All residential gas 'mains' are fed from a higher pressure line at the origin of the main. The pressure is lowered by means of an adjustable regulator. If the pressure in your neighborhood is that low others must be expriencing problems also. Ask the utility why they can't increase pressure at the regulator. (The 'Main' I am speaking of is the pipe that runs down your road and all the service pipes to the homes hook to it.)

fxdbdennis makes some good points; However, if money is limited, two tankless units would be expensive. I think the important thing is to understand what the goal is. For some people, unlimited hot water is the priority and they are willing to pay for expensive tankless unit(s)to get it. Some even add a recirculating pump that makes the hot water to the faucet almost instantaneous at the cost of significant standby losses. Other people are interested in energy savings (even if it doesn't make financial sense). Still others want a system that is economical over the life of the system -- be that tank or tankless. What I understand from windyhill_gardener is that they are looking for a way to not vent through the chimney-- something that can be accomplished with a powervent tank unit or a tankless unit. I also hear that they want to lower their utility bill, again something they could do with either type of unit-- with a small advantage to the tankless unit. I also got the sense that budget is somewhat of an issue since they have not replaced their old 80% or less furnace yet and that this is an older home where running a new gas supply is not likely. I am also guessing that they don't have major hot water uses (jacuzzi tub) since they have an existing 40 gal tank. If my assumptions are correct, my advice would be to go with the 50 gal powervent unit and put any additional money towards the furnace. This would provide a significant quantity of reliable hot water at a good efficiency while keeping installation costs reasonable and allowing for the elimination of the chimney. It's hard to justify spending an additional $1,000 -$1500 for a single tankless unit in order to save $50 a year on the gas bill when the furnace needs to be replaced. With all of that said, I want to be clear that I am not negative on tankless units. For some applications they are a good choice.

The $700 or so to replace your gas line from the street is chump change compared to the 3K you will need to spend to upgrade your electrical service from the pole to your house and replace your current panel. All of that will need to be done before taking your home to all electric. Then your utility bills will go up. Electric costs more for the same amount of heat than gas does unless you are somewhere that hydroelectric is the primary means of generation. Now, if you were to install a heat pump system for your heating and cooling, you would see a reduction in energy costs for heating and cooling your home, but you would still need the new electrical service in order to be able to do that, plus the costs of installing the ducting and heat pump system which would be 15-20K on a small house. You would never achieve payback for the expense for the amount of time that you live in the house. If you want to save costs on the gas line replacement, dig the trench from the street to your home yourself. If piping in the interior of the home needs to be replaced, you can do the distribution system piping as well. Black iron is only moderately difficult to work with for someone who can do a bit of math. Then you can call a plumber out to do the connections at the street and at the regulator outside of your home. That is assuming that your insurance company and municipality do not have an issue with DIY work on gas systems.

Klem, I am a control freak by nature and these kinds of systems I worked on what seem like a life time ago when I was working in facilities maintenance for what is now mostly a defunct cell phone manufacturer. Realize this system does not provide the heat to the structure... it is not a boiler type system. It is your hot water system on steroids. You already have the hot water that is in the hot water tank. The pump is merely a way to circulate it. The pump would come via commercial type application. The thermostat is merely to turn the pump on or off via a predetermined set point. Bell & Gossett are a well known brand some 20 years ago for this kind of application. As far as power problems go, the best way to alleviate those concerns would be to install a generator that would kick on to run your furnace, power the water heater and the pump and controls of the system. You could size the generator as big or as small as you want depending on what your goals are.