Getting into hot water, Part 3

When the oil boiler went away the hot water tank did also, and this gave me an opportunity to re-locate the new water heater directly below the two bathrooms. This reduces the wait time to get hot water to the tap substantially, and the tank is now only half the distance from the kitchen as well.

When hot water leaves the tank it goes through a Caleffi thermostatic mixing valve (TMV). I installed this in case I decide to install solar DHW, because solar can make very hot water that carries a scalding risk. A TMV is set to a maximum discharge temperature, which it holds regardless of the inlet temperature. The TMV creates this mixed temperature by mixing hot water from the tank with cold water. Colleagues of mine have found that some TMVs, even when set at maximum temperature, still mix in some cold water. I tested the Caleffi and it doesn't do this, which is a nice feature in a solar DHW system - when the water temperature gets right down to the minimum usable temperature you don't want cold water mixed in, because that will cause the back-up heat to come on.

Once the mixed water leaves the TMV is goes to the distribution system. The DHW distribution system in House 5 is a home run system. There is a manifold for the hot water (and one for the cold water) that has a series of connections with shut-off valves, and each connection has a cross-linked polyethylene (PEX) tubing run that goes to an individual fixture. In my house, these are 1/2 inch, but if I were to do this myself from scratch they would be 3/8 inch for most fixtures. One of the advantages of a homerun system is usually shorter wait times for hot water than occurs with a traditional trunk and branch system. The smaller the tubing, the faster hot water gets to your tap. Having a shut-off valve for each fixture allows an individual fixture to be repaired or replaced without having to lose the function of any other fixture.

Here's the manifolds above the DHW tank, the cold manifold is behind the hot one:

Another efficient approach to getting hot water quickly to each fixture while wasting minimal water is to use an intermittently operating recirculation pump (Taco D'Mand). My very smart colleague Gary Klein has written extensively about the benefits of one of these systems (Gary Klein powerpoint) and here's a simple schematic he drew of one:

Notice that the hot water line continues back to the DHW tank after the last fixture branch. When you are in the bathroom, you push a button (it can also be set up with a motion detector, to work automatically) and the pump turns on, sending hot water to you at several gallons per minute. When it reaches the pump, the pump turns off, and you have hot water in a very short time, and almost no water went down the drain while you waited. If the water in the line is already hot from a recent draw, the pump doesn't come on. This clever system is a variation on the old continuous recirculation systems that waste both thermal energy (because the recirc loop is always hot) and electricity (because the pump runs all the time.)

The larger and more spread out a house is, the more benefit from this type of system. It really makes sense to look at each house and evaluate the best approach. My colleague Michael Chandler suggests that a modified home run system, where a single 1/2 inch line runs to a bathroom, and each fixture uses a 3/8 inch branch off of that, is the best, at least for a modest sized home. That approach means that once someone used hot water in the sink, for example, there is hot water available for the shower or second sink with no waiting.

Comments

[Bob Lemaire]

Marc,

The link to Gary Klein's report is bad. It was apparently appended to the blog URL.

Your post is timely. I'm just now replacing a satellite hot water booster tank in the kitchen with a recirculation system. Research I did turned up user dissatisfaction with the Taco system because the sensor maintained 90 degrees and most still waited for the warmer flow. I'm going with a B&G Ecocirc that has an adjustable thermostat and ECM motor for about the same money. The recirc loop will only service the bathroom sinks and the kitchen sink and dishwasher, that's 95% of the wait and waste problem. The showers are only used twice a day, are in proximity to the tank, and waste little with uninsulated 1/2" home-run pex.

[Marc Rosenbaum]

here's the PDF URL, paste it into a browser:
www1.eere.energy.gov/solar/pdfs/sda_saving_water.pdf

[Mark Franklin]

Another solution to activating the pump is to use a flow monitor to turn on the pump when any hot water faucet is turned on. Here is a link to a product that controls hot water recirc lines based on flow:

http://www.fasterhotwater.com/Pump%20Manager.shtml

[Water heater prices]

Hi Marc,

im wondering how much cost this sistem? And did you do all work yourself or you get profesional that they do for you?
Btw great system and great job.