Condensing Combi VS Condensing with Indirect Tank

djwcmurphy

Hey everyone,

Looking for some help on which way to go. I'll try to make a long story short and I'll be happy to answer any questions.

I have roughly 3000 sqft house that is heated by hydronic baseboards in three zones, one for each floor. Currently they are powered by an electric boiler and the cost is killing us. We had a couple $1600 bills over the winter. We live in British Columbia, west coast of Canada and electricity is very expensive here. So, I am looking to switch to natural gas. Speaking of location, our winters are considered mild, at our coldest we get down to about 28F/-2C. Maybe a bit lower in the last couple years

My questions is whether I should look at a combi condensing boiler or a condensing boiler with an indirect tank. I should mention the bottom floor of the house is a suite, so there is a tenant in there who I have no control over when/how they use their hot water. They have a washer and then just domestic taps, no dishwasher.

A contractor I am talking to right now is suggesting a Navien combi condensing unit, which after a short look online I am very skeptical of. All I see are nightmare stories from both homeowners and contractors on the units themselves and the support from the company.

Any insight and advice would be greatly appreciated!

NY_Rob

Most here seem to prefer discrete boiler and indirect vs. combi units.

djwcmurphy

Discrete boiler? I've never heard that term

NY_Rob

^ Discrete: separate, distinct, individual, detached, unattached, disconnected, discontinuous,...

I'm sure you got the idea anyway


FWIW- if you do go with a mod-con and an indirect- get a larger indirect as tenants (at least the ones I've had) take 30min showers.

djwcmurphy

Thanks very much Rob. Can you (or anyone) elaborate on why that's the preferred setup?

hot_rod

In your case with an unknown or possibly changing HW demand in the rental, a tank would be my choice.

Not too big however, people tend to use as much HW as is available. especially when they and not paying directly for it. If you put in a small 30, they learn how much is available and live with that. if it is a 120, same theory, but more fuel cost for you.

Combos are nice for small footprint and when the DHW load is known, and know more that 3- 4 gpm at constant draw.

NY_Rob

I like the fact that the user can choose the size of the indirect they actually need vs. using the one-size-fits-all small internal combi tank.

My family is pretty conservative, we take short showers and there's only four of us. Accordingly we went with a 30 gal indirect and we've never run out of hot water yet.
You have a tenant- and they're on your boiler- you're not charging them for actual hot water/gas usage... they will take 30min showers. You need a larger indirect vs. me.... no problem because you can choose whatever size indirect you like.

The combi's (to me) look like they're more difficult to service as everything's crammed into one cabinet.

Leon82

The drawback of most Combi are the minimum btu fire rate is often very high compared to a high turndown standalone boiler.

If he math works out a Combi will work well.

Lots of small zones a boiler may be a better choice

hot_rod

Seems all the combos turn down 10-1 just like the same sized boiler. Although the smallest combi that will provide close to 3 gpm is oftem a 110 or 120,000. So 12,000 at low fire.

The ramp delay and anti cycling control functions on the Lochinvars can minimize cycling if a buffer is not possible.

djwcmurphy

Thanks for all the info guys.
My only other big question on this is what about the summer. The idea is great for the winter, but this would mean I'd have to have the boiler on all summer right?

Leon82

It would only fire when the tank dropped below the set temp.

flat_twin

I can't speak to multiple showers going on at once but our single shower cannot deplete the 40 gal indirect while it's recovering. It is essentially endless hot water. Summertime NG usage for us (two adults) is 9-10 therms per month.
We could not be happier with our modcon boiler and indirect WH after years of a fuel oil boiler and electric WH.
With the electricity usage you describe, your ROI for switching to NG should be relatively short.

NY_Rob

djwcmurphy said:

My only other big question on this is what about the summer. The idea is great for the winter, but this would mean I'd have to have the boiler on all summer right?

Regardless if you went with a combi or mod-com/indirect either would still have to powered up 24/7/365.

Neither have standing pilots, so zero gas consumption when they're not firing and in standby/sleep mode they only draw a few (under 10) watts.

Henry

Hi, I am in Montreal were it is somewhat much colder. Unless your house is of recent construction, your flow rates from your faucets would exceed the capacity of a small combo unit. Here the combo units are only trouble. HTP is available in BC. I highly recommend either a UFT 100 or if venting lenght is a problem, the EFT 110. The UFT has a 10 to 1 turndown. So far, all of our installs are: install and forget about a callback! I would also install a SSP 30 tank with a 5 gpm circulator. The tanks are S/S as well as the coil.
htproducts.com/literature/SuperStor_Pro_Brochure.pdf

NY_Rob

^ coil in the SS tanks are copper IRC, the tanks are stainless though.

njtommy

Navien is a Price point boiler for sure. Their are much nicer boilers and combis on the market just like every thing else is priced high to low end.

I have a Navien combi boiler at home and I've really like it. I've had no problems with it as we head into the 4th winter with the unit. I would definitely install another Navien unit I've had really good luck with them. Not just for boilers, but also tankless water heaters. What really matters is the installation of the boiler and making sure it is setup correctly along with being piped correctly.

Depending on your DHW demands and your heating loads a combi may work for you depending on your smallest zone like Stated above. Naviens are 13k or 18k low firing rate for combis. The boilers range from 8k-15k lowest firing rate.

Now for DHW if you need more then what the combi offers you could always add a Navien tankless water heater in and tie them in series with each other to handle the load. Not a big deal just an added cost.

Just a heads up as with any combi boiler and or tankless water heater it does take a little longer to get hot water to the fixtures. This can be annoying to many people.

djwcmurphy

Thanks again for all the responses guys!

I'm looking into an IBC boiler with indirect tank. Either the SL20 or SL14. However, my guy is quoting a 50gal tank. Seems like maybe a bit much? Me, my wife and our 2 year old daughter plus some yet to be determined tenants. No more than two adults though. Or is this a better safe than sorry situation with tenants when I have no control over their hot water usage.

hot_rod

Never had a customer complain about too much hot water If the price is not objectionable between the 30 and 50, I'd go with the 50.

Brewbeer

Need to determine heat loss to determine boiler size. Also need to determine output capacity of the installed baseboard radiation relative to the heat loss to figure out how hot the boiler needs to supply water to the baseboard emitters.

I have a Lochinvar WHN055 mod con boiler, and 40 gallon indirect. Family of 5, two adults, 3 kids turning into teenagers. Only time the hot water goes luke warm is simultaneous showers. Back to back showers have never been a problem.