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Why does it take so long to heat up water?

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Brad White
Brad White Member Posts: 2,398
I think the other respondents so far have touched on the potential issues (Jim Lockard in particular) but want to nail it down for my own mind. Andrew brushed a point I would want to take further:

Has the house been maintaining temperature? That is the gauge, not whether or not the boiler reaches setpoint. On a mild day the boiler may only need to get to 120 before the thermostat is satisfied.

Joe- does the burner ever shut off? Or is it, as you say, constantly operating burning for four hours using gas at over $4.00 per therm? Just want to be clear on what the real issues are versus what might be exaggeration born of frustration.

If the boiler were firing fully for that time, 360,000 BTUs total input and if 80% efficiency call it, 288,000 BTUs net to water input would be delivered or 72,000 BTUs per Hour.

If the water does rise from say 80 degrees starting temperature to 160 F as you said was achieved, that represents a 20 degree per hour rise, less the heat delivered during that time. Idle thinking I just had to do..

Say your small-mid size house is poorly insulated and has a design heat loss of 70,000 BTUH if it is zero outside. On a recent day (no idea where you live) the temperature was 40 degrees, the heat loss would be about 30,000 BTUH. Need to know these things.

With 72 MBH going in and 30 MBH going out, you should be able to heat your house from setback to comfort in about 25-30 minutes.

Absent that "withdrawal" from your thermal piggy-bank, that 80 degree rise over four hours and 288 MBH net input to the water means you are heating 3,600 lbs. of water or 432 gallons more or less. That seems like a lot to me, for such a small house, even with big old gravity HW piping. (We just filled a 50,000 SF elementary school boiler and HW system with about 1.5 times that. The water from flushing had to be carted off-site so I know this.)

I have to ask that question.

If the system is really running full tilt and not heating the house, I would want to know what your stack temperature is. Could the boiler be sooted up to a point that most heat goes up the stack and not into the water? Mr.Lockard is giving you good advice to get that checked.

The advice about adding TRV's, outdoor reset and other features is still good advice, but let's find out what the real problem is, if any, first.

What are the real numbers and what is the comfort level in the interim?


My $0.02

Brad
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"



-Ernie White, my Dad

Comments

  • Joe_100
    Joe_100 Member Posts: 4
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    Why does it take so long to heat up water?

    I bought a house with hot water heat last year and was not at all impressed. It takes 4+ hours and $15 worth of gas just to heat it up to 160 degrees from a cold start. Is this normal?

    I have a 1400 sq ft twin house with a 10 year old 90,000 BTU boiler at 82% efficiency. There is a ciculator pump which seems to be working and the radiators are large 36" tall 1920's style. The supply and return pipes are large cast iron but they neck down to 1" copper near the boiler.

    What can I do to get it to heat up faster? Is it because there is a large amount of water in the big radiators and pipes? or is there something else wrong?
  • ALH_4
    ALH_4 Member Posts: 1,790
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    Constant Circulation

    The cause could easily be the water volume in your system. It takes 1 btu per pound per degree Fahrenheit to heat water, plus the water is losing heat as the boiler is trying to warm it. Have you insulated your piping? That should help keep it warm between cycles to improve response.

    Outdoor reset with constant circulation and thermostatic radiator valves would help. A tekmar 356 or 363 would do the trick.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
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    It takes a long time to heat water because it can hold an extraordinary amount of heat. 'tis just one of the "miracles" of water that, by the way, allows life....
  • Dave_4
    Dave_4 Member Posts: 1,405
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    Good Answer

    Nice answer Mike. Just remember Joe, as long as it takes to heat it up, those radiators will be giving off heat after the burners shut down. Take Andrew's advise and insulate the piping where you don't want the heat.

  • jim lockard_3
    jim lockard_3 Member Posts: 5
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    Tune up

    Joe -your boiler maybe running just fine, or there may a problem with the fire side. Having a service pro do a check of your system would not be a bad thing. Best Wishes J.Lockard


  • Cast Iron involves significant thermal mass as well.
  • Joe_100
    Joe_100 Member Posts: 4
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    Andrew,
    Can you explain how the tekmar unit works and the benifits? I looked up some info but got lost in the technical documents. Does it basically monitor the temperatures and modulate the pump to provide an optimum delta T through the boiler?

    Thanks everyone for all the great responses so far.
  • Joe_100
    Joe_100 Member Posts: 4
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    I had a 'tune up' performed when I bought the house, right before the heating season and he checked out the burners and gave it a clean bill of health.
  • ALH_4
    ALH_4 Member Posts: 1,790
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    Controls

    The tekmar control would vary the temperature in the system based on the outdoor temperature. It does not have control over the delta-T. This Essay explains the functions available pretty well. The primary benefit for you would be that the boiler would only have to warm the fluid a few degrees to match the heat load rather than from cold to hot. The boiler should be checked by a pro to make sure it is functioning properly. A mod/con boiler would be a great match for this system at some point. It may be a better investment to buy a Triangle Tube Prestige than to add all the piping and controls to control your cast iron boiler well. Zoning via thermostatic radiator valves will help by only sending heat to the zones that need it.
  • zeke
    zeke Member Posts: 223
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    It would help if you give us the starting conditions, like the intial temperature of the house and the outside temperature during this 4 hour heatup, so we can make an intelligent assessment of the problem. Also you have to give us an idea of the ability of the system, once heated up, to maintain tamperature at, say 0 degrees outside temperature. All this data is necessary before one can draw any conclusions.
  • Jim Davis_3
    Jim Davis_3 Member Posts: 578
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    What do you mean by tune up??

    Many contractors call cleaning equipment and looking at the flame a tune-up. Did the contractor give you some kind of combustion report or readings? Is there a hole drill in the flue? Did he show you what tool he used to test the equipment? If not you only got vacuumed.
  • Joe_100
    Joe_100 Member Posts: 4
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    They did a carbon monoxide test along with another test and gave me a report with the efficiency, etc. What do you mean by the hole drilled in flue?
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
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    Slow to heat

    may mean the circulator is too big and moving the water thru the system too fast, if everything else is ok.



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  • Gordo, that is not possible.
  • ALH_4
    ALH_4 Member Posts: 1,790
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    Flow rates

    I'm with you, Rob, but isn't that what this Hot Tech Topic says is possible? Maybe I am missing the point of the article.
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
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    Yes it does

    That was an extreme case, but it shows this is definitely possible. Took me a while to figure it out, at the time I didn't think it was possible either ;-)

    If there's little or no delta-T (difference in temperature) between the supply and return of a hot-water system, and all the radiators are bled and their shutoffs open, the circ is probably too big.

    This Hot Tech Topic goes into more detail on circ sizing:

    http://www.heatinghelp.com/newsletter.cfm?Id=125

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  • joel_19
    joel_19 Member Posts: 931
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    ???

    Joe why are you turning it down and how much??? turning it way down when you leave will probabaly not much money.

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  • You're right that the circ is probably too big if the delta T is too low, but that doesn't mean it's not picking up heat. Delta T shrinks with more flow, but input/output doesn't shrink. If you double your flow, you half your delta T, but your heat output is not much different in most cases (and typically it would be higher, due to the average temperature rise across your emitter).

    Well, *maybe* you'll lose *some* output if the return water to the boiler rises in temperature, raising the average temperature through the boiler, but unless that water is the temperature of your heat exchanger, it will transfer heat. If the heat exchanger IS the water temperature, it has to be because you are pulling enough heat out of it to keep it at that temperature!

    I would suggest that whatever problems were fixed in the tech topic, one of them was not water moving too fast to pick up or release heat. You may not have been extracting heat from the boiler due to the flow CHARACTERISTICS through the boiler, there may have been other factors I am not aware of involved, but all else being equal you will never, ever reduce output by increasing flow unless that increased flow bypasses something for some reason. Like it won't "turn" into the radiators (I don't even know if that's possible, but I assume there must be some inertial property that would allow for that), or it's only flowing through half the boiler for some hydraulic reason.

    That's my two cents, and I've been wrong before, and absolutely no disrespect intended, but I'm just not buying that explanation. High flow will do a lot of things, but preventing heat transfer is not one of them last I knew, just lowering the perceptible temperature change across a heat input or output (narrowing of delta T).
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
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    It has to do with

    the amount of time the water spends in the boiler and radiators. If the time is shorter than it should be, the heat transfer will suffer. I have found that "right-sizing" a circ actually makes the system heat faster!

    If you've been around for a while and have ever done a tune-up on a car with breaker-point ignition, you remember the term "dwell". This was the amount of time the points were closed, allowing a magnetic field to build up in the coil. If the dwell was insufficient, you didn't get a good spark.

    Same principle here.

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  • You may be heating faster: all I'm saying is, it's not because fast water picks up heat more slowly than slow water. If your HX is 300 degrees and the water going by is an average of 180 (to throw out completely made up numbers), you transfer heat at the same speed, whether the temperature range of the water going through is 178 to 182 or 150 to 210. The only difference there (assume constant burner) is flow rate... not output.

    If you slow down flow, you might have a useful temperature water leaving the boiler right away, instead of having to heat all the water in your system up to a useful temperature before heating (which, in a large cast iron radiator system, must be a whole lot of water between those radiators and the huge pipes!!!!). But what you're doing is only heating a portion of the water in the system, or to put it another way, you are not using the whole system as a buffer tank any more, which all has to heat up before the house will get warm (edit: in a very high flow scenario where the apparent temperature rise out of the boiler is very low).

    If you know what the water volume in the system is, figure out how long it would take to go from cold to hot if the water were flowing fast enough that you had to heat it all up. I bet it'll be illuminating.

    But fast water, slow water, it doesn't care. Heat (BTUs) will tranfer in or out just as fast either way. What that will do to the outlet temperature may change depending on flow, yes, hydraulics might change; but not BTUs transfered to the water or out of it again based solely on water velocity.

    Again; I've been wrong before, but it's going to take more than anecdote to flip my mind. Nothing personal, I'm just pretty confident in this particular property of water is all.
  • Bruce Stevens
    Bruce Stevens Member Posts: 133
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    I would think that

    if the water was not transferring heat to the radiation then the boiler would be bouncing off it's high limit, now if he has a -.10 draft the heat just might be going up the chimney it has to go some where or the oil/gas pressures, nozzles/orifices are off and not as big a flame as there should be, or the boiler is too small and system radiates faster than the boiler can keep up, any thoughts on those issues??
  • hot_rod
    hot_rod Member Posts: 22,158
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    I'm with Rob on the flow speeds :)

    I don't disagree with Steamheads findings on circ size, and agree the correct size should be determined and used on any system.

    But the numbers and laws of thermodynamics exist to prove clearly heat is released faster as the water moves faster.

    "Convection is governed by the surface contact area, the delta t between the fluid and wetted surface, and a number called the convection coefficient" John Sigenthaler wrote.

    I still believe in Steamheads experience, it has more to do with the fluid path thru the radiator or boiler as Rob alluded to, with the higher flow rates.

    Siggy wrote two articles on this controversial subject in PM mag June 1997, and recently March 2007. Both are archived at www.pmmag.com.

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
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    It may be limited

    by the amount of heat-transfer surface available, the fact that you're not heating all the water at once (which would reduce heat losses in the return lines, insulated or not) or something else, or a combination of the above factors. But whatever the reason, within the confines of actual systems as found in the field, it is definitely possible for over-pumping to hurt total system performance.

    Another example of this can be found in the zone valves/zone circs debate. If a single circ and several zone valves will provide enough circulation, using an individual circ for each zone might be overkill. We recently reworked a system where the second and third floor baseboard loops each had B&G 100 circs, pumping thru flo-checks and disconnected Taco 570-series zone valves. We pulled those circs out, removed the flo-checks, added a first-floor zone of cast-iron rads to supersede a heat pump, reconnected the wiring to the zone valves and pumped-away the whole thing with a Taco 007, which delivers about 2/3 the flow of a single B&G 100 at a 3.5-foot head. Everything heated great; why would we need bigger or more circs?

    I recently right-sized a circ, took out a Taco 007 and installed a Grundfos 15-42 which delivers roughly 3/5 of the 007's flow at a 3.5 foot head. After I did this, the lady said it had never heated so well. She'll probably use a few less therms per degree-day this winter.

    On both systems, delta-T ended up at 15 degrees or so. There was hardly any delta-t in either system before we worked on them.

    Resist the urge to oversize and overpump, and systems will work better.

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  • [Deleted User]
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    Something's not clicking here....

    By my estimation, if your appliance is using 90,000 btuH of gas per hour, and if it were to run for 4 hours, that would equal 3.6 therms. At your estimate of $15.00 worth of fuel in 4 hours of run time, that puts you at a per therm cost of $4.16 per therm. Unless you are in Germany, something's not right. Cost per therm here in Denver is less than 1 $/therm.

    What kind of gas are you running in that beast. Methanol alcohol??? ;-) Is that a HEMI in that thang???

    As for flow and thermal efficiency in boilers, I believe Frank is probably corrrect in HIS assesment of HIS situation. He was dealing with an older fire breathing dragon that was originally designed for use as a gravity boiler if I'm not mistaken, and increasing the flow through that large passage wayed, multi pathed, paralleled beast could have created a path of least resistance that did not include the areas surrounding the combustion chamber.

    As it pertains to the newer ultra high efficiency boilers, I would have to agree with Robs statement regarding average emmiter AND generator temperatures. Higher flow rates are beneficial, to a point...

    As with any hydronic question, the correct answer is "It depends", and the other only true statement is "Let me get back to you..."

    You need to have an expert with a combustion analyzer check things out for you to see what your true net thermal efficiency of the appliance is, and IGNORE the little number that the analyzer spits out at you. You can hit that number a hundred different ways.

    True thermal efficiency is based on the net oxygen reduction in the burn process, accompanied by the resultant rise in water temperature based on flow through the heat generator, and tempered by the flue gas temperatures seen at that point in time.

    ME
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
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    Mark, the boiler in that HTT

    was as you say. But I've tried it on much more modern cast-iron boilers too, including my own, with similar results. I wouldn't have done those two HTTs otherwise, but you knew that.

    On a primary-secondary setup, we would size the system loop circ this way. The boiler circ might be sized differently, especially on some mod-cons.

    Once the boiler has heated the water, its job is done. After that, the system must move the heat to the radiation. It may be that slowing an excessive flow rate mostly makes the radiation work more efficiently, for whatever reason (though along with changing the return location, it made that particular Spencer work much better too!). But this raises the efficiency of the complete system- boiler, piping and radiation together.

    Here are pics of the old Spencer from the HTT and the new Solaia and indirect we replaced it with.

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  • Jim Davis
    Jim Davis Member Posts: 305
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    Efficiency Report

    What kind of reading are on the combustion report. The calculated efficiency has little to do with actual efficiency. Need an Oxygen(o@) reading and a flue temperature.
  • Andrew Hagen_2
    Andrew Hagen_2 Member Posts: 236
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    Circs

    Doesn't a single speed 15-42F have something like 3 feet more head than a 007 at 3.5gpm?
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
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    More like 2 feet

    13 on the 15-42 vs. 11 for the 007. But at the lower heads we find on existing systems, the 007 pumps more water then the 15-42.

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  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
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    More like 2 feet

    13 on the 15-42 vs. 11 for the 007. But at the lower heads we find on existing systems, the 007 pumps more water then the 15-42. At 3-1/2 feet of head, the 15-42 delivers 12 GPM, the 007 20 GPM.

    BTW, the reason I used 3-1/2 feet of head is that it was cited in the old B&G handbook as the figure to use on gravity conversions.

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  • Steamhead, no one is arguing that overpumping is a good thing. It stinks. It can cause many problems. You've fixed some and you're absolutely correct to be looking at pump size, and the clients that hired you to help out should be very, very happy they did; you fixed their problem. AND, water cannot move too quickly to pick up or release heat. It can definitely move too fast for a lot of things, but not for that.
  • ALH_4
    ALH_4 Member Posts: 1,790
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    3.5ft

    Oops, got it backwards.
This discussion has been closed.