Here you may find inappropriately used technical terms. Please leave a message at my guest book pages, should you be aware of better words.

Emitting no CO₂ is best, even when in doubt about man-made global heating. No CO₂ means clean energy and this is better for man (and mankind) anyway. Hence, using electricity in a smart way is better than burning fossile fuels. Many homes however are still equipped with a natural-gas powered furnace and then a few terms come to mind shoud:
Modulation, outdoor temperature sensor, comfort switch, regulated combustion process, low temperature heating, heating support by solar (see Sailer GmbH or oertli)

Modulation: a furnace can work more efficiently at lower power; a 25 KW furnace could be powered for only 5 KW giving a more constant temperature.
Outdoor sensor: a sensor that observes the outdoor temperature and therewith the energy loss of the home, needed for a ...
Regulated combustion process: the furnace delivers lower temperature water to the radiators only to supplement for energy losses. Calculation
Comfort switch: prevents unnecessary standby heating of water when nobody is home.
Frost protection: a switch that starts the furnace disregarding thermosats

If possible, choose a non-fossile furnace; one that uses solar energy, wood pellets or a heatpump. A good heat pump uses 1 kWh of electricity to pump 4 kWh of energy from a ground loop to deliver a total of 5 KWh of energy into your home (heating or tap water). In this case we say the furnace has a COP of 5. An oldfashioned electric boiler has a COP of 1; you put in 1 kWh of electricity and get 1 kWh of heat in the tank. Even block heating or industrial waste heat cannot deliver energy to your home that efficiently.

Furnaces delevering both tap water and home heating, are usually chosen in a power range that may be needed for worst case tap water heating situation. E.g. for a 60 °C (140 °F) rain shower requiring 20 lpm (5 gpm) and cold water of 5 °C (41°F). May homes have 27-35KW furnaces. The required power for hot water preparation can easily be calculated by multiplying the number of liters by the required temperature increase (in °C). That number is multiplied by the heat capacity of water. Finally factor in the time in which this amount of energy has to be delivered. This wiki explains that to increase the temperature of 1 liter of water by 1 degree Celcius, 4180 Ws is required.

Suppose the temperature of cold tap water is 15-20 °C. The shower delivers water of 40°C. To prevent legionella growth, the furnace delivers water of at least 60°C. The incoming tap water needs to be heated from 15 tot 65 °C; the δT is 50 °C.

Water of 65 °C runs from the furnace to the shower thermostat valve. There water of 15 °C is mixed in to reach a shower temperature of 40 °C.
Should the solar boiler hold warm water of 40 °C, the net temperature makes heating unnecessary yet the above routing of the water uses/wastes natural gas. It does allow us however to calculate the required furnace power.

Using a water conserving shower head, I'm using only 10 liter water a minute. I'm not deploying shower waste water heat recovery, so all energy must be delivered as hot tap water. The 15 °C cold tap water has to be heated to eventually(^(*)) mixed water of 45 °C, so an increase in temperature of 30 °; the required power in Ws per liter*liters*degrees per time unit (minute=60seconds) equals 4180*10*30/60=21KW. Should we use 40 °C solar boiler water rather than 15 °C tap water, we only need 4180*10*5/60=3.5 KW !

Power requirements when using a Shower waste water heat recoverer (1, 2, 3, 4). Heat recovery up to 65% is possible. That will save 3983 MJ annually which is the equivalent of 1106 KWh or 116 m³ natural gas. Should 65%be recovered, that simply means a reduced power requirement of 65% or a power requirement reduction from 21KW to 7KW should we have cold tap water or even down to 1.2 KW in case we use solar preheated water. Few furnaces can modulate below 5KW unfortunately. On the other hand, a 1.2 KW electric heater (a not so wise but sometimes very handy method of after-heating) isn't rocket science. Many a close-in boiler has a higher power.

Warm enough water in the solar boiler means we (Soft-I/O, that is) open(s) the comfort switch and we enjoy solar-heated water only. The few liters of cold water that precede the warm water are often used for something else, like houseplants, rinsing the dishes, brushing teeth) or are used by the hot-fill dish washer for the first rinse, which according to rumour should be done cold to prevent sticking. After brushing teeth, the warm water has arrived to have a comfortable shower.

In those cases where we want hot water to be availablle quickly. we have remote controls that instruct the home automation to flick the comfort switch for 5 minutes, regardless of the solar boiler temperature.

^(*) To the boiler it does not matter wether to heat up 5 liters by 50 degrees (15→65 degrees, wich is combined with 5 liter of 15 degrees to make 10 liters of 40 degrees shower water) or that it should heat 10 liters by 25 degrees (15→40 which goes directly to the shower). In the first case you will use more natural gas, as the furnace is not in condensing mode; flue gasses are warmer than the condensing temperature of 57 °C.

According tot the heat loss calculation of our home (using a known temperature difference, a known amount of natural gas, the energy contents of the natural gas and a known furnace effieciency), a furnace of only 7KW for our dwelling should suffice.

Children can easily get burnt by hot radiators, the radiators smell, give an arid atmosphere and have a long afterglow (the temperature overshoots), all of which drawbacks are prevented by reduced furnace power. Olino had an interesting article about saving with a high efficiency furnace.

Unintended (?) side effects of a passive house: it is a quiet dwelling as thermal insulation also muffles the noise. The taped gaps and absense of fissures in boarding also prevent noise coming in. Also the air is cleaner; to reach passive house standards, high efficient heat recovery ventilation is needed and these come with filters. Thus breathing air is always filtered. That leads to reduced vacuuming and thus saves energy.