I am looking to replace my rusting gas hot water tank (15 years old) and leaking gas boiler (30 years old), and am trying to determine what the most cost-efficient long-term solution (30+ years) would be (ie - installation fee, maintenance cost, gas bill...).

A bit about my house:
- 3300 sqft 30 year old detached (5 bed, 5 bath)
- Vancouver, Canada
- ~$13/GJ gas price ($300 per winter month, $60 per summer month)
- 5 zone radiant heat on both floors
- Poly-b pipes

Questions:

1) From my research, there are three systems I can use. Are there any other systems/options I have missed?
-- Standard efficiency hot water tank & standard efficiency boiler
-- High efficiency hot water tank (aka hot water on demand) & high efficiency boiler
-- Combi system high efficiency with a storage tank (I was told I would need a water storage tank with a home larger then 3000sqft)

2) Is one type of system (standard efficiency, high efficiency, combi system) more reliable then another? I figured the high efficiency and combi systems are more complex, therefore can break more easily.

3) Would the maintenance for each system be similar? What should be done to maintain the system, the frequency, and the cost?

4) Generally, how long does each type of system last?

5) What do you think the gas savings would be with each type of system? From what I understand, a standard efficiency system is roughly 80% efficient, where-as a high efficiency would be around 95%. In that case, would I be saving just 15% on my gas bill if I purchase a high efficiency system?

6) I have a few installers coming next week. Are there any specific questions I should ask them?

I would get a Hi Efficiency boiler and a 'better quality' storage tank type water heater.

FIRST

you need to get PROPOSALS from the 'installers'

THEN

select the proposal of your choice, turn it into a set of 'specs'

AND

put said specs 'out to bid' so you are comparing apples to apples (you may simply contact the other 'proposers')


LICENCED - BONDED - INSURED contractor + a building permit as required


remember: the cheapest price is seldom the lowest cost

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

check your area for energy rebates.
my area MA offers very generous rebates some in the thousands.
in my opinion, a high efficient combi unit is the only way to go.
efficiency numbers are tricky. you also have to look at volume of water in the heater. some high efficiency units have only a few gallons capacity along with a thin wall stainless chamber so
when there is a call for heat they come up to temperature very quickly.
they also have the technology to reduce the gas flame by as much as 80% so you aren't pushing wasted heat out the flue pipe.
BTW, if you do add a storage tank, make darn sure it is stainless steel.

Make sure the company you go with offers service 24/7/365.Company's that do will cost you more but will be there for you when it's -3 outside. The BBB is a great place to start, call and ask them for 3 local company's. avoid the
Craigslist yahoo's ,there are so many schools out there pumping out Hvac tech's that go into business after there 16 weeks of schooling and never work for someone to sharpen there skills.