Help on domestic hot water w wood furnace

Working wood Posted By Working wood, Aug 7, 2018 at 8:37 PM

  1. Working wood

    Working wood
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    I have a wood furnace with a domestic water coil which circulates through a 80 gallon preheat water tank. When hot water is drawn through the house the preheat tank feeds into the main hot water heater. My problem is that my circulator pump circulates from the bottom of the preheat tank through Wood furnace then back to top of water heater. Problem is that when I am using a lot of hot water the cold water gets circulated from bottom to top and goes directly to house water heater, not getting the true hot water in tank, more of a cold mix. I'm thinking of reversing flow of water circulator. What r ur thoughts?
     
  2. cumminstinkerer

    cumminstinkerer
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    @Working wood you need to do like I have and add a check valve to prevent the circ from circuit from back feeding and that should stop the issue.
     
  3. Working wood

    Working wood
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    Well if you can visualize the top of my water heater/preheat tank. I have a cold coming in hot going out. Then I have the circulator pulling water from the bottom drain going through the wood heater then entering where the hot water goes out into the gas hot water heater in the house. When cold water enters the bottom of the preheat tank it gets drawn from the circulator and pumped right to where the hot exits the preheat tank. I was thinking about reversing my circulator so that it will pump top to bottom. I have a family of six and in the past I would turn the circulator off when we were using lots of hot water like at night for showers or washing clothes.
     
  4. Working wood

    Working wood
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    I don't believe a check valve would work in this application being that the hot water outlet is teed off with the circulator
     
  5. maple1

    maple1
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    More details might help - what exactly you have for furnace & coil, flow diagrams, pump type & flow info, temp data in & out of tanks & coil, etc..

    It could simply be that your hot water use is more than your coil can make. I would expect that if that were not the case, during heating season you should have two tanks of hot water by the time morning comes, which would carry us through the whole next day.
     
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  6. maple1

    maple1
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    Where is your gas heater in that diagram? I think in one spot you said the recirc was T'd into the hot-out from the gas heater, but the diagram shows it T'd in at the storage tank out. If the gas heater is after that T & the storage tank, it should finish the job of heating the semi-pre-heated water?

    I am also thinking that with what sounds like pretty heavy hot water use, there shouldn't be much water running around the recirc loop when using hot water, compared to total hot water flow to the taps. What do you have for a pump there?

    EDIT: OK, I re-read and think the gas heater is after that T?
     
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  7. Working wood

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    Gas heater is after that diagram. Circulator pump only circulates through preheat tank and Wood stove. I also have a ball valve on my circulater loop which I can control the speed of water going through. Temperature could be 150° I could barely touch the copper line with my hand. Once you start drawing hot water for a period of time cold water circulates through that pump and takes it right out to the T and hot water line and copper pipe. The hot water line becomes cold fairly quick. I'm asking is if I can change the direction of the circulation pump would it be better in this application. I am feeling that bringing hot water from top to bottom will eliminate cold water mixing with the T at top . With all being said this is a pretty flawless set up. I am only trying to make this more efficient. My gas water heater barely runs in the winter . I have been doing this for a very long time and just trying to perfect it
     
  8. Working wood

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    One more piece of the information. This set up works very well as long as I turn off the circulator pump when we are drawing a lot of hot water
     
  9. maple1

    maple1
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    So what do you have for a circ pump? I am thinking it's pumping too fast.

    I also have a recirc but it pumps thru a flat plate. At very slow rate. If it pumped fast it would also do similar to what yours is doing, except the flat plate heats water up real good so wouldn't be as noticeable. With a slower rate you will get a bigger temp rise thru the coil, and not send as much water to the tank outlet around that loop when using hot water

    Have you actually measured water temps? It might not be getting as hot as you think - I don't think I can hold my hand on 120 piping for very long. Or 120-130.

    Reversing the flow IMO would be a big catch-22. You would be sending hot water to your coil and likely not gain much heat rise or heat recovery on it when it passes thru the coil - a big efficiency hit. The way you are circulating seems to be the right way to do it. No chance you are feeding cold water to the preheat tank via the outlet fitting is there?
     
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  10. Working wood

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    I can control my circulator pump with the ball valve. I have it turn down to a very low flow. Yes I understand what you're saying about reversing the flow but when all said and done the circulator pump is just keep in the 80 gallon tank at even temperature anyway. Yes I am sure on the temperature because I have a gauge or thermometer plumbed in the waterline.
     
  11. Working wood

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    Do you think a better option would be to T it in the cold water line on the preheater? If you would look at my diagram now it's in the hot water line leaving the tank if I put it on the cold waterside it would just push it down to the bottom of the tank
     
  12. maple1

    maple1
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    I would likely try that before reversing the circ. But you still might take an efficiency hit if the bottom circ feed inlet is anywhere close to the cold infeed inlet - that would mix up the temp of the water entering your coil, and you generally would want the coldest water hitting the coil for best efficiency.

    I think the thing that would be most apt to help your situation while at the same time not hitting overall efficiency would be to put a smaller circulator on. Mine is a B&G Ecocirc - an e^3-4 model. Has a variable speed dial on it. That would cost a few bucks though. I don't know what it's flowing but I'm guessing no more than 1gpm.
     
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  13. Working wood

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    One reason I don't like the idea of circulating through the cold water inlet is because I am afraid with lack of use at times there will be a hot pocket of water on top which will exceed The expansion valve. This has happened to me before when I had my circulating pump on a thermostat.
     
  14. maple1

    maple1
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    I'm not seeing how that could happen any more than it could now? The hot water should then be coming into your preheat tank at the bottom. Instead of at the top.
     
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  15. Working wood

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    Well if I'm pumping water from the top of the preheat tank through the cold water inlet that does not release water until it gets closer to the bottom of the tank tube correct? And then I would be pulling water from the very bottom drain port of the pre- Heat tank
     
  16. maple1

    maple1
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    Right. (I think). You would both be pulling cold from and returning hot to or near to the bottom (via a dip tube) of the tank. Then natural convection should see the tank mix (at least partially) from the hot rising when it hits the bottom of the tank. So can't see a hot water pocket develop that would kick off the blow off. Also not sure how it would happen if there was a stat in the system controlling the circ - the circ wouldn't be pumping all the time then so not as much heat should be getting into the preheat tank. Unless - it was circulating very slowly by convection when the circ wasn't running, and sending very hot water into the top of the preheat tank.

    All around and if it was me, I would swap in a smaller pump like mentioned, I would still pull from where you are pulling now, but I would return the hot from the coil into the gas heater. Somewhere around the top - like maybe T into the T&P fitting if it was on the side. That would in effect add that much more volume to what you can heat up overnight when not using hot water. But I am not sure how much you would have to gain, as the weak part in all that might be the BTU capacity of your coil. I can heat up my 80 gallon DHW tank in around half an hour when my boiler is running - that is giving it around a 40° temp rise, ballpark. Which would usually last us a day. Unless using a real heavy amount of DHW.
     
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  17. Working wood

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    I thank you for all your help today. To answer your question yes when the pump would go off through convection a hot water pocket would develop at the top of the heater and the expansion valve would go off. Remember when heating with wood I can only afford a slow process of heating water because there's very little control of this when the temp gets to high. I really didn't follow your last statement by saying connecting both heaters together. I don't think that would be a very good idea being that it would be difficult to ever get the temperature high enough. I also think it would be better to keep both tanks independent.
     
  18. maple1

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    I was looking at it from the point of view of maximizing the btus you can get in your tank(s). Which might not directly compare to absolute highest temp. Example if you raise 100 gallons by 20, that's better overall btu transfer than 50 gallons by 30. And more btus transferred is less gas burned. It would also put the hottest water directly into your gas heater and prevent it from cycling in periods of no or little use. And working with bigger volume would lessen the 'too hot' issues some.
     
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  19. brenndatomu

    brenndatomu
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    I agree, I think the water is circulating too fast...hot/cold water not stratifying properly. Not sure that tempering tank is plumbed quite right either...can you get pics of your setup?
    Can you raise the tempering tank up on blocks so that the cold line is flat, or better yet, downhill...and then the hot line is even more uphill...that will allow better thermo-siphoning without using the circ pump. If you can plumb so that there is no/very little horizontal, that helps too...in other words, always have rise or fall or the lines, no pure flat horizontal.
    How many ports are on your tempering tank? Was it an electric or gas WH before?
     
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  20. brenndatomu

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  21. electrathon

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    Your system is very similar to mine. I have a coil in my woodstove providing water to a preheat tank. Your return line from your furnace to the preheat tank is going to the wrong port. It should be going to the cold water "in" line. It will add warm water near the bottom of the tank. The "hot" will rise up to the top of the tank and be ready to fill the water heater, preheated. A simple change that I would recommend (other than correcting the misrouted plumbing) is to put a solar pump control on it. Your pump will run far less often, saving energy and extending your pump life. My pump turns on when the bottom of the water tank is 15 degrees cooler than the water in the stove coil.
     
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  22. electrathon

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    I have been thinking about this for the last few days. Do you have an expansion tank on this system? There is not one in your drawing. You need one, bad. I question if your valve is opening due to heat or pressure. Hot water expands a lot more than you would think it does. A cold water filled 50 gallon tank expands to about 51 gallon once the water is heated (that water must go someplace). You are heating over 120 (normal recommendation for a hot water tank, more heat, more expansion). If you are truly reaching 160 degrees you should consider a tempering valve for safety (anti-scald) reasons.

    The target for my preheat tank is 120 degrees. I was getting a little more than that so my solution was I added 6' of baseboard heater to the return loop. The lost heat from the hot water ends up in the basement, where it is needed. It settles in at about 115 to 120 not when I am burning. The main difference in yours and my system is I have an instant hot water heater. I turn it off most of the time in the burning season. I also use a solar controller to cycle my pump.
     
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  23. maple1

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    I would expect it to be temp instead of pressure, in this case. When the circ stops, the flow slows way down & the now much hotter exiting water goes to the top of the tank with very sharp stratification. Which I assume is where the T&P valve is. If this is a well system the cushion tank usually can handle the expansion in a hot water tank, but not sure we know that or not. Also not sure if enough water is getting heated up enough that would increase the pressure that much - if so I would expect that to be the case whether the circ was running or not. But depending on the entire system configuration and exactly how much of a temp rise over the whole volume there actually is - an expansion tank could be needed
     
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  24. electrathon

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    The circ pump is running all the time, not cycling. Properly set up the return water from the stove should be going into the cold IN line, then the heat will migrate up in the tank, mixing as it rises. The coldest water has settled to the bottom, it leaves and goes into the stove to be heated and returned to the cold water supply line. Effectively the preheat tank is just being supplied with water that is a little warmer than it was. Then it keeps doing this till the tank settles in on the max temp it can obtain for the set-up. If there was no circ pump and the water is circulating by convection it is set up proper. You need the cold bottom and the hot return to make it work. That is most desirable, but sometimes you must use a pump. Then you change the return location.

    The reason that it is supplying cold water out during water use is the pressure is slightly higher in the pump loop than it is in the tank. So on demand the water is coming out of the loop and drawing water from the coldest part of the tank, the bottom. Returning the water to the cold IN port will fix the issue.
     
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