Is an indirect HW tank on a conventioal boiler an efficient configuration?

Jim100Flower

There is not a good spot to vent a condensing boiler in my odd-shaped house so I’m planning to install a conventional natural gas hot water boiler instead. Is an indirect HW tank on a conventioal boiler an efficient configuration? It seems inefficient to fire up a 165K BTU boiler in the summer just to raise the temperature in the storage tank by a degree.

the_donut

It’d be hard to heat the water and house at that low output on design day if you used all your stored water.

SuperTech

I consider an indirect tank the best way to produce domestic hot water, unless you have the option to heat a storage tank with a plate exchanger.

It beats traditional direct fired tank heaters, especially with natural gas.

NY_Rob

CentroTherm has their "Innoflue Flex" system that will let you vent through your existing chimney if you want to go with a mod-con and have no sidewall venting options.




http://www.centrotherm.us.com/Products/InnoFlue-Flex/Chimney-Kits.aspx

KC_Jones

Jim100Flower said:

There is not a good spot to vent a condensing boiler in my odd-shaped house so I’m planning to install a conventional natural gas hot water boiler instead. Is an indirect HW tank on a conventioal boiler an efficient configuration? It seems inefficient to fire up a 165K BTU boiler in the summer just to raise the temperature in the storage tank by a degree.

I read one of your older posts stating this is a 3500 sq ft house.

165k is going to be oversized for sure. If you are worried about efficiency I would be worried about getting the boiler sized properly. No way you need one that huge. I take a WAG and say half that size would be more appropriate.

The only way to properly size the boiler is to have a room by room heat loss (Manual J) calculation performed. That or you can use one of the free programs available, Slant Fin has a nice one that works on a tablet or smart phone.

Robert O'Brien

What NY Rob said

gschallert

Robert O'Brien said:

What NY Rob said

+1

There is more than one modcon mfg that allows for existing chimney flues to be used as a chase where sidewall venting is not feasible. My in-laws replaced a CI boiler in Fall 2016 with Vitodens vented up existing chimney. In answer to your question about pairing indirect with CI, that's what I have and though it's not as efficient as modcon I still prefer it to a stand alone fired DHW tank. Boiler fires about twice a day to maintain the tank temp when not drawing. I have extra insulation on already well insulated SS indirect.

bob eck

https://youtu.be/zo2bFE_Om1Y

Video on installing Innoflue chimney liner kit.
Must run Innoflue hard pipe from appliance to the chimney liner kit.
No other appliance can be vented into the chimney. Old chimney is just being used as a chase.

NY_Rob

Looks pretty straight forward... solves a lot of code related issues when sidewall venting your mod-con.

Wonder how that plastic/rubber chimney cap will hold up over time exposed to the sun, weather, etc...?
I've had rubber seal fail on the plumbing stack flashing on the roof in several locations.

flat_twin

Temperature diff on our 38 gallon indirect is 9 or 10 degrees. It came with a basic Honeywell aqua stat. I can make several short draws of hot water before the boiler goes to dhw mode. It probably only runs twice a day just to make up for standby loss.

Wellness

As for efficiency in the summer, why not get an indirect with electric backup. Not only do you not have to turn on the boiler in warm weather but you don't have to worry about the gas water heater flue. That's what I did.

Zman

By all means the indirect is an efficient choice.
Consider your options:
If you have venting issues a demand heater won't work.
Electric is expensive
Stand alone traditional gas heaters are < 70% efficient and have significant loss through vent.

Your indirect will run at the boiler efficiency ~84% and if installed and insulated correctly will have minimal standby loss.

The boiler will not fire to raise the temp 1 degree. The differential should be set more like 10 degrees.

As mentioned above, don't even think about a single stage 165k boiler for 3,500 sq/ft. A correctly sized boiler will likely be 1/2 that. Either go 2 stage atmospheric or line the flue and go mod con.

It sounds like your heat loss calcs are coming out high. Consider double checking assumptions and/ or do a degree day calc using utility bills.

Jim100Flower

@Zman, thanks for suggesting the 2 stage boiler. I did not know they made them. Sounds more efficient.
They're supposed to do a heat loss calculation. 165K is my current boiler.
I just replaced my standalone gas HW last year so I need to be convinced the indirect will be a big savings on the gas bill.
I don't want to vent through the (lined) chimney because there is a nice copper architectural feature capping the side-by side stacks for the heat/HW and the fireplace that would have to be removed.
And I'm not convinced the approximately 10-15% savings on the gas bill would offset the ModCon's higher install cost, higher annual maintenance, and higher repair costs, even if I could vent it.

Zman

I am not sure I would toss out a brand new gas heater to put an indirect on a conventional boiler.Maybe just stub for future indirect.

To help with boiler sizing, why not just figure out how much of you are using now? You could just clock your meter for a day next time you have a cold snap.

Jim100Flower

@Zman, I will check the meter for a day next time we get a cold snap. Just for reference, I used 333 therms, actual, in 29 days during December-January. That averages approximately 48,000 BTUH. But that average does not tell me peak demand and it wasn’t the coldest January on record. During the cold snap my boiler kept up with demand and seemed to be running most of the time, but I wasn’t watching it.

hot_rod

> @Wellness said:
> As for efficiency in the summer, why not get an indirect with electric backup. Not only do you not have to turn on the boiler in warm weather but you don't have to worry about the gas water heater flue. That's what I did.

And you could add some PV to offset electrical cost. There are some direct PV to DHW tank systems like the Sun Bandit

Solar tax credit probably applies

D107

A number of contractors on the Wall have stated they use regular polypropylene inside chimneys to vent to the roof. What are the advantages of inno-flue? Cost?

Eastman

@Jim100Flower

Do you still have six separate zones of baseboard?

Jim100Flower

@Eastman, yes.

Zman

David107 said:

A number of contractors on the Wall have stated they use regular polypropylene PVC inside chimneys to vent to the roof. What are the advantages of inno-flue? Cost?

Poly pro and PVC are not the same at all. PVC was designed for water, Poly pro for boiler vents. Which one should you use?

In fairness to the OP, this should probably be a separate thread...

D107

@Zman not trying to start a separate thread. Got my terminology mixed up. Since the Inno-flue was mentioned on this thread I was asking why it was superior to the non-Innoflue Polypropylene that many Wallies say they use.

Zman

If you are going to line an existing flue with Poly, you really need to go with flex. There are several brands making flex poly.here are some differences in connections, it usually come down to local stocking dealers.

rbeck

The flexible is best. If you use ridged pipe you are supposed to have inspection doors at the joints. The second
thing applies to all plastic type venting. You cannot put plastic in a chimney where there is a second chimney beside the one you are using. You would be required to use SS in that case.