only reason you might of "gotten away" with keeping the house warm is becuase it eventually got the space warm where as steam would simply condense before it got there. Mad Dog

zeke

under



quote" Date: March 12, 2007 09:53 PM
Author: John Ruhnke
Subject: RE:

Zeke,

Short cycling is not a consequence of reduced boiler water temperature. Proper control logic can control the cycling of the boiler. In fact hotter water tends to be more of a problem with cycling then warmer water.......
---------------------------------------------------
Hate to dredge this up , but I strongly disagree with you. High boiler temps DECREASE CYCLING, the opposite of which you said; since the boiler has higher capacity, it heats up slower and loses heat slower. I have verified this analytically as well.