Indirect WH vs Gas Energy Comparison

Steve_35

trying to come up with some way to quantify the savings realized from using an indirect fired water heater vs a gas fired. Has anyone found any resources that would help with this?

It seems if a typical gas WH only retains about 60% of the energy it consumes an indirect with a standby loss of 1/2 degree per hour connected to a 85% boiler should use 2/3 or less of the energy a gas fired WH uses.

Thanks.

ED_27

Indirect v gas

One thing that stands out for me, is that new indirect Mega-stor has a lifetime warranty for the homeowner. How long does a gas fired water heater last ???

Steve_35

There are several ways in which an indirect fired water heater is better and more economical to operate than a gas fired. It's just quantifying them that's tough.

An indirect has a much lower standby loss. Generally less than 1 degree an hour. Some down to 1/2 a degree an hour. A gas fired water heater has cool air moving through it most of the time. That air is warmed by the water you just paid to heat and then the warm air rises out the chimney.

A gas water heater will lose efficiency faster over time due to mineral build up in the base of the water heater. This happens to be the spot where the greatest heat transfer should be taking place.

A gas water heater is less efficient when firing than most modern boilers are.

You have two pieces of equipment to maintain rather than one.

The recovery rate is better with an indirect than a gas fired.

Constantin

This is all true...

... yet many people shy away from indirect water heaters because of the added intial expense. For the price of many a "life-time warranty" indirect, you could buy a number of inefficient gas water heaters, run them ad nauseum in a residential setting, and still come out ahead, economically. Never mind the fact that Americans move every 7 years on average and therefore usually don't have long time horizons.

Further distinction has to be made between indirects with gobs of HX surface (Viessmann Vitocell 300's, Monitors stuff, Phase III from Triangle Tube, etc.) and those with smaller ones (some Superstors, Amtrols, etc.) A small HX will make little difference with a small boiler, as the recovery will be limited solely by the boiler. Once you get into higher-capacity boilers, the recovery rate can be limited by the HX inside the indirect.

I'd always go for a indirect with large, smooth HX's because they allow the heat transfer to occur with greater efficiency (i.e. lower ΔT required) and scale will have a more difficult time clinging to the HX. However, if your water is soft and scale is never an issue in your area, then water heaters with "rippled" HX's might work just as well.

However, the reason that you sell an indirect to most people is likely to be one of convenience. It reduces the complexity of their home, it may recover better, and it'll almost certainly be more efficient, but those are usually nice-to-have's not must haves when compared to the trophy kitchen, car, or entryway.

Steve_35

> ... yet many people shy away from indirect water

> heaters because of the added intial expense. For

> the price of many a "life-time warranty"

> indirect, you could buy a number of inefficient

> gas water heaters, run them ad nauseum in a

> residential setting, and still come out ahead,

> economically.


This may be true if you install the water heaters yourself, but if you pay to have them installed it's about 2 gas water heaters to an indicrect.

Constantin

You're absolutely right...

... yet how long does a standard gas water heater last in relation to the length a occupant is in a house? IIRC, the average life of a standard US water heater is something like 10+ years (varies by region, of course). By the time it comes to its end of life, the house will have changed hands 2+ times, on average.

Don't get me wrong, I installed an indirect in my house. However, I doubt that I'll be able to justify the &Delta$ in an NPV calculation that ONLY compares installed costs to installing a direct vent water heater. I did it for the CO, the reduced service fees, higher efficiency, the solar heating system, and the other operational costs that come with having yet another combusting appliance in the house.

Yet, many homeowners don't consider operational/life cycle costs... judging from some of the stories I have read here, you're lucky if they even consider having someone come in for annual maintenance of their equipment!

hr

I'd challange you on this point

"I'd always go for a indirect with large, smooth HX's because they allow the heat transfer to occur with greater efficiency (i.e. lower

Ted Robinson

Gas HW heater efficiency

When I measured the flue temperature of my gas unit, I was not happy to see it near 300 degrees. With 140 degree exiting hot water, you can seen they are not highly efficient!

Some day I may get some quotes to see what an 'indirect' gas fired system would cost to install.

Constantin

... where is the disagreement?

There is no doubt that the finning does a great deal to improve the HX surface area w/o adding a lot of material. The below first hour recovery results speak somewhat for themselves... (200°F boiler water, 90°F rise)

• HTP Superstor (finned tube) - 60 Gallon Tank - 298 Gallons/hr - 198kBTU boiler
  
• TT Phase III (tank in tank) - 60 Gallon Tank - 410 Gallons/hr - 270kBTU boiler
  
• Vitocell 300 (looped tube) - 53 Gallon Tank - 285 Gallons/hr - 215kBTU boiler

This is why I stated that finned HX's work great in areas w/soft water. Elsewhere, they may plug up in no time and non-finned HXs may be superior. Also, these performance numbers are on the agressive side. I would like a look at first hour recovery rates with boiler loop water temps below 140°F to better simulate a condensing/energy efficient boiler environment.

Under those circumstances, I would imagine that tanks with HXs that penetrate deep into the tank would do better than tanks that have a small coil at the bottom and which mainly rely on generating strong convective currents via huge ΔT's between the coil and the interior water.

Larry Weingarten

a thought

A good gas heater has an energy factor (EF) of about .62. A good electric heater will be about .90. Electrics and indirects have similar heat loss, no? If you take that .90 and multiply it by the combustion efficiency of your boiler, say .85, you get .765, or roughly 76% of the energy that was in the fuel made it out of the tank as hot water. This says nothing of the energy losses in the plumbing... that's another thread. A more efficient boiler would give a better percentage. Another thing to consider is the energy penalty of pump and control which would bring that hypothetical .76 down a bit.

Yours, Larry

Ron Schroeder

standy by loss

Another lost for the indirect is the standby loss of the boiler. My smart 40 phase III seems to run the boiler up to about 145 degrees F. When the boiler is standing by, it cools down to about 100 degrees in about 6 hours. 10 gallons (~83 lbs) of water and ~300 lbs of cast iron ... (1*83)+.11*300)*(145-100)=5220 btus. That is enough energy to heat about 10 gallons of water from 60 to 120, or one five minute shower.

Steve_35

Part of the year that would be a consideration

The difference in this case is this is an atmospheric boiler with an automatic damper so the standby loss goes into the buiding envelope. With a gas fired water heater most of the loss goes up the chimney.

Constantin

That's an interesting point...

... in the winter time it's certainly no big deal to add a little heat to the basement... thinly-insulated boilers make for pretty good small radiators, I guess. However, I bet there is a significant flue loss unless there is a damper and even with a damper, there is going to be leakage.

In the summertime, the heat is wasted, and worse, it may need to be extracted from the building via AC. That's when thinly-insulated indirects and the boilers feeding them are going to start costing you money... ditto for conventional gas water heaters, of course. Worse, conventional water heaters usually don't have dampers or insulated flues, so a lot of heat will end up in the house.

For instance, if we take the above example of the boiler losing 5000 BTU to the house every day and assume a 10 SEER air conditioner, that's 500 Watts, or 50 cents per day (@ $0.10/kWh). Multiply this by a 100 day summer season, and you have yourself an extra 50 bucks of cooling per year that may not have been necessary if the boiler wouldn't have such high standby losses.

Never mind the other issues such as make-up air, infiltration, etc. Some day, my dream will come true and I'll be able to use my Vitola in a sealed combustion set-up. In the meantime, I'm treating my mechanical room as if it was part of the outdoors, i.e. I'm sealing the heck out of it.

Bruce M.

You are all on the wrong track

A gas fired on-demand water heater like a Noritz or Rinnai is the way to go. You have to run a boiler in the non-heating season in order to keep your DHW tank warm. You very often have to oversize the boiler if you have a DHW load. The on-demand is usually direct vented and in warmer climates it is mounted outside.

leo g_13

in actuality

that is one of my selling points. By having the indirect tagged along with the boiler, the boiler gets continous use throughout the year, which to me makes better sense then having it sitting, gathering dust and pumps getting stuck, etc. Plus the houses nowadays that we install our systems into, would need 2, possibly 3 on-demands just to keep up with the master soaker, plus all of the other baths, laundries, nanny suites etc.

I just feel, that having 80 gallons of heated water, in a very well insulated indirect, ready to go, is the most customer satisfying.

Leo G

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Steve_35

We did a new home several years ago and had a problem with the relief valve dumping water every time the boiler fired. This was in July and no one was living in the home at the time. The boiler dumped water every other day. The only demand was the DHW. Every other day it made a call for heat. So the standby loss in the envelope due to the boiler was over two days instead of one.

Besides, in upstate NY we only have about 10 cooling days a year.

Steve_35

We've yet to oversize a boiler to account for an indirect

Tankless heaters are fien for some people. Many though don't want to change their lifestyles to the degree necessary with a tankless heater.

In addition, tankless heaters may require you to dump more water down the drain before you have hot water at the tap.

Steve_35

My thought exactly, Leo.

I'd rather keep the system operating year round. And have a single piece of equipment to maintain.

hr

Best case indirect partnering

would be a low mass, high efficiency condensing boiler to drive the indirect.

Much less standby loss in an Ultra or Munchkin than a 300 lb cast iron hulk

Boiler room summer time overheating issues go away with low mass low temperature boilers also. Of course it never hurts to insulate the pipes in the mechanical room, especially the boiler loop and piping to the indirect as this is a 365 day loss!

Also with a good, high exchange surface indirect, DHW could be produced with lower (than 180- 200° F) boiler temperatures.

Again I feel the Phase 3 or Ergomax could provide reasonable DHW with 150- 160 boiler temperatures.

It's time for all the indirect manufactures to lower their tables to show output with boiler temperatures starting at 150 or 160 to be in step with the condensing boiler market.

I realize most if not all condensors will step up to 180 or so the recover DHW quickly. But how quick is quick enough?

I have the time to allow my DHW generation to happen with 150°! But ONLY after the solar has it's up to bat

hot rod

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Constantin

It all depends, IMHO...

.... instantaneous water heaters have their place, so do indirects.

An indirect is a great insurance policy, allowing you to cover large dump loads like jacuzzis, Amazon-like showers, etc. with relatively small boilers. The recovery may be slow, but you end up with a heating plant that is sized for the heat loss, not the hot water needs of the occupants.

Indirect water heaters are also great at dealing with irregular flow. The giant buffer helps smooth demand and offers very little, if any, flow restriction. Lastly, when they're insulated well, the heat loss is neglible.

Instantaneous water heaters do well in single-demand applications like a bathroom or kitchen. The flows are pretty predictable and the temperatures are therefore easy to control. However, on-demand water heaters are likely to scale up a lot faster than an indirect because the ΔT across the HX is very high... nothing that proper TLC cannot handle.

From what I have heard, on-demand heaters also usually have a lot of trouble with either low-flow or high-flow situations. There usually is a lot of pressure drop across the HX, ganging in paralell is the norm to cover high-flow rates, which makes controlling the temperature properly at low rates very difficult. I'm not saying it cannot be done, but buffers do have their place for a reason... they give you leeway.

In an ideal world, you only have low-mass boilers that can modulate 16:1 like the NTI Trinity 400. Then you can produce the "oompF!" when it's needed and let the boiler modulate down to very low firing rates when it's not. A 400 trinity coupled with a 40 Gallon Phase III or Ergomax would produce hot water and heat anything but the most ridiculous of homes...

So, why complicate life with two appliances that need servicing, that can break, etc. when one condensing, modulating boiler can take over both duties and hence save you on installation cost, maintenance, and quite possibly energy cost?

Lastly, a large buffer gives you the opportunity to recover "waste heat" like the "Aquifier" (desuperheater), solar DWH heating, etc. and store it until you need it. Some people don't need such features...