Oversized combi for when we take over other unit?

HomeownerDustin

I'm getting quotes to convert our 40+ gas boiler (monoflow to hydronic convectors) and DHW tank to combi.
We have a 2 unit home, live in 1, and intend to take over the other in a couple years. I floated the idea to the plumber that ideally I'd like to just tie in all the zones to that one Combi unit when we take it over. He suggested we size up from Navien 130 to Navien 150.

Current needs in our unit (for the next 1-3 years): 1000 sq ft, 1 zone, 5 convectors, 1.5 bath, 1 kitchen, 1 laundry.

Needs after we combine: 2200 sq ft, 4 zones, mix of convectors and baseboard, 3.5 full bath, 1.5(?) kitchen, 1 laundry.

Other alternative is to keep the upstairs DHW tank, so the HE boiler would just be taking on 3 more zones.

**Main questions:**
-Is it a very bad idea to get the Navien 150H combi for our 1-3 year current needs (ie does the bigger size pose a problem)?
-Would the smaller Navien 130H combi be able to handle 3 more zones (an additional 1200 sq ft) easily?

jad3675

Do a heat loss calculation before you choose your boiler.

KC_Jones

As said do the heatloss, you will be in for a shock.

That said, rough estimate for heating. The 130 would probably be enough to do double the area you will have, after taking over the other unit, so sizing up would be kind of crazy IMHO.

This is the compromise of using a combi, it doesn't really 100% fit either job it's being asked to do. If you size it for heating, you might not have enough hot water, if you size it for hot water, you will have an oversized boiler for heating.

HomeownerDustin

I'm in the midst of the heat loss calc on slant/fin [update: 56,000 BTU/hr for my unit per slant/fin app, and 33,500 per the GBA article below]. Am I in for a shock of how oversized the system is? My current old HydroTherm boiler is rated for 70K btu/hr. I didn't see the BTU's written on the side of the other unit's Burham series 2.

Would I gain anything by doing an indirect HW tank in terms of efficiency and reliability? I'm certainly open to trading a bigger boiler for an indirect tank, if that even makes sense.

The biggest issue I'm dealing with (why I need to convert to mod/con combi +/- indirect tank) is my chimney liner is too small for 4 heating machines, and I need to get at least 2 out of there ASAP. Currently have a 5" liner for 2 boilers and 2 DHW tanks, and we likely can't fit larger than a 6". No evidence of back drafting in the year I've had the place, but it's undoubtedly undersized.


Lastly, the original question is really this: will having an oversized mod/con (150 vs 130) lead to short cycling, or is the issue moot on these advanced systems with a big TDR?

jad3675

HomeownerDustin said:

Would I gain anything by doing an indirect HW tank in terms of efficiency and reliability? I'm certainly open to trading a smaller boiler for an indirect tank, if that even makes sense.

With an indirect, you'd be able to size the boiler properly to the heat load of the structure. There's a few more moving parts with an indirect - zone valves/circulators, depending on how your plumb it.

John

Hot_water_fan

I'm in the midst of the heat loss calc on slant/fin. Am I in for a shock of how oversized the system is? My current old HydroTherm boiler is rated for 70K btu/hr. I didn't see the BTU's written on the side of the other unit's Burham series 2.

My money is on that 70k boiler being enough for both sides and an indirect. Check the bills using this method to be sure:

https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

A combi doesn't provide much other than space savings. An indirect will have high capacity but not much better efficiency compared to a regular tank.

Jon_blaney

Watch the Youtube videos about these Navians.

HomeownerDustin

Update on my unit's heat loss based on slant/fin app: 56,000 BTU/hr (set at -15 F outside and 68 F inside). My insulation is poor and will be somewhat addressed later this year. Per the GBA article, our BTU/hr is 33,500.

Space saving is definitely a benefit to why I'm doing this. The basement is small, and my workshop needs storage space.

Hot_water_fan

Update on my unit's heat loss based on slant/fin app: 56,000 BTU/hr (set at -15 F outside and 68 F inside). My insulation is poor and will be somewhat addressed later this year.

Is your existing boiler is 70,000 input or output?
If 70 input, its output only about 56,000 btu/hr, so it should be running non-stop when it's extremely cold.

Generally, online heat loss calculators aren't great and almost never come close to matching fuel usage (reality).

KC_Jones

-15 design temperature? Where are you located?

56,000 for 1,000 sq ft would be 56 btu/sq ft, while it's not done by square footage it's sometimes a good reality check and that number seems really high.

For example I live in a 100+ year old house, and mine is ~38 btu/sq ft with essentially no insulation and original single pane windows. I live in south central PA, that's why I ask where you live because that -15 design temp is really low, about 22° lower than me.

Jon_blaney

Buy what you need for today. Things change. Would you buy pants on sale that are two sizes to large because you know you are going to get fat. Life is two uncertain. Don't get caught up in the combi craze. Two little cast iron boilers running in tandem may be the way to go.

HomeownerDustin

I've made some edits above, and per the GBA article above, I'm calculating 33,500 BTU/hr for my 1000 sq ft unit based on Jan 2021 usage. We also strategically use electric heaters sometimes. My 99% degree day is 8.8F, and inputting -15F was crazy. I'll probably do a few more months' calculations to confirm that usage. I like the empirical method described by the article. It took more than 15 minutes the first time though.

I know the other unit is leakier than mine because they also have the poorly insulated roof, lots more leaky windows, and more exterior wall due to being on 2 stories. That said, I'd doubt the total heat loss for the house is greater than 90,000 BTU/hr. I'd hope the 130 BTU unit would be enough for mine.

I've always said I'd be happy with my current old HydroTherm conventional boiler that's rated at 79% efficiency. That thing just keeps on trucking, and the breakeven point for upgrading is 15-20 years, based on quotes I've seen.

But again, the impetus to convert to direct vent is driven by the chimney liner issue.

Hot_water_fan

I've made some edits above, and per the GBA article above, I'm calculating 33,500 BTU/hr for my 1000 sq ft unit based on Jan 2021 usage.

That sounds more reasonable. I think wall hung mod-con with low-boy indirect installed directly underneath makes the most sense. A 100k btu unit would turn down to just under 10k and cover both sides well. Even if you don't replace either boiler now, you can add an indirect and remove the two water heaters.

HomeownerDustin

Do you recommend a specific of mod con or some options? I see the forum generally isn't a fan of Navian. Last year, I did get a quote for a Viessmann Vitodens 100 that included new radiators and indirect tank, and it seems like a real bargain compared to quote i just got on the Navian. That said, the payoff if I assume saving $100/month on average was 20 years. Is this normal?

Hot_water_fan

That said, the payoff if I assume saving $100/month on average was 20 years. Is this normal?

This is the wrong value here: you're replacing a boiler to prevent back drafting right? How much is that worth to you?

The key is capturing all costs and all benefits, and making sure they're incremental. You can save high % of gas, but as to future prices that's a guess and you have to use enough gas that the % matters.

hot_rod

A 130 combi would get you 2.5- 3 gpm of DHW, plenty for most families unless you have a big soaking tub with a fast fill valve?

I think all the combis available now have at least a 10-1 turndown, so I doubt that is over sized for many, if any homes?
Not many cast or copper boilers available with a 10,000 BTU/ hr output for small jobs.