"The last option would be to use it to heat the cold water coming into your shower." I know about drain heat recovery options, but I had never though of putting it on the cold water side for a shower. Thanks, moandtg, for contributing. I will look at it in a different way. This sounds like the best option. When you start stacking water heating options on top of each other, you don't get additive economy. For example, if you save 50% with solar pre-heat and 20% with your shower hot water in series you will not get 80% savings. It is a little hard to compute the exact figures because it will depend on the weather. In addition, the higher the temperature change, the more efficient the heat transfer is. By pre-heating the water going to the solar, you actually decrease the efficiency of the solar heating process. That is why you are better off installing a cost-effective way to heat the cold water. Note that I am not saying that drain heat recovery devices are cost-effective. I have not evaluated them. If you pre-heat the cold water to the shower, you should get additive heat efficiency. In addition, the plumbing will be greatly simplified and less costly. The heat losses in the between the heat recovery unit and the other water heater will be gone as well!

What you have there is a very desirable feature, and one that I highly encourage my clients to allow me to install. Years ago when they still did laundry in Hot water it was common for water heaters to be set at 180 or 190degF, especially if the water heater was supplying a dishwasher because the health codes require 180degF water for the sanitizing rinse. The Medical profession says infants, children and ppl with sensitive skin can be seriously injured by water temps exceeding 130degF, so the Plumbing codes were ammended limiting the temperature of domestic hot water to not more than 125degF. Now most laundry detergents work fine in cold water and nearly all domestic dishwashers have a built in booster heater to provide the 180DegF sanitizing temp. (When you turn the "power saver" switch on it disconnects the booster heater to save energy). Within a short time it was realized that trained plumbers could be relied upon to set the water heater thermostat to the code prescibed 125degF, but there was no way to prevent the homeowner from coming behind the installer and turning the temperature up, so in order to protect themselves from future liability most domestic water heater manufacteres installed thermostats that only go to about 140degF, and instead of marking the temperture on the control knob they just put an index at 125degF, indicating the correct temp to operate the heater. There is only one small problem with that solution. The human body temperature averages 98.6degF and most ppl like their shower slightly warmer than body temp, which means an average shower is 105 to 115degF and some even like a Hot shower which is in the range of 120 to 125degF. The problem here is that back when the water heater was firing at 180 a typical shower was approximately a 60/40 mix of Hot & cold water, but once the source temp at the water heater was turned down we now have approximately a 95/5 mix of hot & cold water. Keep in mind that even with a code required 2.5gpm shower head a 10minute shower consumers 25gal of water, and if that water is coming from a water heater that is only producing 125degF water, it is very easy to run out of hot water before the shower is completed. Now add to that the fact that most new homes have at least two if not more bathrooms and it is quite common for two or more ppl to be taking a shower at the same time so the supply of hot water is critical. The standard solution today is to install bigger & bigger water heaters, in fact, where a three BR house previously had a 40gal water heater it is not uncommon to see an 80gal heater or even two 60gal heaters in homes today. Now you have to understand that while you are drawing the 125degF water off the top of a typical water heater, that heater is also being constantly filled with 35 to 55degF cold water from the bottom, and as that cold water enters the tank is dilutes the stored water, which results in loosing temperature in your shower quicker. The WATT's tempering valve is actually a mixing valve, so you can turn the temperature up on the water heater and as the super heated water enters the Watts vavle it is mixed with cold water so the output from that valve is at the code max of 125degF. The end result is that you have 25% to 35% more usable hot water from the same size of tank. Now in regard to your original question. I cannot see how it is all hooked up at your house so I cannot comment on the second cold water line, but I have attached an illustration of how we typically install a tempering valve and I am certain you can connect it with just one pipe coming down from the ceiling. The only thing you have to remember is, the horizontal line from the cold water supply to the tempering valve must be connected to the coldwater line before the water heater main water shutoff valve.

HI, I'm not sure what was used to submit or edit the original question, but the entire post wasn't very clear. I also don't know if this is being asked by someone in the USA: Codes vary by country. I'm going to give you some general answers to the questions that I think you are asking: 1.) Don't "rig" a heater to use it in a way that it wasn't intended. If it was designed for outdoor use, don't use it indoors. 1.A) Don't even think about rigging the venting. Don't substitute materials, don't improvise. You will find yourself dead. 1.B) Don't even think about rigging the gas. Again, you will find yourself and your neighbors dead. 1.C) Don't even think about rigging the electrical. You will find yourself dead 2.) There are ways to increase the capacity of a single tankless heater by using multiple units. The units must be made for this purpose. You must have the gas supply to accommodate such an install and it does not make sense to use multiple small units when a single larger unit is available. Again, don't even think about stringing together multiple units without them being expressly designed for that AND doing the work by the book. If you currently have a 40 gal tank, I can't imagine you would need more than ONE 200k BTU/H unit, which is as large as they typically come. 3.) There are ways to use a primary heater (for example your 40 gal tank) and a point of use heater. We do this when a master bathroom is at one end of a house and the tank heater is in the garage (really bad design) but it's not common. What you are describing though sounds poorly thought out. 4.) The setup you are describing would not save energy or operating costs. 5.) A competent plumber can address your needs - but isn't going to help you rig a system. If the issue is running out of hot water, then a larger tank heater can be the most effective way to resolve that issue. If the issue is lower operating costs, then a single tankless gas unit or a hybrid electric tank unit can be a good choice (but come with higher up-front costs). Again - I can't say this any more clearly: Do not use a heater in a way that it wasn't intended. Do not improvise or substitute materials on the venting, gas or electrical supply. Do not string together multiple units unless they were designed to operate that way. There are installation and use manuals available online for any of these heaters. Follow the design and installation instructions. If you aren't very familiar with plumbing and water heater installation, use a professional.

The response from Stax is not correct. The expansion tank is to prevent pressure from exceeding design limits when water is heated in an otherwise sealed system. However - once even a small amount of water is used, that pressure would drop. If your toilets are filling faster, etc. it's not because of the expansion tank - you would fully draw down any extra cushion that the expansion tank provides in something significantly less than the capacity of the tank. An expansion tank has a total capacity of about 2 gal. If the tank wasn't working at all, the pressure drop would be almost instantaneous when you opened a faucet. What's more likely is that you have a pressure reducing valve (PRV) and it's not working correctly. It's probably coincidental that it happened when the tank was replaced - perhaps when the water was turned off. The answer is to use a pressure gauge and see what your static pressure is (you can turn off your water heater to take the expanding heated water out of the equation). If it's above 70 psi or so you want to repair / replace your PRV.