It is a shame the search feature does not seem to work in these forums... I would guess there have been mentions of these boilers in the past. I am looking for reliable LPG combi boilers for a large house circa 1964. 420,000 BTUH according to the HVAC engineer. I imagine 4 zones of heat plus demand HW from each end of the house thus 2 boilers. Zones 1 & 2 are calculated @ 250k and zones 3 & 4 = 170k. the engineer prefers Weil-Mclain the installer prefers Navient. Open to any and all thoughts and opinions. Robb

We don't recommend brands online. Frankly, you would be best served by doing some independent research yourself. As far as searching the GBT forum, just use Google and put GBT in the search field along with whatever it is you are searching for. Please make sure your heat loss analysis (ACCA Manual J or equavalent) is accurate before selecting the heat source. Heat sources are commonly grossly over-sized in the HVAC industry and this is best avoided.

We like Navian
We like Weil-Mclaine
We understand the need to have a boiler sized for the domestic water load at each end of the house will be different than the actual heat load
We think this is not such a bad idea as the turn down on the heat side of both boilers is 10 to 1
We like that
We do not sell Navian
We do not sell Wiel-Mclaine
We like going with the plumbers preference as both boilers are great proven products and the plumber will be working on it as a service agent.
We assume your HVAC engineer has clues about heat load all figured out

A heat load of 420,000 BTU/hr seems pretty high (CRAZY high) even for a 54 year old 10,000' house. Are those the zone heat loads, or is that the combi-boiler specs?

The larger low mass combi boilers have fairly high minimum-modulation numbers, which can make them less than ideal for hot water delivery at low-flow taps.

HTP's Versa combi boilers wiht the 199K burner can deliver 135 MBH of heating (160F output max), and can support even the tiniest trickle of hot water flow as well as 10+ gpm gushers. Whether that's a good fit here depends. (The biggest Navien wall-hung combi is "only" good for 120 MBH of space heating.)

A big burner Weil McLain (or other) + indirect fired tank can solve these issues, if your heating loads are really that big, but that's a custom designed system, not a "combi-boiler".