thermostat for hot water boiler system

john_181

Hello,

I would like to ask: what is the best thermostat I can get to control a gas hot water boiler system.  It is a 430,000 BTU system.

The reason I ask is I just installed a thermostat and to just bring up the temp one degree - it has the boiler running for hours and the return water is as hot as the supply water. 

Do they make a thermostat that will call for heat and then after a set time it will cycle off the gas and wait to see if the system reaches the setting before refiring the burners?



Thanks

John

Jamie Hall

I'm not a hot water guy --

I'm a steam guy.  Once in a while I'll look at hot water...



But on first look, this doesn't sound like a thermostat problem, but a circulation problem.  If your return is as hot -- or even close to as hot -- as the feed, you simply aren't taking any heat out of the water and putting it into the radiation.  Basic physics.  So I'd look at my circulation flows and directions first... could be something as simple as a valve not opening somewhere.

Timco

Tekmar

I own a Tekmar tN-4 system with tekmar t-stats. Simply put, fantastic control. It calculates the lowest possible water temp with each heat call, figuring in indoor & outdoor conditions to make that decision, and is always learning. It has every control feature you would ever want, and you can view water temp, outdoor temp, or any reading from any t-stat. It will also modulate & control a mod con boiler, and has built in priority for DHW. The best part is it pulses the pumps up to temp and mine has never overshot the setpoint. You tell it what types of emitters you have.



Tim

john_181

more info

They are cast iron square tube radiators.  Also, I just installed a ODR with the system that works great.  I just changed the thermostat and this problem started.  I am trying to buy one that is "smart" not an old run of the mill.  Do you think setting the thermostat to be very sensitive is better than less sensitive?  so it does not overshoot the setting?

Timco

overshooting

The issue is your emitters are high mass, and thus by the time the t-stat is happy your rads keep on giving until they stored heat tapers off / radiates out. It takes a much smarter t-stat and system that controls how fast your system comes up to temp and works to just reach that point and no more. Unless the t-stat controls the pumps and firing rate you may just need to set the t-stat to 3-4* lower than you want the temp in the house.



Tim

Mike Kusiak_2

Anticipator

Sometimes in a high mass system such as yours, an old style mercury or mechanical thermostat may in fact work better than a fancy new one. These old thermostats had a anticipator setting, which caused the thermostat to end the call for heat before actually reaching the setpoint temperature. It did this by incorporating a small electrical heater in the thermostat itself. When the thermostat called for heat, the internal heater was activated and raised the internal temperature of the sensing element before it could respond to room temperature, thereby ending the heating cycle earlier. The anticipator setting was adjustable and could be set so that the actual room temperature would not overshoot the thermostat setting.



This is probably the type of thermostat you had previously, which compensated for the overshoot which you are now experiencing.

Mark Eatherton

The thermostat is speaking Greek, and the Tekmar only speaks Canadian....

and neither knows the other exists....



Too many variables for each to keep track of.



The thermostat (even the most intelligent AIRS type) assumes that when it calls for heat, the heat source is going to projectile VOMIT btu's into the space. Now that you have an ODR in the mix, that statement will only be true at design conditions.



What starts happening, is that the thermostat realizes that it can never seem to catch up to the load, so it will only set back for a VERY short period of time, and begins trying to satisfy the AM set point, shortly after the beginning of the PM set back. Consequently, as in your case, it becomes useless as (enter example here) :-)



If you are doing set back for energy conservation measures fuhgedabowdit. The major conservation effort is now coming from the source. If your home actually drops in temperature that fast, it indicates a critical need to plug holes, replace windows and add R value to the walls.



If you are doing it because you like a cooler sleeping environment, if your system is conducive to it zoning wise, just keep the sleeping areas set for a lower temperature.



If it is NOT conducive due to piping methodology, consider installing non electric TRV's with bypass at the heat emitters, and keep the sleeping areas set for a lower operating temperature.



I agree with Mike K., put in a good old non solid state thermostat with an anticipator circuit for the ultimate in cruise control....



One of these days, someone will come out with a control logic that will allow for deep set back and reasonable recovery, but the fact remains that true radiant heating systems, which primarily affect mass temperatures, and not air temperatures, are NOT conducive to deep set back and reasonable recovery.



ME

john_181

old thermostat

So I guess the old thermostat type will work better for this big old system - go figure - LOL.  Looks like I am of to Lowes to see what they have.  One any better than the others?

Mark Eatherton

Tough to find at the Big Box stores....

Most of them have gone to a solid state stat.



Look for something with a bi-metal sensing element. It should have an adjustable anticipator. Set the anticipator for around .32 amps to begin with.



ME

Jamie Hall

I still think...

from the description of "the return water is as hot as the supply water" that there may be another problem in here.  The return should, as I understand it, be cooler -- 20 degrees or so cooler -- than the supply.  The question I would ask is: are the heating elements in the space hot?  If not, why not?  If they are, why isn't the space heating up?

john_181

radiators

yes the rads are hot in all the rooms.  the return water was 130 and supply was 135 last night.  I also believe the new metal thermostat guard i installed does not have enough holes in it to allow enough air flow.  Today I am going to drill some holes in it and place the thermostat on standoffs away from the wall. 

Also, the system is an old 432,000 Btu boiler from the 1930's.  It is oversized for the building - I have turned off 3 of the 5 burners and it still works good.  With the full 5 burners I believe it to waste more gas - am I right here?



I will look for a thermostat today and will set it as you reccommend - Thanks

Mark Eatherton

It depends....

When you say you shut off burners, how exactly did you do that? If you just shut off the gas supply to the burner and didn't block the secondary air going through the combustion chamber, you may be wasting more fuel than having all five burners lit up.



The only correct way to find out for sure would be to perform a combustion analysis under both scenarios, and compare the readings of the two.



Also, a 5 degree DT sounds close to normal to me at those low operating temperatures. The only time you should see a 20 degree DT would be with the system coming out of a DEEP cold condition, at design condition. Otherwise, the DT will be all over the board, depending....



Got pictures?



ME

john_181

more info

I did just turn off the burners gas supply.  What else could I have done or ???

I do have pics - will post later today.

Mark Eatherton

Things to be done...

Unless you have and know ow to use, a combustion analyzer, there really isn't a LOT you can do to an atmospheric appliance. And the majority of the work needs to start outside of the mechanical room.



Start with a heat loss.



Once you're done with that, do an EDR study to make sure that the radiators are capable of handling the heat loss.



Now, look at the boiler, and see how it is mathematically sized in relation to the calculations you have performed.



Next step would be to perform a clean and tune up on the old beast. Do an analysis BEFORE and one AFTER.



If the boiler is severely oversized, you might want to consider replacement with one of the new high efficiency modulating condensing heat sources. It will reduce the fuel consumption by a minimum of 30%, and possibly as much as 60 percent or more.



You've already thrown a fine control at it, but if it is EXTREMELY oversized, there is only so much good a control can do...



Turning gas burners off should be left to a professional so you don't expose the boiler to un-necessary thermal stress, and cause it to die an early death.



Let's see what you've got in the basement.



ME