heating hot tub and dhw

heating hot tub and dhw

PostBy: lew On: Sun Aug 19, 2012 8:47 am

I installed an AHS 130 2 winters ago that was set up to heat 2000 square feet(fairly well insulated) using a heat exchanger in our forced hot air funrace. The boi8ler water exited the heat exchanger and went through a plate heat exchanger to heat our DHW. the circulator pump ran 24/7. During the winter I used 60 pounds a day. At the beiginning of this summer I bypassed the hot air furnace heat exchanger and directed all boiler water the DHW plate heat exchanger. Wow, that thing loses a lot of heat without any insulation when it was running 24/7. used 50 pounds per day. I decide to tear everything off the boiler and start new. Now I have a circulator pump feeding just the DHW plate heat exchanger with an aquastat controlling the temp. set at 140 degrees. I also insulated the plate heat exchanger. The heat exchanger for house heat is also on a seperate circulator pump (obviously not in use now) the will be controlled by the thermostat instead of running 24/7. I also added a hot tub to the situation on its own circulator pump controlled by a thermostat. So far this summer, running the hot tub and the DHW I use 30 pounds per day. Quite a bit ahead of where I was. I'm happy with the results, but could I be happier? Can this consumption rate be improved upon, or am I in the ball park?
lew
 
Stove/Furnace Make: AHS
Stove/Furnace Model: ahs-30

Re: heating hot tub and dhw

PostBy: ryan20strohl On: Sun Aug 19, 2012 9:44 am

it sounds like your doing well as long as you insulated everything well i dnt think you can get much better then that
ryan20strohl
 
Stoker Coal Boiler: efm stoker and allen stoker
Coal Size/Type: rice and buck
Other Heating: allen stoker
Stove/Furnace Make: EFM
Stove/Furnace Model: 520

Re: heating hot tub and dhw

PostBy: Lightning On: Sun Aug 19, 2012 9:54 am

Hey I'm curious about your hot tub, I recently installed a hot tub and somehow I want my coal heat to help keep it warm. Right now it probably is running about $30 in electric a month to run it. In the winter I expect it will be more and since I'll have the coal burning, there has to be a good way. I'm not sure heating the hot tub water directly is the right answer. My idea is to put a 4 ft baseboard water heater under the tub where the pumps are mounted to keep the whole cavity under the tub warm. It would be driven by circulator OR maybe it would gravity feed. I'm fearing this hot tub could be a power hog in the winter since it is outside.

Any suggestions?
Lightning
 
Hand Fed Coal Furnace: Clayton 1537G
Coal Size/Type: Nut/Stove Size Mix

Re: heating hot tub and dhw

PostBy: Rob R. On: Sun Aug 19, 2012 10:14 am

Hi Lew - if the piping is well insulated and the circulators run on demand, there isn't much else you can do.

:idea: Is your DHW piping after the plate exchanger insulated?
Rob R.
 
Stoker Coal Boiler: EFM DF520
Hand Fed Coal Stove: Hitzer 50-93
Other Heating: Dad's 1953 EFM Highboy

Re: heating hot tub and dhw

PostBy: lsayre On: Sun Aug 19, 2012 10:23 am

To heat only our homes DHW (not including a hot tub) we are going through about 13.8 to 14.4 lbs. of coal per day on average in our AHS S130 (I add coal every 13-14 days and then back calculate for the average usage over that time period). Our homes hot water comes directly from the boilers DHW coil, via a 3-way thermal mixing valve (Taco 5003 series). There is no direct or indirect hot water tank (we have an electric hot water tank, but it is shut off and valved off).

During the summer there is no circulation of boiler water with our set-up, with the sole exception of when (or if) the boiler trips the dump zone (which does not happen very often at all with the dump zone aquastat set to trip at 215 degrees. I've only directly observed it to happen once this summer). The only water that moves during the summer is the water traversing its course through the DHW coil and then off to the users in the home (sinks, showers, dish washer, clothes washer).

After the blower fan shuts off at 180 degrees, our boiler seems to subsequently over-shoot to a temperature of about 212-213 degrees and then reach a comfortable equilibrium between heat being created within and heat being radiated away to the surrounding air, thus remaining comfortably close to but not tripping the dump zone circulator. When my dump zone aquastat was set at 210 degrees the boiler dumped rather often, but at 215 degrees it does so exceedingly rarely. This fine line of equilibrium seems to be a key secret to using hardly any coal at all.

With a hot tub, 30 lbs. of coal per day may be about right.
lsayre
 
Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Anthracite Pea
Other Heating: Resistance Boiler (if I ever get it fixed)

Re: heating hot tub and dhw

PostBy: lew On: Sun Aug 19, 2012 12:23 pm

Lightning, I don't think I would gain much for heating my hot tub with a baseborad heater set underneath the shell. First my shell is insulated, thus blocking any heat from the water I'd want to heat. Second, it would be a poor heat transfer from the heated air to the shell then to the water. Eventually it would work, but very slowly. Maybe not fast enough in the winter. what I did was to purchase a water to water heat exchanger made for pools and spas. they are made of stainless steel. the chemicals in pool/spa water are not good for cast steel or brass/bronze (according to my limited research). I disconnected the 220 electric from my hot tubs electric heater and used one leg of the 220 (Now just 110 since it is only one leg) to run my circulator pump. Then I plumbed the water that comes out of the Little Giant pump that circulates the water through the hot tub electric heater to first go through the heat exchanger used in conjunction with my coal boiler then returned that water to the elctric heater (that's now inoperable due to no electric) this way I didn't disrupt any of the electronics that are in the hot tub (there's more than I can figure out). Works great. the only trouble I have is that it will keep the hot tub about 2 degrees warmer than it is set at. This is because I am getting hot water from the boiler rising from the cellar and continue to slowly flow through the heat exchanger even though the circulator pump is shut off. I am planning on putting in solenoid valve in the future to cure this and possibly save alittle more coal.

I have yet to insulate the plumbing from and to the hot tub and hot water tank. I have the insulation, just haven't had the ambition lately. Thanks for the thoughts and comments. Makes me feel like I am on the right track.
lew
 
Stove/Furnace Make: AHS
Stove/Furnace Model: ahs-30

Re: heating hot tub and dhw

PostBy: lew On: Sun Aug 19, 2012 12:26 pm

Isayre, I calculate my coal usage the same way. I have the older (Ithink) round hopper. When first purchased I filled it with bags and it came out to 300 pounds. That is now lasting 10 days like clockwork. I disconnected my Barometric damper because of puff backs and CO problems. I have a manometer now and need to install it to play with the barometric damper again. thanks for the input on your coal usage. That helps a lot.
lew
 
Stove/Furnace Make: AHS
Stove/Furnace Model: ahs-30

Re: heating hot tub and dhw

PostBy: Lightning On: Sun Aug 19, 2012 4:42 pm

lew wrote:Lightning, I don't think I would gain much for heating my hot tub with a baseborad heater set underneath the shell. First my shell is insulated, thus blocking any heat from the water I'd want to heat. Second, it would be a poor heat transfer from the heated air to the shell then to the water. Eventually it would work, but very slowly. Maybe not fast enough in the winter.


I plan to keep the electric on for the heater in the hot tub, I am just looking for a non evasive way to "help" keep it warm. I'm a little nervous about splicing into the plumbing on the tub itself or bypassing the heater thats in it. My tub has about an inch or so thick layer of the sprayed on insulation on the bottom side of the shell. I agree that heat transfer wouldn't be substantial but keeping that cavity under the tub warm should help cut down power consumption. I guess all I could do is try it lol :)
Lightning
 
Hand Fed Coal Furnace: Clayton 1537G
Coal Size/Type: Nut/Stove Size Mix

Re: heating hot tub and dhw

PostBy: rudy On: Tue Dec 10, 2013 5:09 pm

Using an electric spa heater is by far the most common method due to its relative simplicity.

An electrical heating element, similar to that which you would find in your emersion heater or kettle, is mounted in a tube. The pump in your spa pushes water through the tube and when the flow sensor indicates that sufficient water is passing over the element, the thermostat or spa control system turns the element on.

In an average family spa the electric heater will raise the temperature by about 5 – 6 degrees an hour, so how long your spa will take to heat up from a re-fill obviously depends on the temperature of the water going in.
rudy