Mod Con. or Combi?

Steltz

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? 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, or a true 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

hot_rod

I'm not sure the combis have a wiring or control connection for an indirect, my Lochinvar Nobel combi does not. So you would need to externally wire a priority demand signal for the indirect.
If the indirect is adequate for your home, go with a mod con, not combi. The combis do complicate the boiler some and need routine maintenance on the HX and valves inside.

Or stick with oil and go with an Energy Kinetic system energykinetics.com

bio_guy

Before you do much, you are going to need a load calculation. I'd include or even use alone a calculation based on fuel use with the oil system. You or your oil dealer must have that. All you need in addition to that is degree days from a nearby weather station or that same dealer.

It is challenging to run hydro coils with condensing boilers. What is the hydro coil output at condensing temperatures? Probably not much and not enough. Maybe you can figure out how to cool the return water further by running it through the radiant or the radiators after the coils before it hits the boiler again. Heavy radiators work well with low temperature water. I don't know if it has ever been done, but if you could figure out how to use the radiators and floor radiant as dump zones without overheating those areas, you might make it work with the higher temperature hydrocoils. It might, however, really confuse the native programming in the mod con.

Do they even make a combination boiler big enough for what you want to drive with it? Here is an idea, I don't have a good picture in my head of what combinations of heat source you plan in different areas, but I guess two in some and one in others. You need very hot water for the hydrocoils and cool for the radiative heat. For the DHW there is no reason to go over the temperature of your anti-scald if you have enough storage capacity. Use more than one boiler. Some time ago, a field study was done in Minnesota testing the idea that in old homes a small condensing boiler could be installed alongside the existing low efficiency gas boilers. They were set up so the condensing handled the load unless the weather got very cold where they kicked in. I believe they simply piped them in series. You don't necessarily have to do that. There are more than one option with a couple of boilers with different controls and piping, basically series or parallel if interconnected, and completely independent.

You could keep the old oil boiler and press that into service on the coldest days with an undersized mod-con. The trouble with that might be fuel storage. A gas gun for the old boiler might not be that expensive, but chimney modifications might be.

I've always liked HTP Versa-hydros on paper. The heat exchanger is unique as it uses the natural tendency of water to temperature-stratify to operate well in condensing mode. It's cousin the Westinghouse WGR0 series is much less expensive but no heat exchanger to do double service for DHW. Both are very forgiving of DIY because they require no special piping. (They can be drop-in replacements for old ~80% bang-bang boilers.) The WGRO only put out 76 kbtu. I don't recall for the HTP. If you have more than one boiler, you have some backup against mechanical failure as well.