Let me start by saying
that I am not a big fan of basements (or mechanical rooms) that are accessible
only from the outside. That said; let's assume that this is the only reasonable
solution. If I understand this correctly, there would be a stairway along the
side of the house that descends to the level of the mechanical room floor. At
the foot of the stairway is a landing with a drain. There is another drain in
the floor of the mechanical room.
Since this is new construction, there is no reason why this wouldn't work out.
The foundation will be properly sealed on the outside and will have a footing
drain so it is unlikely that any water will ever get into the mechanical room
unless there is a plumbing leak or the water heater gives out and develops a
leak. If possible, it would be good to have a roof over the stairway and
landing, but perhaps that would not be possible. The rockery will have to be
constructed so that any ground water that may be behind it will be drained off
through footing type drains away from the house.
My question would be where do the floor and landing drains go? Are they tied
into the sewer or septic system or are they plumbed into a gravel sump, or are
they day lighted somewhere lower on the property. They could drain into a sump
with a pump that is activated when the water level rises. One inconvenience you
might run across with the interior drain is the need to keep the p-trap wet.
Odor from sewer or sump can enter the room if the water level in the p-trap
falls low enough due to evaporation. You will definitely need to keep an eye on
the drain outside, especially in the autumn when leaves tend to blow into areas
like the landing and cover the drain.
The mechanical room will of course need to have ventilation to allow proper
combustion. If this is a propane furnace, there is also that to consider. Since
propane is heavier than air, if there is a leak, it can pile up in the room
rather than dissipating as natural gas tends to. I think that you would need to
have some sort of escape route for the gas, but that is a question for the
mechanical contractor.
I have never had a project that involved a heat pump, but I like the idea. As
the price of gas continues to rise, and the summers get hotter, I think this
type of device sounds better and better. I like the idea of having some option
to cool the house on the few days we have here that are too hot.
I don't really like on demand hot water. I have seen it used on a small scale as
well as for spa heating, but the heaters I have seen seemed kind of cheap. I'm
sure the technology has improved, but I'm not really familiar with it. A well
insulated and efficient traditional water heater with tank just seems like a
good idea. In an emergency, you always have 40 or 50 gallons of drinkable water
stored in the tank. If you decide to go with a heat pump, it wouldn't take too
much space to put a traditional water heater inside the main part of the house.