Water Heater instead of Boiler

john_215

I understand,Doug, and agree, if the application is limited to dhw use; but the application is space heating and the subtractions do not apply in that application since the heater and its vent are in heated space.

Trevor Thompson

Water heater inplace of Boiler

I have an American 4 square. 2.5 stories. In the basement is a very, very old boiler. It was built in 1915, what I'd like to know is could I install a 40 gallon hot water heater on a closed system and install a circ. pump to heat the house? A lot of info I've found speaks to a drinking water/heating situation, which isn't what I want. Any issues that you know of? Anybody done it before?

Andrew Hagen_2

Water Heaters

There are a few issues:

1. It sounds like you could easily have a higher heat load than a 40-gallon water heater can supply. A good first step would be to perform a heat loss calculation so you know what size boiler you need.

2. Tank-type water heaters are not very efficient. I suspect efficiency is your goal in replacing the old boiler.

3. If you have a separate water heater for your heating that is not connected to your domestic hot water, there is no drinking water legionella worry.

Have you invested in reducing your heat loss? That's almost always the best first step. How tight are your windows and doors? Single or double pane glass? If they are single pane, do you have air-tight storm windows? In your case, that old boiler should be replaced as soon as possible. You will very likely realize a significant efficiency improvement. Do you have cast iron radiators? Thermostatic radiator valves (TRV's) would be a good upgrade when you do the boiler. I encourage you to install a modulating condensing boiler since, in the long run, fuel is not getting any cheaper.

Trevor Thompson

I have invested into reducing heat loss from the cheapest up to most expensive. Insulation has been added to the attic as well as basement. The worst Windows and doors have ben replaced. That means I'm up to the boiler.

I have 8 cast iron rads in the house. How do I perform a heat loss calculation?

Would the value be worth it in the long run? A gas boiler is far more than a gas water heater. would I make up the savings over 10 years time?

jp_2

pay now or pay later?

you can spend the money now, get a good mod/con, then end up with small monthly gas bills OR you can put in a huge hot water tank and see high gas bills all the time?

I prefer to waste money on the system, rather than waste money on energy.

which water heater are you thinking of using? how are you determining which one to buy?

Trevor Thompson

just looking.

At this point I'm not. I'm just running the idea through my head.

What would casue a water heater to be less efficent than a boiler? Other than keeping the water hot when it sits, wouldn't they nearly be the same?

It is a pay me know or pay me later idea, but you have to figure out which pays more. There isn't any point to spend 1500 dollars more up front to save 10 dollars a month.

As far as how to pick which water heater, take a look at BTU ratings, efficency and cost and there you go.

Like I said, has anybody done it? What was the experience like, any benefits or drawbacks?

jp_2

water heater efficiency

you can buy water heaters that are as efficient as hi eff boilers, but they cost about the same.

now take a $300 water heater from home depot, about 65% eff.

how many BTU's do you need? you need to figure out how many btu's you need on the coldest part of winter to keep the house warm.

at the top of the page is a menu, click on resources, then heat loss calcs. get an estimate of your needs and come back and tell us.

or take a look at last years bills, what would you save per month if that bill dropped by 20%?

off hand nobody in-the-know would use a standard water heater to heat an averaged sized house, unless its super insulated and requires a real small heating unit.

mark ransley

I know my house needs 180f water when its near zero out to heat the radiators and house, now is near 40f out and it runs to 145f. What is your boiler set at now, a WH probably wont have the btu output or temp needed to heat when its real cold out unless you get a big commercial unit. You do need a load calculation done. To know WH tank overall efficency read the EF rating, that is "Energy Factor" Just because a unit is 82% efficent its overall EF is likely near 55-65 EF so 35-45 cents of every dollar heating water in a standard water heater is wasted. You can get a boiler with a real 93-98% efficency rating and include a HW set up to save even more.I know my radiators, large cast iron need 130- 140 to get heat out and warm up the place, some baseboard needs much higher temps. I dont think a WH is designed for that load but it could work. A top efficency boiler will save in utilities more than a regular tank by a large margin. But some areas down south that need minimal heat it might be ok.

Andrew Hagen_2

Payback

The average water heater with its relatively small burner output has to be inefficient in order to keep from destroying itself. The burner could be running beneath a 40-50°F tank of water and then through a length of exhaust pipe. The flue gas cannot condense inside the water heater or exhaust pipe or it will rust and destroy itself. Conventional cast iron boilers can be piped to avoid flue gas condensation while keeping the efficiency higher. Modulating condensing boilers are designed to condense the vapor in the flue gas and recapture that latent heat.

There is a link at the top of the page under "Resources - Heat Loss Calcs" that will direct you to Slant Fin's software for calculating heat loss.

Water heaters can and have been used for heating purposes. As long as the water heater can reach the temperature you need to heat your home, the biggest drawback is the relatively low efficiency. The catch-22 about reducing your heat loss is that as you burn less fuel, higher efficiency saves you less money. You will have to make the judgement based on your fuel bills, the projected efficiency improvement, and home resale value.

Will the output of a water heater carry your heat load? From a quality and longevity standpoint, a cast iron boiler would be a much better choice than a water heater. Don't forget to figure fuel price inflation into your payback calculations. The price of natural gas increased 15% this year, here in Montana.

Larry Weingarten

Trevor

Hello: In reading this discussion it seems to me you're not really asking the right questions, because you don't have the experience in this field that many others here do. That's why you're here asking questions.

However, it might be best for you to find a local contractor (from this site) to walk you through the thinking that will result in an efficient, long lived system. Hiring someone to consult will save you many dollars and much trouble.

The idea that a heat loss calculation may not be money well spent, when it's pretty clear you don't have a good handle on what your house is doing energy-wise, tells me you're thinking of jumping into a pool that likely has rocks just under the surface.

This place has helped many. Following the steps Andrew laid out will result in a system you can be happy with.

Yours, Larry

joel_19

simple answer

No,period

Doug_7

Pay Now or Pay Later ?

The efficiency of a conventional direct fired natural draft domestic hot water tank is only about 40% to 50% in terms of heating water for use. I know this because I have measured the efficiency.

When it fires, 25% of the heat goes straight up the flue.

When it is not firing the standby losses due to holding a tank full of hot water are about 15%. It continues to draft heat up the flue and radiate heat.

A DHW recirculation system with insulated pipe will lose another 20%. With un-insulated pipe the heat losses are much higher as the piping system acts as a radiator 24/7/365.

When you use a conventional DHW tank for heating, the efficiency gets worse. A conventional DHW tank only has to fire 10% of the time to look after DHW load.

To look after a heating load it may have to fire 80% of the time on cold days. They are not designed or rated for long periods of continuous firing required in heating systems. A conventional DHW tank may last 10 years but in heating service the life will be reduced to 3 to 5 years. Been there - done that.

A condensing boiler gets 95% efficiency and avoids standby losses entirely. It also has a very long service life.

The simple truth is that using a conventional DHW tank instead of a high efficiency boiler for heating is false economy based on short-term thinking.

Run the numbers for fuel cost and service life.

Regards,

Doug

Doug_7

Long Term Cost of Ownership

For the two different boiler efficiencies, you can calculate the cost of heating fuel used annually and over 10 years.

The average energy use of a house is about 10 Btu / Sq Ft / Heating Degree Day. (U.S. Department of Energy survey)

If this was a 1,800 sq. ft house in a region with 5,000 HDD's per year you would use 90 MMBtu / year of energy.

Assuming fuel costs $10 / MMBtu ($1 / therm), you can provide this heat with a >90% efficient ModCon boiler at a fuel cost of about $1,000 per year or with a <60% efficient water heater at a cost of $1,500 per year.

The difference is $500 per year or $5,000 over 10 years.

You can plug in your own numbers for your actual house energy efficiency; your actual floor area and your actual annual heating degree days.

In any case, boiler efficiency has a pronounced effect on your annual fuel cost. Also remember - fuel costs will go up again, just as soon as the economy recovers.

Think long-term-cost-of-ownership, not initial cost.

Doug

Mike Thomas_2

I've Done It

I did it for a friend. A 24 x 40 well insulated garage. Standard water heater would heat the garage just fine. Our first sign of problems was when he noticed everything around the bottom of the water heater was black and sooty. You guessed it, flame roll out. Flue gases condensed and rusted the water heater so bad the rust would fall down on the burner and flames everywhere. He would vacuum the rust out twice a heating season. After the 3rd year and almost burning the garage down more than once, we put a Munchkin in and he has never been happier...

john_215

Of course, in principle, Doug is right, but we have to be careful in converting words to numbers. Water heater efficiency is calculated weighted by its purpose--dhw; and considers standby losses, cycling time and stack losses, none of which properly apply to a heater used as a boiler especially when installed in and with its chimney in, heated space, and running constantly.

Apples to apples here might be water heaters to water heaters not water heaters to boilers.

Charlie from wmass

A water heater is around 35- 50 K BTU and is simply not made like a boiler to extract heat from the flue gasses and transfer them to water. Also I would check the warranty and the fact a water heater may not be listed as a heat source for a dwelling so any home owners insurance would not cover the heater or damages caused by anything that did go wrong with the unit. Boilers are boilers.

Doug_7

Measure overall DHW system efficiency

I am talking about the total domestic water heating SYSTEM efficiency - not just the efficiency of the tank itself.

You would be surprised what you can learn about the actual efficiency of the total hot water heating system, using some simple field measurements and some simple calculations. I am referring to a conventional natural draft DHW tank.

Connect a Red Lion timer to the burner, and use burner rated capacity to convert elapsed time into Btu's.

First measure the total burner hours per 24 hour day of normal operation with normal hot water use.

Then measure total burner hours per 24 hours with zero hot water use. This can be done at night. This gives you the total system heat losses per day - which is made up of standby heat losses and recirculation heat losses.

Then measure total burner hours per 24 hours with zero hot water use and zero recirculation. This can also be done at night.

Now you can break down the total heat losses into the standby heat losses and the recirculation heat losses.

You know the overall efficiency of the tank from the manufacturers literature. i.e. How much heat goes into the water versus straight up the flue.

Calculate the numbers - The results will surprise you. And this is after insulating all the DHW piping to reduce the recirculation heat losses.

See attached Pie Chart.

Doug

Unknown

65% efficiency is a vast understatement of water heater efficiency in a heating application. that might be the energy factor, but that is for a modest amount of domestic usage only, including jacket losses, which fall rapidly as a percentage figure as load increases.

http://dsp-psd.pwgsc.gc.ca/collection_2007/cmhc-schl/nh18-22/NH18-22-106-108E.pdf

about 80% measured for a water heater in a heating app.

that said, the heat load calculation whether that 80% is enough, or whether it's worth reaching for another 10 to 15% in a mod/con. that is the only way to answer whether it is worthwhile to do the upgrade (well, besides doing a lifecycle calculation).

Unknown

Doug,

your measurements are exactly the opposite of the experience and measurements of other in the field, and it also makes no sense whatsoever that the efficiency would get worse when heating. the only reason the efficiency of a water heater is measured at in the sixties is because it's a tiny amount of usage compared to the standby loss the EF test requires. I could see someone saying it wouldn't IMPROVE, but it certainly couldn't get worse.

never mind the dozens of pros who have done so and would refute your info anecodotally, I posted a link to an actual study that refutes your measurements. you might want to look at your methodology.

you are correct about fuel cost, service life, and load numbers being important. but I think your continual draft scenario isn't the case with today's water heaters anymore? I'm not sure about that, but it's certainly not true for all models of water heater at least.