Will Cold Water Supply to my Bypass Humidifier Hurt my Heat Rise of my Furnace?
I have a Rheem hybrid hot water that feeds into my bypass 98% efficient furnace. I am in Colorado and I set my humidifier to average 35% (peaks at 40% in the morning dips to 32% at night). After many years of adjustment this seems to be about right for our skin while balancing hot water usage.
However in the morning we can run out of hot water in the morning because of the humidifier uses much of the 50 gallon tank when the furnace goes from 63F at night up to 65F in the morning. During this 3 hour ramp up in temp is when the vast majority of the humidification happens. That also happens to the time when showers are run. I have tried switching the hot water heater to high demand mode for this time of day which helps a lot.
But there is another issue, I exhaust the cold air from the Rheem from the basement into the vented crawl space. I have measured 393 ft per minute out of the vent when it is running, which I think equals about 77CFM of conditioned air leaving our house in the 6″ vent. Since our house as a blower door score of 5ACH50 it is easy to see the cold air entering the house were I have air leaks with a thermal imaging camera while it is running. The hot water tank is a much bigger issue on this than our range hood since the it runs for hours not just minutes a day.
So I have cold showers and air leaks in the morning due to supplying the humidifier with hot water.
I measured the heat rise of my furnace it it is consistently 55F at all 4 stages of heating on my furnace. My question is if I change the humidifier supply from hot to cold water would my heat rise of my furnace lower because of the cold water running through the bypass making my furnace run less efficiently?
I realize that the humidifier will use more water by changing the supply to cold water but at least my wife will not be mad at me.
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Replies
There's no reason to be exhausting the water heater outdoors. The air you're venting is being replaced by outdoor air which is colder and drier. That makes that much more humidity you need replaced.
Something else is not right if your humidifier is draining the hot water tank.
Let's say your house is 20,000 cubic feet -- 2500 square feet with a ceiling height of 8 feet. Air has a density of 0.7065 pounds per cubic foot, so the air in the house weighs 1,530 pounds. At 65F, at 100% RH air contains 93 grains of water per pound of air, so 1,530 pounds of air would hold 142,000 grains of water. There are 7000 grains to a pound, so 142,000 grains of water is a little over 20 pounds.
That's at 100% humidity, changing the humidity by 10% would mean adding 2 pounds of water to the house, or about a quart.
So something is quite wrong with your humidifier.
At 5 ACH50 you may not need humidification at all. I would try re-routing the water heater and see what happens with the indoor humidity.
This is a good point. The air coming off the water heater will typically be warmer than the outside air in the winter and cooler in the summer so you'd want to keep it inside the envelope pretty much all year long.
The heat rise of the the air in your furnace may slightly lower but I don't see how your furnace would be any less efficient. It may need to run slightly longer in order to raise the temperature to the setpoint but it likely wouldn't be much. Is this diagram how you have it ducted? The humidifier is an ultrasonic type? If it is steam you may lose a small amount of electricity heating the water since it won't be partially via a heat pump but instead electric resistance.
My town is letting me beta test a new water meter. It seems to track my usage pretty well, but it sees my humidifier as a slow leak. If it is accurate my humidifier uses about 8 gallons a day but sometimes up to 22 gallons. I will turn the humidifier off it is less than 10F outside for the night because of condensation on my windows. After having it off it will use over 20 gallons in a morning to get the humidity back to 35%, that is when I really have shower issues. Also the new meter gives me the temp of the water, in summer it is 69F and in winter it is 42F coming into the house, so the hot water heater has to work a lot harder to make it to 120F.
So to answer the original question, it takes about 50 BTU to raise a pint of water to hot water temperature, and about 1000 BTU to vaporize it. So the difference between hot and cold water is minimal.
Based on the water usage, it appears that at least 95% of the water going into the humidifier ends up going down the drain. So you're wasting a lot of water, and if that water is heated a lot of the energy in it is wasted.
My advice is still to stop the water heater from venting outdoors and then see if you still need humidification at all.
DC, thanks for your advice. I am working on a plan to dump the cold air into a condition storage area next to my utility room. I was getting some errors from the water heater that read something like 'low suction temperature' when I was dumping the cold air in the utility room that is why I was not dumping it in the utility room. (I also added foam board to the concrete walls in the utility room but that only helped raise them temperature in that room a couple of degrees F.)
I will also re-plumb the water supply for the humidifier to the cold supply and report back.