Asking for advice for combi boiler replacement and hydronic system upgrades

Seasoned_Amateur

New user with a new post! I installed my hydronic system a little over 15 years ago. I purchased everything from a radiant heat supply store near me and used their design. I did research things at the time, but now I know more and would do things differently, as is always the case.

For the last couple months while working from my home office, I've heard someone slamming their car door outside, only to see no one out there. Then, last week, when I was standing outside, I heard the sound coming from my boiler vent. I thought, I should get a tech over hear to analyze and adjust my combustion. Then, I realized that my Laars HT330 is over 15 years old and probably reaching the end of its lifespan. I've also started to notice that it's DHW flow detection is getting less sensitive. And, it's DHW output (3.6 gpm@80deg heat rise) has always been less than we've needed for our 2 showers. I decided that our money would be better spent on replacing, but feel free to tell me that I'm a fool.

I've been looking at different Combis to replace this unit. You'll see from the photo of my current set up that space is tight, though all minimum dimensions are respected. I like the Rinnai I-Series for it's good DHW output, quality build, concentric vent port, and similar location of inputs as my current set up. One of the things I noticed when reading the specs and instructions is that it has 1" CH connections and that their optional primary-secondary heating kit connects to a 1-1/4" manifold. After doing some reading, I realize I would probably need to upgrade my primary loop to 1" from the existing 3/4". (My Laars has 3/4" connections for CH and may have benefitted from a 1" primary, but it has worked great for 15 years.) My current primary loop does not have a manifold, but is just a loop. I'm assuming I would just purchase this Rinnai kit or make my own to fit my space better. I made a schematic of my CH system as an attached PDF. All pipes are 3/4" except for the 1/2" loops to staple up zones and radiators.

I'd love to have the 5.1 gpm of DHW that the Rinnai i120C offers, but that will mean a very oversized combi (120k BTU) for my heat load which I've calculated at about 50k BTUs. The i060C and i090C are a better match for heating, but only have 4.1 gpm of DHW. I also realize that maybe Rinnai has come out with new models in the I-Series (I-Plus) that are similar spec, but with updated features that I should be considering. My current Laars is 126k BTU and can't do CH and DHW at the same time, like this Rinnai can. So, maybe I've been using a 126k BTU boiler for heat this whole time?

So, my questions are:

Should I pipe a new 1" primary loop given my configuration? (You're gonna say "Definitely!)

Do you have a recommendation on which size boiler to purchase or should I be looking elsewhere?

Thank you in advance!

Laars Hydronic system.pdf

hot_rod

You alway size to the larger load, in your case it is the DHW production you want. The boiler you are looking at derates to 15,000 in heating mode, so it is not oversized at all. Most mod cons can be locked at that heating output.

Actually you pick up some efficiency by firing a large mod con at low firing rate. You have a large hx surface area exposed to a small burner flame and more condensation.

Most tankless water heaters and combi are rated at a 77° temperature rise so make sure you are comparing apples to apples.

Confusing is that i120C is actually a 199,000 in dhw mode?

Seems like is should be a i199C :)

Screenshot 2024-11-22 at 4.45.58 PM.png

Seasoned_Amateur

Very helpful @hot_rod ! These model numbers get really hard to keep track of. I think I would be able to keep it more straight if they named them after dog breeds, instead of numbers and letters.