So due to a combination of events involving an out of town trip, an old crumbly brick chimney, and an old water heater in my house, I had to make a decision rather quickly about having hot water in my home or preventing my children’s death of carbon monoxide poisoning. I went with the latter and shut down the 15 year old 40 gallon tank water heater and started shopping for a new unit.
As an architect, a technophile, and as a person interested in being green, I decided to get pricing on a tankless whole house water heater as an option to line up against the price of a standard 40 gallon tank unit. Because of the location of the old tank heater and accessibility (or lack thereof) to an exterior wall for a “B” vent, the water heater would have to move or I would have to rebuild the chimney. Easy decision for me – move the water heater – I was already going to have to pay money for the heater, I was not ready to do both at once.
The new location, in the laundry room was tight, which helped my decision to go with a wall mounted tankless unit. The installation cost was 60% more than a tank unit, but some of that cost is in new exterior venting, a new 1″ gas line, a new outlet, and new hot and cold water piping. And there is (was) a $300 rebate available through my utility.
So now I have (in theory) an endless supply of hot water for my 2-1/2 bath home and I’m learning how to use the system. First, no trickling of hot water for shaving – too low of a flow (<.5 gal./min.) and the heater won’t kick on. Second, and this is the big one, watch out for the “cold sandwich”.
So the “cold sandwich” is a term to describe a situation unique to tankless water heaters. In a tank system, if you turn on the hot water in the morning, you wait a few seconds as the cold water gets pushed out of the pipes and then you have hot water. If you turn off the hot water, then turn it back on again, the hot water in the pipes keeps on coming, pulling from a large tank of hot water – so until that tank is empty, the water will be hot.
In a tankless setup, when you turn off the hot water, the hot water heater turns off. When you turn the hot water on, a little cold water goes through it first so it can determine flow and temperature so it can regulate the amount of gas to burn. This, in turn, puts a section of very cold water into the hot water stream. If you are not too far from your hot water heater, this is not a big deal, because you know when to expect it and you wait a second for the hot water to get there again. In my house, the hot water heater is now right next to the laundry and utility sink, right below the kitchen sink/dishwaher, and only a few feet from the powder room, leaving the two 2nd floor baths the only problem areas. But these are really big problems!
Imagine in the morning, you are at the sink, washing your face. You finally get the hot water flowing and then you turn off the sink, jump in the shower and it feels so nice. 10 seconds into your shower, you get 5 seconds of ice water! That’ll wake you up, but it’s not pleasant. So the short term solution is to turn on the shower before turning off the sink, ensuring a continuous hot water flow so the heater stays on. The long term solution is to add a small holding tank to the system – potentially with a recirc pump that runs on a timer for just the mornings. Although it would cost more money, on top of the investment already made, I don’t want to have any more “cold sandwich” mornings!
