# Hot water tank pre-heat?



## mayhem (Feb 4, 2008)

So I've had an issue with my hot water system since new 5 years ago, its always bugged me but now I want to actually do somehting about it.  

Background info:  Oil boiler, Weil-Mclain Gold 4 chamber...about 180,000btu.  Hot water tank is a medium to large size (I'm afraind I don't know the gallon capacity).

Problem:  Every time we use the hot water in the house the oil burner kicks on.  Sounds normal and I wouldn't complain if it kicked on when I take a shower...but this thing runs every time I use warm water to wash my hands.  Its got to be costing me a significant amount in oil.  When it kicks on it seems to run the boiler for a significnat period of time...like 10-12 minutes...and then the power vent does its post purge for another 7 minutes, so I'm probably using way, way more energy than is truly necessary to heat up less than a gallon of hot water.

The way I fugure it, when i use the hat water I obviously has to top off the tank.  Where does this extra water come from?  Why, from my ice cold artesian well of course!  The cold water rushes into the hot water tank and hits the thermostat (which I believe is located at  or near the top of the tank where the fresh water inlet is) and kicks on the boiler if I use more than a couple cups of hot water.

Logically, there must be some way to pre-heat the water before it hits the main tank, but I'm stymied as to how to go about it.  The goal here is a net energy and cost savings, but I'm not certain how to define what I'm spending right now in extra oil and electricity to heat a small amount of water.  I can tell you we're a family of two adults and one 5 year old.  We wash our clothes in cold water, take short showers and generally try to conserve energy and oil...but we will go through a full tank of oil for hot water between winter seasons (in other words, zero oil used for heating the house during this time)...which at today's prices is pushing $800 just for the oil and probably another $50 at the least for the electricity to run the boiler and power venter while heating the hot water.  Extrapolate it out and its not hard to think I'm spending somewhere between $1500-2000 per year just for hot water.  Seems a bit too much.

I saw a post a week or so ago for a home made solar hot water system that the poster is using to fill a second tank that he has in-line with his oil fired tank, this would probably be ideal for my situation and I'm considering it, but wonder what the other alternatives are that others have used to resolve this issue in their homes...I consider it very unlikely that this is a unique problem.

Things I've done so far:  The only thing I've done was to reduce the temperature in the hot water tank to its minimum setting.  Helped a bit in that I can run the hot water faucet for 45 seconds instead of 20 seconds before the burner kicks on.  Other than that I've done nothing.  My water tank is not wrapped, though its barely 5 years old and the body doesn't get more than luke warm, so I'm not sure if a wrap would help...my pipes to and from are alo not wrapped in any insulative material.  

Suggestions?


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## steam man (Feb 4, 2008)

Awhile back I could have written most of your post myself. I have a similar size cold start boiler with a power vent that bugged me when it seemed to cycle too much. A number comes to mind that 60% of energy useage is used to heat domestic water for a "typical" family. I work on large control systems and not the small house stuff but the principle is the same. At first my hot water tank temperature differential would swing too much from hot to cold. I found the sensor did not fit the well properly and fixed that. If your "large" tank is calling for heat immediately it almost sounds as if the temperature sensor is reading the cold water coming in. My tank has a diffuser type of arrangement in the bottom of the tank and the incoming cold water doesn't directly hit the aquastat. The question comes to mind is why would a sensor at the top of the tank be immediately affected by incoming cold water since it should enter low in the tank? I would think there would be a dip tube of some kind to do this. What kind of indirect tank do you have? A gallon or two of cold water doesn't sound like it should make the mix temperature drop that much in a large tank.

I also went to a smaller burner nozzle to minimize the cycling since the boiler would heat up faster than the 40gal hot water tank which would require the burner to cycle a number of times for a small water useage. I haven't suffered any heat/load problems since then. I also set my power venter purge to minimum approved (3 minutes) since it seemed a waste of energy (144 watts). It used to have a thermal purge shutdown which to me made more sense but Field Controls recomends the electronic type timer. How much does the boiler cool down in its 7 minute purge? Mine doesn't cool much since there is no airflow  with the burner shutter closed. I am wondering what your boiler differential setting is, i.e., low limit on, hi limit off is. It probably is fixed like mine (180 deg @ not sure 10 deg diff?) but there could be an adjustment to widen it out a bit depending on your controls. What do you have on the Weil Mclain for controls, burner control and circulator controls? Also, my aquastat is at the bottom of my tank along with the cold water diffused inlet, however, that just starts the circulator and the burner only kicks in if the boiler is below its minimum setting. 

I do have a preheat tank setup from my wood stove that can circulate through the 40g tank and 120 gal tank. I put in a switch to isolate the 40g tank so the circulator/burner won't come on. See https://www.hearth.com/econtent/index.php/forums/viewthread/13691/. The past month I used about 1 gallon of oil a day. Any preheat system is going to help. I think solar is the best use of this application since it works most of the year (slow and steady). I have been thinking about installing a Beckett Heat Manager (a smart control that claims to reduce cycling-it seems to widen the boiler differential) or updating to an outdoor reset control. Using a minimum amount of oil now probably won't result in a great payback adding anything else now. Some newer controls also will do a post-purge with an off delay timer into a heat zone (hot water tank) to reduce the standby losses in the boiler itself. Taco has controls that do this now and others. I am thinking about just adding my own off-delay timer to my hot water tank circulator since I see a rise in heat in my boiler when it is satisfied just to make use of the extra BTU's. Wrapping your tank and pipes can't hurt.

Give us some details of your equipment. Maybe some heating professionals will chime in.


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## d.n.f. (Feb 4, 2008)

http://www.motherearthnews.com/Green-Homes/2007-10-01/Build-Your-Own-Solar-Water-Heater.aspx


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## Nofossil (Feb 5, 2008)

mayhem said:
			
		

> The cold water rushes into the hot water tank and hits the thermostat (which I believe is located at  or near the top of the tank where the fresh water inlet is)



If that's the way it's plumbed, then there is a really simple fix: Move the cold water inlet to the bottom of the hot water tank, where it belongs. Cold water goes in the bottom, hot water comes out the top. The tank is designed to maintain thermal stratifications such that most of the tank can be pretty cold and still be delivering hot water out the top.


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## Llywelyn (Feb 14, 2008)

Hey,


Saw this post and thought I would chime in.  Sometimes it isn't possible to change the heater from a top feed to a bottom feed.  They are designed at a top feed for a purpose.  Top feed water heaters have a tube that runs from the top to the bottom, so even though the cold water coming in, is threaded into the top of the tank, it is actually going to the bottom of the tank before mixing with the water.

As far as preheating your water, perhaps using some sort of flat panel exchanger would be a solution, though I agree with a previous post that solar really is the way to go.  If you are figuring on $2000 a year to heat your water, that would easily pay for one panel to heat your domestic water.  

another interesting idea I have seen is something that is being developed here in eastern canada, though i am sure there are other versions of it elsewhere.  It is a large copper coil that is installed in your main sewer line.  It basicly extracts the heat from the water that is going down the drain and uses it to preheat the water that is being dumped into your tank to replace it.  So the hot water going down your drain is preheating the water entering your tank.  Of course, your sewer system has to be set up for this sort of thing, but most conventional systems should work.  This doesnt' do all the heating of course, and is only a booster to the tank.  I think I have the link around here somewhere.

Well, thats my two cents.


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