Mod Con. or Combi?

[WP_Homeowner]

Hi all,
Newbie homeowner who fancies himself a DIYer here so I apologize if I'm over my skis here a bit. Some quick info on my home:
* 6 BR/4.5 BA (4 tubs - and my children and my wife tend to bath. 2 kids often bath at same time). House is in White Plains, New York (Westchester)
* Inherited big oil boiler and did a pretty big house reno 3 years ago. Left the boiler (stupidly though money was tightening) and converted big radiators to a 3-zone hydro-air system (2 air handlers, one in basement next to boiler and one in attic for 2nd and 3rd floor). Coils in each handler are rated at 70k btu's. We have a gas line in the house with enough capacity (250 CFH).
* I recently ran (actually did it) a new zone to a huge radiator that was in a large semi-finished basement room (I calc btu load at about 17-20k, call it 20k..).
* I plan on running radiant heating for the first floor in the basement joists (don't know how many feet of PEX yet).
* Also plan on running some runtal radiators in the family room that is often cold (has 13 windows that are ehhh - all other windows were replacement vinyl). Haven't solidifed these plans.
* Currently have an IDH (believe a Burnham stone lined) with the oil boiler.
* I have not done a heat loss calc. yet...

So to my question. What to do? Combi or mod con (or other suggestion)? I plan on keeping my IDH either way as I don't think the Combi could keep up with our peak water demand (nor do I want to chance it....). In my mind, the further question is does it make more sense to mod con with IDH or combi with IDH. And if someone thinks it makes sense to go combi, should the IDH be a water zone and the combi feeds the IDH directly with hot water (if even possible), or a boiler zone as it is currently setup. And then whichever we I go, I would love some suggestions on boiler sizing (in general for my situation and for possible future plans).

Sorry for the long post. Please let me know if there is any other info needed.
Thanks,
David

 

[Dana]

If you're planning to run the same air handlers it'll take a pretty beefy mod-con/combi, even if running them with an entering water temp (EWT) of 140F, where the heat output would be about half that of an EWT of 140F.

Run a room-by-room, zone-by-zone Manual-J-ish load calculation of the "before buildgin upgrades" version of the house, using a better than average freebie like LoadCalc.net or CoolCalc.com, using AGGRESSIVE (per the Manual) assumptions about air leakage & R values. Those tools will still overshoot reality (often by quite a bit), but won't be 2x reality. Then compare the whole house number to what you get from a fuel-use based load calculation (which is almost always going to be a bit smaller than even a competently executed pro-tool Manual-J), and scale the zone loads accordingly. Unlike Manual-J, a fuel use load calc is a measurement of the heat load, including distribution losses, etc.

With air handlers you'll probably be operating them fixed temp (no outdoor reset), and you may need to tweak other radiation to work well at that water temp, and you may or may not need to mix down for the radiant floor zone, depending on the load of that zone, the type of radiant, and how many BTU per square foot it needs to deliver. If they're 3 x 70K at an EWT of 180F, figure on 100K of boiler output to run them at 140F or less (and have a shot of getting condensing efficiency). The whole house design load might not even be 100K (probably isn't), but don't upsize any more than you have to. A 120KBTU/hr-in / 115K/hr-out mod con is way more boiler than 95% of the houses in NY would really need.

With 4.5 baths most combis aren't going to cut it on the hot water delivery end. A mod-con with a big indirect tank (big enough to fill your biggest tub) would deliver faster fill times and overall better hot water service. Size the boiler for the heating load - don't upsize for the indirect, just make the indirect the priority zone.

Until you get the load numbers squared away there isn't much relevant advice to add.

20K is a lot of load for a basement, unless there is no foundation wall insulation and it's leaking a lot of air. If there is any reasonable way to bring the foundation insulation up to current IRC code it's going to be "worth it", even in parts of the basement that aren't going to be finished fully-heated space.

 

[Sponsor]

 

[Fitter30]

Windows ,doors and insulation have to be considered. Utility company rebates, fed tax rebates? Running times and comfort level should be noted. This has to be one of the worst times with the cold wave.

 

[Dana]

Windows ,doors and insulation have to be considered. Utility company rebates, fed tax rebates? Running times and comfort level should be noted. This has to be one of the worst times with the cold wave.

The current Polar Vortex disturbance cold wave has not really hit the northeast (yet). It's primarily a midwestern plains state phenomenon right now. It's a bit unusual to see temperature reports this week where even gulf-coast Houston is dramatically cooler than New York or Boston. As of this minute after warming up 10s of degrees Dallas/Fort Worth is still 3F colder than my location in central MA (experiencing normal cold for February), 2F colder than WP_Homeowner's White Plains NY (also quite normal temps for the season.)

The infrastructure problems in TX are pretty alarming- I don't think we have a handle on the extent of it yet. Between both water mains & gas pipelines freezing (let alone the hundreds of thousands of homes with frozen plumbing) it's going to take more than just an extended thaw to re-normalize. This is a terrible week to have an outdoor mounted tankless water heater in TX, but that's just the frost on the tip of the damage iceberg. In the northeast homes and infrastructure & homes have pretty much been designed & maintained for sub zero cold snaps, not so much in the southern plains.