Water Heater or Boiler

Gordy

TDR is the ratio the boiler can step down firing rates. You won't always need full output. The higher the TDR the better. 80 k boiler with 10:1 TDR can fire as low as 8 k.

The tank on the indirect will be same size as a tank style conventional WH it's heated by the boiler with an internal heat exchanger. An indirect will have the efficiency of the boiler in the mid 80's % while making DHW and stand by losses are lower than a conventional gas WH.

steamedchicago

giceman1337 said:

Thanks Gordy. So if I am understanding the indirect tank doesn't need to be as big as normal domestic water heater because the boiler will be able to continuously reheat the water within the indirect water heater?

Is 10:1 and 5:1 TDR the modulation efficiency of the mod/con? I assume higher is better.

Remember that a 50 gallon conventional gas water heater only has an input of about 40,000 BTU hour. Most houses end up with a bigger boiler than that to deal with heat loss.

TDR is "turndown ratio". Modulating boilers modulate the burner, to more closely match the current load. 10:1 means a a 100,000 BTU/hr boiler can output as little as 10,000 BTU/hr; 15:1 would mean 6,666 BTU/hr min output. Ceteris paribus, higher TDR is better.

mikeg2015

Boiler for Hydronic + Water Heater for Domestic

I'll switch to indirect when I replace my boiler. But I'm on steam. Have an electric tank now.

giceman1337

Thanks guys. All your help is greatly appreciated.

I think I want approximately 4 GPM.

I did a little research and planning on my DHW needs. All of my shower heads are <= 2 GPM. I'd like to be able to run 2 showers simultaneously. We typically wash our clothes with cold water, and we can control when we run the dishwasher.

So a 40 Gallon indirect tank would supply at least 10 minutes of 2x showers. I think the output temps are usually 125F and would run the shower at 104F, so technically only using ~85% hot water, so the 10 minutes becomes 12 minutes. That should be more than enough.

Next question is recovery time and/or sustaining hot water.

I found a <a href="https://us.v-cdn.net/5021738/uploads/editor/v1/ycrbhbygrbnk.png">source that says my incoming water is 42F.

My tank water would be 125F or would I have it higher for input into the indirect water heater?

I found this calculation for Tankless systems (I assume boilers are the same):

System Delivered BTU = 500 x GPM x System Water Temperature Change

GPM = 4 System Water Temperature Change = 125 - 42 = 83 BTU = 500 x 4 x 83 = 166K

I don't know if I need to account for efficiencies to account for.

Gordy

With an indirect you can store at 140-150, and Mix down to 120 it extends usage, and keeps legionella in check.

jumper

When boiler goes down it's nice to still have DHW.

giceman1337

Jumper, are you implying have a separate water heater, or just having sufficient water in the indirect tank?

Gordy

Either one will give you a reserve. However an indirects is made more efficiently through the use of a boiler. Low to mid 80’s percent verses a standard tank style gas WH at maybe upper 60’s. Electric Trumps however they are terrible at recovery.

NY_Rob

The nice thing about a mod-con and indirect is that you throw the full output of the boiler at the indirect so recovery is fast. My 80K BTU mod-con satisfies the DHW call when someone is still taking a shower sometimes. I keep my indirect at 135F FWIW.

giceman1337

Thanks guys.

I'm leaning towards an 80K BTU boiler. My plumber has installed quite a few Navien NHB boilers. I'm looking at the NHB-80 which has 10:1 TDR.

I'm still not sure on what indirect water heater to go with. I've been looking at NTP SuperStor, but the cost was relatively high and I'm not sure what key factors to look at on the indirect system.

jumper

giceman1337 said:

Jumper, are you implying have a separate water heater, or just having sufficient water in the indirect tank?

Separate water heater. I can fix or replace it quickly. I don't care about DHW efficiency. Figure DHW cost is about a dollar a day.

This opinion is from somebody who tried a variety of methods to reduce fuel costs for multi-residential buildings.

giceman1337

@Jumper, is your claim that Boilers break down for often than a typical gas water heater? Do you have examples or data that backs this up?

Everything I've been reading the new mod/con boilers are extremely efficient and decently reliable. There may be some routine maintenance depending on water quality.

What's the history and experience of others on the reliability of boilers vs gas water heater?

hot_rod

Combo Boiler for Hydronic and Domestic

not much to go wrong with a tank type WH, although they have gotten more tech these days. Plenty of water heaters run 10 years or more without any attention.
I doubt many high efficiency boilers can make that claim. The warranty may be based on occasional check and combustion analysis. If you pay a service tech to perform a yearly check, in 3 years or more you paid for a tank type wH.
There are not many inexpensive parts on mod cons


When a mod con goes to high fire to cover DHW, you probably are not running the 90% plus efficiency anymore.

To Jumpers point, the average family spends around 300 per year on DHW.

Rich_49

Combo Water Heater w/ Tank for Hydronic and Domestic

I have 2 words for you . Some may disagree though , too bad . I am also attaching performance results from a new home install just finishing up second full year operation .

VERSA HYDRO ( HTP)

Worth looking into the possibility dependent on home construction but in any case will give you 90K central heating at about 160* SWT requirement .

The attached spread sheets are from a 4000 sf well constructed home in Michigan . All radiant , 41000 load with all zones calling at 7* design . Oh yeah it's 14 zones and never short cycles , imagine that . The idea that one must use an H stamp device in what is increasingly becoming low temp systems still amazes me , especially from the esteemed company here . In short , yes , you can use a water heater if designed properly , without any danger . Probably the lowest maintenance system you'll encounter also due to the HTP tanked units HX design

You could also easily build a discontinued HTP that was called the Versa Flame using an HTP Pioneer ( low maintenance / hx ) with a flat plate heat exchanger for DHW with a flow switch . All great options .

BobC

@jumper I agree completely,if your hotwater needs are modest a gas hot water tank makes sense. When I replaced my boiler in 2012 I considered an indirect but promptly backed down when I realized what they cost.

I use 6 therms a month for hot water (averaged out over a decade or more) , how many centuries would an indirect have to last to make it worth my while. My 10 year tank is 12 years and counting and I;m going to replace the anode rod again this week, this will be #4 for that tank.

Bob

Gordy

BobC said:

@jumper I agree completely,if your hotwater needs are modest a gas hot water tank makes sense. When I replaced my boiler in 2012 I considered an indirect but promptly backed down when I realized what they cost.

I use 6 therms a month for hot water (averaged out over a decade or more) , how many centuries would an indirect have to last to make it worth my while. My 10 year tank is 12 years and counting and I;m going to replace the anode rod again this week, this will be #4 for that tank.

Bob

6 therms a month! you must have a luxurious Roman soaking tub sipping champagne, eating strawberries, and chocolate in candle light

Gordy

If you sift through this years posts alone many navies woes. However one can’t say if they are owner, or installer created.

BobC

To be honest the 6 therms includes whatever I use for cooking.

Bob

giceman1337

Maybe I'm slanted because I have a crappy under-powered electric water heater. But we have a decently large house with 4 bathrooms and 4 people to supply DHW. I am sick of being the 3rd or 4th person taking a shower and getting mostly frigid water. It's been so bad we have had to basically have planned shower schedules so we don't deplete the water heater.

I liked the possibility of the boiler providing nearly unlimited hot water as well as servicing my radiant heat.

I'm certainly not opposed to using separate units for DHW and radiant heat. The water heater would take up about the same space as the indirect heater. I will need an extra vent line out the side of my house, unless it is possible to common vent the boiler and water heater in the same vent.

Rich_49

Combo Water Heater w/ Tank for Hydronic and Domestic

giceman1337 said:

Maybe I'm slanted because I have a crappy under-powered electric water heater. But we have a decently large house with 4 bathrooms and 4 people to supply DHW. I am sick of being the 3rd or 4th person taking a shower and getting mostly frigid water. It's been so bad we have had to basically have planned shower schedules so we don't deplete the water heater.

I liked the possibility of the boiler providing nearly unlimited hot water as well as servicing my radiant heat.

I'm certainly not opposed to using separate units for DHW and radiant heat. The water heater would take up about the same space as the indirect heater. I will need an extra vent line out the side of my house, unless it is possible to common vent the boiler and water heater in the same vent.

Please see my last post Giceman . You can have all that with either a boiler or a water heater pulling the duty . just not any water heater though

Gordy

An electric water heater is truly the bottom of the barrel. Next to boiling your own water for the next draw.

Take a serious look at the HTP versa hydro @Rich mentioned in his post. There are many tools in the tool box. Don’t use a crescent wrench when a properly sized open end, or socket is available.

jumper

giceman1337 said:

Maybe I'm slanted because I have a crappy under-powered electric water heater.

Not so much underpowered as too small. A 300 gallon electric doesn't run out.

hot_rod

Combo Boiler for Hydronic and Domestic

When you have a large demand you either store enough to cover it, called a dump load. Or generate it at the rate you use, the tankless approach.

If 4 people shower back to back that is where you need either HP to keep up or have enough stored for dump load.

In the commercial DHW business it's called a Hunter Curve, or Revised Hunter Curve. Here are two examples from a recent Caleffi webinar. See how the load profile differs between a school and hotel.

Which is you family closest to

Lochinvar has a nice calculator at their website showing how you can extend the draw by increasing the tank temperature and mixing down to say 120, Gordy mentioned that already.

It really comes down to what you want and what you have to spend getting it.

I've found most people use all the DHW you give them, or don't.