Thoughts about condensing boiler as back-up?

rickster05

Considering adding a 'cb' as a back-up to my Tarm wood boiler. Going to make 2-3/4 helix's to add to storage tank in order to only charge tank, not circulate hot water through heating zones. I then pull from the storage tank in order to satisfy heating zones. I do have hard water with high manganese. My plan is to make this a closed loop with water that does not have a scale/mineral load. That said, once it flushed/filled and blead, I will have an auto feed with my well water although I don't expect it will use any.

Will use a Tekmar radiant thermostate to set high and low limits on 'cb'. I like the idea of a 'cb' due to efficiency and ease of venting. Love any thoughts regarding my plan! Especially any 'cb' recommendations regarding service repairs etc.

Zman

What kind of heat system do you have? What temp water is it designed for?
Propane or Natural gas?

rickster05

Have a Tarm wood boiler. Would install a LP condensing boiler with design temp between 160-180 degrees. Only need it to keep 600 gal thermal tank in that range for zones to be satisfied when called on for heat.

Zman

I don't think a mod con boiler would make much sense for you. The big advantages of the mod con is that it will have improved efficiency with low return water temps and it will modulate to meet the load to prevent short cycling. Your application is hi temp and high mass. A properly sized conventional boiler would be just as efficient.

rickster05

I understand what you are saying, just not sure why you are saying high mass? The 2 helix coils would be 180 ft in an insulated tank where the delta would only be 20 degrees? I principle only burn wood but would like the convenience of going away and not having to have someone light our boiler every day?

rickster05

Conversely I could have the 'cb' run in the range of 120-140 degrees thereby allowing it to function the way it is designed. The circs/cb boiler would be running longer but would be more efficient.

My biggest problem is that the is just no way for me to vent a conventional boiler given the stoves/fireplaces I have in my chimney and I like the thought of a 'cb' due to the reduced exhaust temps.

Zman

I think you effectively have a high mass system because of all the storage water you have in the tank (unlikely to short cycle heating that much water).
If you are thinking condensing because it is the only way to vent, there is nothing wrong with that. I wouldn't do it for efficiency.
You might find that you can run in condensing mode on the shoulder seasons and save some $.

rickster05

I now understand what you are saying. On the flip side, I guess I could take the spring and summer season off from lighting the boiler once a week for domestic hot water, and still have the convenience of going away in the colder months just not with increased efficiency...

Would you have any recommendations regarding manufacturers?

psb75

Don't use the cb to heat the storage water. Use it to ONLY heat the house load. Then it will condense more.

rickster05

Alright. It’s now been 3 years later and I’m back to considering an add on boiler. A cb boiler is my only venting option. Water is high in minerals, so I’m still considering adding a closed loop to heat storage tank only from 120-140 degrees. I will only add conditioned water after initial install flush. I don’t see the system requiring additional water since it will be a closed loop. Thoughts?

Jamie Hall

If a condensing boiler were your only venting option -- which I very much doubt -- there's nothing wrong with using one. Maintenance will be higher than a conventional cast iron boiler using side wall venting (lots of options) and capital cost will be higher, both in terms of first cost and shorter life, and you won't get either condensing or increased efficiency. But... if you want to say you have a mod/con boiler, go for it.

Solid_Fuel_Man

I'm going to have to disagree with Jamie on this one...that doesnt happen often! I have a very similar setup in my own home.

I have a mod/con as a backup and couldn't be happier. I had oil as backup before and the low usage caused more problems than it was worth.

Now I can send 70 degree (or colder) return water directly to the boiler if I want to preheat my wood boiler, or heat storage directly with the mod/con.

The condensing boiler stays clean, and gives no trouble from sitting idle for long periods.

hot_rod

There are a number of ways to pipe a system like that. Ideally you pull heat and DHW from the "tank"

A few ideas here.
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_10_0.pdf

rickster05

My set up is similar to these. I have 2 coils from the boiler, heating the tank and supplying the zones and one coil in the tank for domestic hot water. There’s space for another coil for the cb. The cb will be more expensive, but will allow for easier venting, ie. lower exhaust temps. Our home is tucked into the ground, intentionally. Doesn’t allow for a power vented unit (that I’m aware of anyway). If someone knows of one please tell me.

hot_rod

There are direct and power vented cast boilers available. I suspect price would be in line with a mod con, considering the flue piping required.
I put in some Crown, Aruba I think they were called, vented with rated stainless

EBEBRATT-Ed

Yes as @hot_rod mentioned the Weil McLain CGi is a great boiler vents sidewall with SS

hot_rod

In some case the modulation alone is worth the cost between a mod con and single speed boiler. Most all mod cons have ORD option data logging and much more. It may be worth the extra $$ and service requirements , if those functions interest you.

If it charges the buffer directly, the modulation may not be a benefit. if you pipe it direct to load, and it doesn't load the buffer except for cycling maybe the cast boiler is a better option. Price both and see.

Loading a large buffer with a fossil fueled boiler doesn't make a lot of sense, unless DHW is only produced from the tank? That is why a small DHW backup may be used for times when the buffer is off line.

A mod coin or combi piped downstream, direct to load would be the most efficient option for backup.

rickster05

I’d probably have a 20 ft run with 3 elbows in order to vent a conventional boiler. The benefit would be, I could plumb directly to the supply and return and for go another coil, water feed, spirovent etc. etc.

My Tarm boiler is 120BTU. If I add a conventional as a secondary, should I match that rating? I’m thinking ~ 80BTU would be fine. Thoughts?

rickster05

There’s no way for me to plumb a mod com or combi direct to supply and return as I only use cast iron radiators and my water is really high in minerals. From what I’ve researched, that’s asking for a disaster. So it would be a closed loop with a coil maintaining the storage tank for heat and domestic water.

GroundUp

There is absolutely no point in heating 600 gallons of storage with a fossil fueled boiler. The Tarm needs it, but it's a complete waste of fuel for the gas boiler. Condensing or not, bypass the tank while the Tarm is cold and heat the loads directly from the gas boiler. A 3 way Taco Sentry zone valve wired to an aquastat on the tank will do exactly as you need, and fire the gas boiler (piped as primary/secondary) when the tank is below a certain temperature. You can accomplish the DHW side of things with a simple plate HX if you're worried about the combi, just have to open your mind a little bit and get creative. Hydronics is incredibly versatile.

hot_rod

rickster05 said:

There’s no way for me to plumb a mod com or combi direct to supply and return as I only use cast iron radiators and my water is really high in minerals. From what I’ve researched, that’s asking for a disaster. So it would be a closed loop with a coil maintaining the storage tank for heat and domestic water.

You can pipe directly into a closed loop system, it is done all the time. Usually you only fill the system once so minerals are not being added constantly.

If the water is excessively hard, over 10 gpg or high TDS, you can fix that. Either fill the system through a demineralizer, or buy filtered water, typically RO water, or you can add boiler treatment chemicals which keep minerals in suspension.

Boilers have been, and still are filled with standard, hard tap water, and they run for decades. But it is fairly easy to "fix" water before it goes into the system, in fact in many European countries it is required you meet a fill water spec.

I think the cost to address the water would far outweigh the cost of heating a large 600 buffer with your gas fired boiler. I would guess the closed loop part of the system holds less than 100 gallons? So if you bought filtered water at a buck a gallon....

Piping the gas fired boiler direct eliminates that buffer tank loss and allows for almost instant start up, no buffer lag or need to maintain that 600 gallons, hot.