how much mod/con does one family need?

dm8877

Hi – total layman here. This site is a great resource. I'm looking for insight/advice and hope you can help.

We just bought a 2200sf colonial in the woods of Connecticut. We knew going in that the Burnham oil burner for heat and DHW had a cracked chamber and needed to be replaced. We want to switch to propane.

We've had several contractors out for quotes and the ones we want to use recommended a Navien NCB-240. I googled around on the model and what I was able to find didn't fill me with confidence. The contractor also offers IBC boilers if we prefer. We have no special hot-water needs beyond two standard showers, dishwasher and laundry. We wouldn't be using those all at once anyway to spare the septic.

The house is leaky. Built in '65, original windows, under-insulated in the attic (about R20 – I will add more before this winter). I will be improving the envelope bit by bit but it'll be a few years before I can get to all the windows.

House is two stories over a basement/garage and has three zones: ~73' of metal fin baseboard on the first floor and ~85' on the second. The basement/garage has no heat but there is about 600sf of semi-finished space I'd like to convert into a den at some point. Could maybe use a wall unit for just that space.

Anyway, we don't want to buy too much boiler here. The Navien goes to max 120K BTUs while IBC offers models that top out at 84K and 106K as well. I don't want to install anything that's going to end up short-cycling most of the time. Any advice or further reading/resources would be greatly appreciated.

njtommy

I've had my Navien NCB combi boiler for 3 or so heating seasons so far and have installed several others a lot with their NHB boiler. I have yet to have a problem with Navien. They get a bad rap from very poor installs/ guys that don't know how to size boilers correctly.

The NCB 240 like many other combi boilers have a bottom end of 18k btus. If you have a zoned heating system the boiler will mostly short cycle especially if you have baseboard. If you have cast iron rads you maybe fine do tonthe higher volume of water.

Navien and other manufacturers have smaller combi boilers, but you may find your better off going with a properly size mod con boiler and indirect tank water heater.

dm8877

Thanks for that. I was wondering if an indirect might make more sense.

Forgot to mention we have hard water. I don't know if that complicates things.

gschallert

If you're not going to treat the hard water (with softener) don't get a combi, get an indirect and a standard modcon with a 10:1 turndown.

If you're going to put a softener in go ahead and get the Navien combi but the 240 is much too large for the house you describe. The NCB-150E or NCB-180E should be just fine for heat and hot water based on what you describe. I did the math on one of these models for someone else not long ago to determine if the on demand hot water gpm would be sufficient. I'll find it and post the link back here.

What is the current heat loss of the house? Did anyone do one to see how big a boiler you actually need? I have similar home and similar zone sizes, same construction built same period and I just put in a 59K so I find it hard to believe you need 120K which is what the NCB 240 is rated for. If your heat loss is in the ballpark with mine, I'd recommend the NCB-180E since the low end is 14K and you get a boost with DHW GPM.

Paul48

I believe the general consensus is to not use water softened by conventional means, but use demineralized water. You have about 80k worth of emitters. You need to do a room by room heat loss calculation. If that 80k worth of emitter has to run at 180* to heat the house, there is no sense in putting a mod/con in. The house needs to be over-radiated, so the supply temperatures can be low and the return temps low enough to keep the unit condensing. Otherwise you might as well put a 3-pass boiler in, and call it a day. Where are you located in CT? We may be able to connect you with someone that will do the best job for you.

dm8877

We're in the southwest corner of CT (North Ridgefield). There is a water softener in place which we will augment or upgrade as it's letting a little too much iron through right now.

No one did any kind of heat loss assessment. I think they just eyeballed what is currently installed and suggested something equivalent.

NY_Rob

I'm not a heating professional, but I've been told and I've read that for a modern home (post WWII) using 2"x4" stud construction that a ballpark BTU heatloss estimate will come in anywhere between 15-20BTU's per SqFt.
Even using the high end 20BTU's per SqFt your 2,200 SqFt home would have a heatloss of 44K BTU's on design day.

Of course there are exceptions, but unless you have gaping holes and broken windows it should be in the ballpark.

Estimates, aside... you need to get a real heatloss analysis done to see if it would be advantageous to go with a mod-con in the first place. Mod-cons only start condensing with return water at or below 130F which means your supply water would be 150F or so with an average water temp of 140f. Can your current radiators (which were sized to operate on 180F supply temps) supply enough BTU's at 140F to heat your house on a cold day? That will only be answered by performing a room to room heatloss analysis.

gschallert

dm8877 said:

We're in the southwest corner of CT (North Ridgefield). There is a water softener in place which we will augment or upgrade as it's letting a little too much iron through right now.

No one did any kind of heat loss assessment. I think they just eyeballed what is currently installed and suggested something equivalent.

Properly softened water shouldn't pose a problem for the Navien combi so all you need to do is get a heat loss to pick the best size. The 150E is 60K input and the 180E is 80K. Based on my own experience with same home style and construction I suspect you'd be fine with the 150E and based on your DHW requirements that will also be sufficient GPM for your fixtures even in the winter's coldest inlet temps. You should still get or do a Manual J heat loss to confirm. I would also guess based on your baseboard zone lengths you are over-radiated (common as boilers and radiation were significantly oversized back then) which should allow you to condense a good chunk of the heating season. Eyeballing what is there if it's original or old will generally net you a massively oversize boiler. Not good. ;-)

Paul48

I believe @Bob Bona is in your area, if not, perhaps @Charlie from wmass would take the ride. Either would do a great job for you.

Bob Bona_4

Thanks for the nod, Paul. I'm in Ridgefield all the time, yesterday in fact. Would be happy to check it out, OP.

dm8877

That's great. I'll reach out. Many thanks everyone.

bob eck

First have your heating professional check and see if you can heat your home with low water temp water. If you have copper baseboard you might need to run high water temps most of the heating season to heat your house. If you need high water temps to heat the house then the condensing gas boiler will not achieve its highest efficiency. In that case I would look for a good cast iron gas boiler. 84% or 85% AFUE. If your system needs high temp you could also look at the Weil McLain GV90 cast iron boiler that has a external heat exchanger that helps to get the boiler up to 90% AFUE and low exhaust temps so this boiler can be vented with PVC pipe. Just add a good indirect water heater for domestic hot water.
There are many good condensing combi boilers / condensing boilers on the market. Pick one that the heating professional has been using and has installed many and make sure they have parts or the local wholesaler has the parts on their shelf and in stock so the heating professional can get the parts fast to get you back in heat and hot water fast. Of course your boiler will stop working on a cold winters day when you need heat the most. Do your home work.

Henry

The design temperature of house in CT is not below 0F. The coldest nights are usually 10 % of the heating season. So a mod-con with outdoor reset is worth it. Here where we look at -20F , we only have a maximum of 100 hours out of 1100 hours of boiler operation at maximum baseboard temperature operation. At those temperatures most mod-con are still near 87% efficiency. Micro management by modulation and outdoor reset are the key for energy efficiency!

kcopp

If the combustion chamber is the only issue why dump the whole boiler? Combustion chambers are relatively inexpensive compared to a new boiler....

Paul48

I think the best way for a homeowner to make an informed decision, is to wait, and work with a skilled contractor. Last I checked, propane, here in CT was very expensive.