Water heater used for space heating

steamer45

Has anyone had experience using a tank type water heater for space heating. I have installed a new 40 gal./40,000 btu heater to serve a well insulated basement rec. room of approximately 900 square feet. I have installed 60' of 3/4" baseboard and 170' of 3/4 copper tube, with the piping system return entering the cold water inlet (dip tube is in place) through a Grundfos 3 - spd. circulator controlled by a relay & wall thermostat. Supply water is fed into the piping system out of the hot water outlet. The system heats the space very effectively, however, it is quite costly in fuel cost's. Upon start-up, the initial return water temp. is approx. 65-68 deg. (room temp), taking approx. 30 seconds to begin to warm. The system Delta-T is at about 12 -15 deg. The problem is the burner fires within 3-4 minutes of start-up on every cycle and continues to fire until well after the stat is satisfied. According to the manufacturer's specifications (American water heater), this heater should be capable of producing 160 deg. water, however, it never will reach that temperature. I have verified only 145 after almost 30 min. of firing, that with-out the circulator operating. The manufacturer says only that without a tempering valve installed to protect any domestic water usage from over-temp. it is installed out of compliance and will not warranty it (there is no domestic water connections). I think the gas valve/thermostat (Honeywell #WV8840B "smart-valve") is not calibrated accurately (differential is to narrow). Regardless of temperature setting, the system operates the same, Does it seem to you the heater is operating as expected?

Jamie Hall

Well, the question you asked is is the heater operating more or less as expected? Um... yes. Which is poorly. A water heater is not a space heating boiler, and was never meant to be. Wrong tool for the job.

It will never reach 160 for the simple reason that it can only put in just so much heat -- an input rating of 40,000 BTU/h so an output of perhaps 35,000 BTU/h. I'm sure that will heat the space -- that's ample BTU/h for that size space -- but you're just running more water through it than it can heat to that temperature.

Jon_blaney

You may have some kind of ghost circulation so the burner is trying to heat the water in the tank and the rest of the system. With all that radiation it will never catch up until spring. Most systems that use a water heater are built off the primary home system, use a high output unit, and are hot air (hydroid) systems.

Henry

HWT tanks are not certifies as a heating appliance. Therefore they do not work like a heating appliance. Should a problem occur and or damages, your insurance would probably not cover as it is a DYI with a non certified appliance. This from experience as a forensic investigator for insurance companies and expert witness in litigation.

Larry Weingarten

Hello, I've seen a number of these systems and they can work nicely if the temp can be kept low, like 120F. This might suggest using different emitters . Two other random things to consider are to check to make sure the flue baffle is in place. If not, you're just not getting the heat from the unit. Also tank type heaters have an anode and gas produced by that rod can mess with your system unless you have provisions for eliminating air or gas.

Yours, Larry

steamer45

I realize this application is not specifically code-approved, however, the installation manual supplied with this heater illustrates it being equipped with a tempering valve so it can be used to supply domestic water at an approved temperature AND supply some form of space heating device (air handler, baseboard, panel rad. ect.). Granted, at 40,000 btu I wouldn't spec it for that sort of application, however, giving consideration to the fact that, according to Slant Fin's baseboard specifications at 120 deg. water temp, 60' of 3/4" will give off 12,000 btu @ 1 g.p.m.. @130 deg. & 4 g.p.m. - 15,600 btu. This system has a delta T of 10 -12 deg.
Compare that to the 50 gal/40,000 btu, Rheem domestic heater the customer has setting next to this one (same flue, same gas supply) that is supplying his 4 bdrm. 3 bath, 2 kitchens & 2 laundries. City water supply is entering @ 55 deg. so the delta is double what the heating system experiences and yet his domestic heater cycles less than a 1/4 of the times the space heater cycles. The cycle time of the space heating zone is approximately 10 minutes or less with the stat set at 68 f. Even with the space stat off, the burner cycles more often than the domestic heater does.

TAG

What's the heat load of the space? temp of the water heater

My guess is everything is getting cold --- the pump kicks on and the heat is taken from the water heater. But the time the burner kicks on the water is cold. It's going to take some time to bring it back up ...

What expensive ?

You may get better results running the pump longer -- or even constant. lower the water temp and try and match the load. This way you always have hot water going around.

The only time I used a water heater was the Bradford White combi unit --- once for a kitchen floor and another for a bath. I used the smallest pump and one of the Danfos mixers

Larry Weingarten

Hi, I’ll add that most people are only using hot water a few percentage points of the day. Space heating will be a much longer use. That might make sense of what you’re seeing.
Yours, Larry

EBEBRATT-Ed

@steamer45

The baseboard runs the show. If it can output more heat than what the wh can produce it will be very sluggish raising the temperature

hot_rod

There are some water heaters listed as heat sources, they were and are used as air handler sources. They typically have higher BTU ratings 65, 75,000
If your tank is 40,000 input, multiply that times 75.% for efficiency if at altitude above 4000, 2% for every 1000 feet.  Does the burner shut off after 30 minute run without the pump running?  The control measure temperature at the bottom, set at 140 the top of the tank may be 150 or more

The high operating cost is both the system efficiency and the heater itself at mid 70%

jumper

Flow times delta T should equal about 90% of your burner output.

steamer45

Thanks to everyone for your input. To kinda rest the stage, here are the parameters of this situation. Basement rec room is 70' x 16', half or 1 side of total, basement is divided by a block wall. Was originally heated by 3 - 10" x 6" ceiling-mounted warm air registers from the forced air furnace used to heat the entire home. Customer has finished this section of the basement by insulating the outside walls using 3/4" foam board, glued to the block, with firring strips on 24" centers with 1/2 drywall . Basement is deep with only approximately 12" exposed above grade (standard basement windows without wells). With only the forced air system adding heat to this space periodically (customer has been using a wood burner to provide heat on the first floor) the temperature in the rec room could get as low as 56 - 60 deg. My new system employs (as was described earlier) 60' of 3/4" Slant Fin baseboard, distributed periodically along 20' of southern wall and 40' along eastern wall. Supply piping extends from hot water outlet of heater 3' up to ceiling, 20' over to south wall, 7' down near floor level, turns horizontally thru block & into baseboard, extends thru baseboard cabinet 90' to the far end of loop where it connects to the finned tube. From there it returns thru the remaining copper tubing & finned tubing portions all the way back to & thru the pump into the cold water inlet to the heater (dip tube is in place). First, I understand the length of run exceeds the usual standard of 100', however, I used long-sweep ell's at 90% of the turns, so there is very little restriction to flow. By my estimation the entire loop contains approximately 7 gallons of water, plus, of course, the 40 in the heater. Upon start-up of the pump (activated thru a wall stat set to 68 deg.) it takes less than 60 seconds to move all of the loop water (65 deg.) back into the heater. At that point, the heater is experiencing incoming water at 12 - 14 deg. cooler than gas control setting (usual set point is 130 deg. on the gas control). Cycle time for the wall stat is between 6 - 8 min. Yet the burner continues to fire for 5 - 6 min after pump cycle is completed. Even with pump off, on the highest gas control setting, American says it will reach 160 deg. water temp, it never reaches above 140. The highest water temp. ever recorded was 147, that after 30 min. of continuous operation (pump & burner, system still at Delta of 12 - 14 deg.) At that point, I turned wall stat to off, burner continued to fire for an additional 6 - 8 min. until I gave up & reset it back to 130 Deg. Sorry for my belligerence, but the numbers do not seem to make sense. This system has way less load than any domestic application and yet ...

Jamie Hall

The baseboard is taking -- to a first approximation -- the total output of the burner in the water heater. That should be clear enough. As long as the pump is running, it can't raise the water temperature, though it may be able to hold the temperature at some intermediate value, which depends on the characteristics o the water heater and the water heater. When the pump shuts off, the burner will continue to operate to raise the water temperature to the set point on the aquastat. If it won't reach 160 with the pump off, check the aquastat to see if it is set properly -- and tripping at the set temperature. They don't always.

GroundUp

Long story short, the baseboard is taking all the BTU that the burner can produce and then some. You either need to reduce the amount of emitter to raise the water temp or increase the burner size to accommodate it. OR you can leave it alone and let it do what it's doing if it's adequately heating the space. Gas cost is high because you used a 70% efficient appliance for the job. Nothing you can really do about that besides replace it with a boiler like it should have been

Ahead

OR... a larger tank?

I'm messing around spec'ing for an 80 gallon 6kw electric tank supplying for a space heating load of about 17k btu. I don't know what the water volume is in my system (I suppose I could sit down and calculate, which I am now motivated to do!), but I doubt it's as much as 7 gallons; but... IF say 7 gallons then that's roughly 9% of tank volume (it's would be a bit more because the tank I'm looking at has coil loops, which reduces total tank size) vs. the 17.5% you're working with.

Jamie Hall

The tank volume isn't related -- except for cycling -- to a heating load application. The heat loss you are trying to meet is.

It is a completely different situation from domestic hot water. In domestic hot water, you are concerned about meeting a rather large peak load -- even an ordinary shower is on the order to 100,000 BTUh. You usually don't have a large enough burner to do that, so you need to store enough water to meet that peak load for as long as it lasts -- which usually isn't very long (say 15 minutes at a maximum). So you can get away with a big tank -- or a bigger tank -- and a relatively small burner.

Space heating, however, isn't a peak load situation -- it's a continuous load (in your case, you are figuring 17,000 BTUh?). If your power source -- the burner -- can meet that, you don't need a tank at all. If it can't, a tank won't help at all. Simple as that. 6 KW will meet that -- just barely.