Recommended Oil Furnace/Boiler Temp?

Discussion in 'Water Heater Forum, Tanks' started by Tyler_s, Jan 31, 2012.

  1. Tyler_s

    Tyler_s New Member

    Messages:
    3
    Location:
    New England
    I know there are posts on this but I wasn't sure if the temps would work with my furnace (could be a boiler not 100% sure..) The manufacturer is a Vallant. It has a Honeywell thermostat, my problem is that our showers are very hot at first then decline to a lukewarm temp then slowly rise to a somewhat hot temp. The temperature I currently have is: Low-190 High-210.. There is a third black dial which says (Diff) which is set to 10. There's a red mark on the temperature which is at 220 which i assume is the danger zone.. I always lower the temp during the summer since it rarely gets used. But it always sits at that temperature never goes over during the winter. So I'm wondering what a good steady temperature is to be energy efficient but have at least hot showers?

    ALSO: The hot water problems only happen during the winter, never during the summer..
    Thanks
    Last edited: Jan 31, 2012
  2. Gary in NJ

    Gary in NJ New Member

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    90
    Location:
    NJ
    Is this an indirect water tank? If so the temperature of the boiler is not as important as the set temperature of the aquastat. The HWH should be set at ~140 degrees F.

    Is your HWH set as a priority zone? If not your heating zones and HWH are fighting for the same resources (hot water).
  3. Dana

    Dana In the trades

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    It's a nomenclature thing- for most people "furnace" == hot-air furnace, and "boiler" usually mean a hydronic boiler (either steam or pumped hot water.) But a high/low limit control means it a hot-water heating system.

    It sounds like your system uses a heat exchanger coil embedded in the boiler for making hot water. In most cases those systems will work fine when new with a 160F low-limit, but over time mineral deposits from the water builds up on the coil, requiring a higher temperature to get the same amount of heat across heat exchanger. In winter your incoming water temperatures are 15-20F lower than in summer, making those issues more obvious.

    It may be possible to de-lime the coil an increase hot water output. But a better solution would be to abandon the hot water coil and re-configure the controls to run an "indirect" hot water tank as a separate heating zone, and LOWER the low-temp of the boiler so that it will run with less standby loss, higher efficiency. At the current price of heating oil installing an indirect and putting the low-limit to 140F (not lower, or you'll have corrosive condensation inside the boiler or in the flue) will pay off in very few years. (If the boiler is cold-start tolerant and you let it idle to room temp your summertime oil use would plummet.)
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    An embedded coil is sort of like waving your hand over a flame. Move it fast, your hand doesn't heat up; move it slow, you might burn yourself. You don't change the flow of the water with the seasons when you shower, but the temperature of the incoming water varies (sometimes) radically winter/summer. With no flow, the (small amount of) water is sitting in the coil and reaches the same temp as the boiler. Once you start the flow, you get that hot slug (tempered with some cold for safety), then, as it cools off the boiler tank, it drops until the boiler fires again, then it rises.

    This has to be the most inefficient means of heating water unless you have an industrial need for hot water all the time. As Dana mentioned, the inside of that coil will get coated with mineral deposits over time, limiting the transfer of heat (picture waving your hand through the flame, but now it has a glove on it).
  5. Dana

    Dana In the trades

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    Dare I point out that even with an indirect, if you're heating hot water with an oil boiler drainwater heat exchangers are very cost-effective?

    A DWHX would lower hot-water side flow, allowing you to lower the temp of the boiler even if you kept the embedded coil, but an indirect and replacing part of the controls with a smart economizer like the Intellicon 3250 HW+ would likely save more oil overall.

    An indirect would save 75-100 gallons/year over an embedded coil, but the economizer could take another 100-150 gallons off. The economizer works by heat-purging the boiler (to the programmed low temp) at the beginning of a call for heat, and doing additional heat purging of the boiler into the zone toward the end of the zone call by cutting the burner when it determines (from previous system behavior) that the end of the call for heat is imminent, parking the boiler at a lower temp for lower standby loss. It "learns" the system based on how prior burns worked out. At the lower temperature between burns the average standby loss is less, and by waiting until the boiler hits the low-temp before firing the burner it takes that average even lower. With an embedded coil you would still have to set the boiler temp at least 20F higher than what you can get away with if you had an indirect, lowering the effectiveness of the economizer. That extra 20F is a 24/365 condition. With an indirect you could safely program the economizer to low-limit a 140F before firing. Also, instead of whatever that wimpy embedded coil can deliver, you have the stored heat in the indirect plus a much more substantial heat exchanger that can deliver 100% of the boiler's heat output rather than the small fraction you get out of an embedded coil.

    THEN, now that you've reduced the overall standby loss, you can do the math on what further you'd get out of DWHX. The economics are quite favorable heating hot water with oil burners, not so much with natural gas (at current prices, anyway), unless you intend to take endless gusher-showers.
  6. Tom Sawyer

    Tom Sawyer In the Trades

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    Location:
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    Its been awhile since I installed Vaillant boilers and for the life of me I can't remember if they even came with a tankless coil for domestic hot water but if they did.......The high limit should be set at 190 - 200. The low set at 160ish and the differential at 10 to 15. So even if you do that you are still going to get the temperature swings for a couple of reasons but the most common is that either you don't have a tempering valve on the coil or you do and the element needs replacing (betting its a Taco 508 or a Watts 70 ) or........the coil itself is all scaled up but.....usually when that happens you just get warmish water and not much of it either so.........investigate your tempering valve. Now if you don't have a tempering valve then you are going to get big temperature swings and there ain't a darn thing you can do about it cept install a tempering valve
  7. Tyler_s

    Tyler_s New Member

    Messages:
    3
    Location:
    New England
    Thank you all for the replies,

    So It must be the coil since it is rather old, so would installing a indirect water heater be more cost efficient? Along with an economizer also? The indirect would be an addition to the boiler?
    Last edited: Feb 5, 2012
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    21,995
    Location:
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    Think of an indirect as an electric water heater, except that instead of electrical heaters, it has a branch from the boiler to an internal coil or water jacket instead. The indirect is plumbed as a zone to the boiler, just as if it was heating the house but instead is heating the water. Generally, it is treated as a priority zone, which means that when it needs heat, it prevents heat from being distributed to the house for space heating. When the indirect satifies it's needs, the system switches back to space heating. They come in various sizes, shapes (square, rectangular, cylinders) and materials. Some use stainless steel and come with a lifetime warranty. Because it can use the entire heat from the boiler, it recovers much quicker, and depending on your draw volume, you can often get by with a (much) smaller one than if it was heated with electric or gas as a standalone system. The brand of the indirect doesn't have to match the boiler, you can mix and match. I have a Buderus boiler and a SuperStor Ultra indirect, for example.

    You could try cleaning the coil. this would take some work in itself, as you would need to disconnect it, then patch in a small pump, then pump some weak acid through it to try to disolve the years of mineral deposits, then put it back together. It's unlikely that you could find a new coil for an old boiler, but that might be possible. You'd be better off spending that money on something else, like an indirect. Note, many utilities are offering a rebate for the installation of an indirect - check with your energy supplier. When I did my new boiler and indirect (this was maybe 4-years ago), the rebate for the boiler and indirect was about $1200. They're still offering similar rebates, so you may have a good chance of that option.

    An indirect will require some space, but it doesn't have to be right by the boiler (although that usually works out easiest), and you may or may not need some new controls to your boiler, but definately will need some plumbing work.
  9. Dana

    Dana In the trades

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    [​IMG]

    ^^^typical indirect hot water configuration^^^

    Key to making an indirect more efficient than a tankless coil is the ability to now set a lower average operating (& standby) temperature for the boiler, which is what a retrofit economizer will do for you. Cost-wise you'll also be on the hook for a zone relay & pump to run the indirect as it's own separate heating zone. If your boiler is on the smaller side (less than 100,000 BTU per hour) it can be useful to set up a somewhat-smart zone controller that has a "priority zone" option to inhibit the space heating zones from drawing heat whenever the indirect tank zone is calling for heat, to prevent the shower from ever running cold. But most older oil boilers are ridiculously oversized for the space heating loads, and priority zoning may not be required. Is there any nameplate or other indication of the BTU-rating or gallons-per-hour rating on this beast?

    If your house is cut up into a BUNCH of zones you may get better efficiency out of reconfiguring the system for the zones to draw heat off a central buffering "reverse indirect", that stores heating system water at higher temp, but has a large internal coil (much larger than a boiler-embedded coil) for the potable hot water. But that's a whole 'nuther can o' design worms.
  10. Tyler_s

    Tyler_s New Member

    Messages:
    3
    Location:
    New England
    Okay, This is starting to make a lot more sense and that picture defiantly made it more clear (I was actually looking for a picture exactly like that and couldn't find it.) So what it seems I'll need to get an indirect and install that near the boiler. I'm thinking this boiler came with the house when it was built. We have 3 floors, the basement has a zone.. the 1st floor and the 2nd floor have their own zones, I'm not sure if the appliances or the shower/bathrooms are considered zones but we only have one full bathroom (two sinks, bath/shower and a toilet.) The 2nd bathroom only has a toilet and a sink and the washer and dryer is heated by propane (I think the washer is but It may just be the dryer.) Here are some pictures of what our setup is like:

    [​IMG] [​IMG] [​IMG] [​IMG]

    It looks like it's 140,000 BTU, also how would you go about cleaning the coil on a boiler like this? I looked over the internet and I cannot find a detailed instruction video or anything on a boiler like this. I myself probably wouldn't clean it because I do not want to screw anything up but I am curious as to how someone would go about it on a boiler of this type?

    I really appreciate all the help here, thanks!
    Last edited: Feb 6, 2012
  11. ballvalve

    ballvalve General Engineering Contractor

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    Location:
    northfork, california
    I would buy an electric water heater for 300 bucks and leave the boiler alone. Turn it off in the summer.

    That storage tank would be close to the cost of a modern boiler, not to mention the plumbing.
    Last edited: Feb 7, 2012
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,995
    Location:
    New England
    To attempt to demineralize the coil, you'd have to cut the pipes, attach some valves and fittings so you could isolate the coil from the potable water. Then, you'd need a pump and a bucket and some mild acid (a strong vinegar would work or something like CLR). You'd pump that acid through the coil for maybe a few hours, flush it out with clean water, then reconnect the potable water lines. If you're lucky, the acid would have disolved most (all?) of the minerals in the coil. that would do nothing to any deposits that might be on the outside of the coils, but most tend to form inside - because you aren't continually adding water to the boiler's supply, there's limited minerals to deposit, but with fresh water, you're constantly suppling new ones as that flows through. Think of the inside of the coils as the bottom of your teapot or coffee pot. After a lot of use, it gets mineral deposits in it. They make fittings designed for this, and any tankless system should have them installed. You could use the same ones, or cobble up something of your own if you wanted to try. Someone may even sell a kit designed for a tankless to do this, and all you'd need is the proper fittings to adapt it to your coil.

    I also wanted to complement you - very few people actually know about and use the macro setting on their cameras...it makes reading things MUCH easier in your pictures. There are so many blurry pictures posted that are almost worthless...
    Last edited: Feb 7, 2012
  13. Dana

    Dana In the trades

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    That makes GREAT sense in most places, not so much in some of the high-cost electricity markets in New England.

    If the flow through the coil is still OK and it's just the output temp that's the problem, plumbing the output of the boiler's coil to feed the cold input to the electric water heater reduces the otherwise slow recovery time of the electric heater during the heating season, and in the summer the incoming water temps are already much higher.

    The efficiency of an indirect in summer is only about 40-50%, but improved somewhat with a heat purging economizer. At $3.50/gallon oil at 50% effieiciency you're only getting 138,000/2= 69KBTU for $3.50, or 19.7KBTU/$.

    With 15 cent electricity and a 0.90 EF tank you get 3412 x 0.9= 3071 BTU for 15 cents, or 20.5K BTU/$

    ...which isn't very different. In 20 cent/kwh areas it's still cheaper to heat hot water with oil, even in the summer, (but that may change with the increasing demand for diesel in South America, a huge export market for the same northeastern refineries that make your heating oil.)

    During the heating season the average efficiency of the boiler will be much higher at least 65%, since the standby loss accrues to the house when you actually need and want the heat, and that would make heating hot water with $3.50 oil significantly cheaper than 15 cent electricity during the heating season, so pre-heating the cold feed to the HW heater would still be the right way to go. If you have 10cents/kwh or cheaper electricity, STOP HEATING WITH OIL, at least until the price drops. (Separate topic, but have you considered heating at least partially with a mini-split heat pump? It's half the cost of heating with oil if you get a decent one, and pays for itself in 3 years or less on oil savings at current oil prices.)

    If this basement is uninsulated it's a good idea to retrofit R6 pipe insulation to the heating distribution plumbing, even the return lines. (Better yet, insulate the basement walls with a couple inches of rigid foam.), and since that boiler is likely 2-3x oversized, even if it isn't your primary hot water heater a retrofit Intellicon or similar would very likely cut the fuel bill by double-digit percentages.
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