old hs tarm

[sfjohnson]

i have an old hs tarm OT35 model. It is a multi-fuel model and I used wood and an oil burner. I just use it to heat the house, not for hot water for showers, etc.

It seems that no matter what I do, the wood has a real hard time heating the water above 160 degrees on colder days. I have dry seasoned wood and have tried adjusting the settings in various ways, using less or more wood, etc. Still no luck. And when it circulates the water (it is a forced hot water system) it brings the water temp down to 150 or 140 quickly and takes a long time to get back to 160 which leads me to use the oil to bring it back up a lot more than I want to. It seemed to work fine when the weather was a bit warmer... Just really started noticing it this month as temps started dropping more and more.

Any ideas? Anything that I might be doing wrong to cause this? Anything with the Tarm that may need replacing/fixing that is causing this issue? Or is that just they way it is with these models when it gets really cold?


thanks

 

[Dave T]

Welcome to the boiler room sfjohnson,
I also had an HS Tarm and came across similar issues I do not recall if the oil side worked right at the time but I had a defective expansion tank a way to kinda check it is to feel the outside of the exp tank the top should be hot and the bottom should be cool with the boiler running,if your whole tank is hot the internal bladder has been damaged,if you have an automotive tire gauge you can check the bladder by using the gauge on the bottom of the exp tank I think COLD 5 pounds was the magic number but don't quote me on that,if it is low but not at 0 try putting some air into it, NO COMPRESSORS use a hand pump..If the exp tank is bad when you shut the system down then relieve the pressure and go to unscrew the exp tank watch out it will be full of water and quite heavy..Good luck to you,Dave

 

[Hydronics]

I would check the heat exchangers for excessive creosote buildup. Do you have adequate draft?

 

[sfjohnson]

Thanks to you both for your responses. The expansion tank makes sense. The previous owner (I just bought the house in april) left a new bigger one in the boiler room without telling me why (i just found it on the floor when I moved in...) so maybe that was it. I did just go check it and it is hot on top and cool on the bottom though, so that part checked out ok. I'll try checking the pressure that you mentioned tomorrow.

Alslo, the oil part of it works fine, it just runs alot to keep the temp up, but it does keep it up and heats up the house quickly.

One other thing that happened is last night we had a power outage at 11:00 pm. I had just put more wood in the burner before it went out and when the power went out, the wood heated the water right up to 190... I opened up the zone valves manually and it kept the house fairly warm for a while and the water stayed at that higher temp of 180-190 for about three hours which it never does when the electricity is running and circulating the water, so it definetly seems to be something with the circulation or settings is my guess, just not sure what...

the draft seems to be good.

On the heat exchangers, can you tell me where those are/how I get to them? I do have a diagram of the OT 35 from hs tarm, but its not labeled and I am still a rookie at all this...

Thanks again

 

[Dave T]

Just so you know I am not a pro in hydronics by any stretch of the imagination just trying to help.. Is the unit shutting down at 160?Or is it going all out and not getting to temp??All this should be controled by the Sampson air damper, if it shuts the door at 160 then you need to turn the knob or adjust the chain, is the Sampson still open but not heating above 160??Dave
My exp tank reads 12 PSI precharge

 

[Hydronics]

That's a good point, you adjust the draft door closing temp. which controls boiler temp. It's the black knob that says Samson on it. It has a bimetallic spring which lengthens as water temp. increases therby shutting the draft door. The heat exchangers on Tarms are accessed from the top.

 

[Dave T]

I never saw any heat exchangers on my ot 70, the top of the unit was solid under the skin..The only place I did not see is where the domestic coils enter and exit the unit..I posted a pic that kinda shows the top of the Tarm the side with the rope hooked to it is the wood fired side, the other side is the domestic coil side with the oil burner under it(is this where the heat exchangers will be found??)..Dave

 

[sfjohnson]

Thanks for the replies. the fire keeps going, it just never gets the temp above 160 or so even when the samson regulator is open. I've adjusted the regulator various ways, including craking it all the way open and just leaving it wide open all the time for awhile, and nothing I do seems to get the temp up. The fire will be roaring, have good coals, etc. but the temp struggles to get over 160.

I've also adjusted the aquastat settings lower and higher to just try different things. Those seem to control the ciruclation as well as when the burner kicks on. In other words, even when the oil heater part is turned off, if I turn the low setting of the aquastat down all the way, the water will still circulate even if it is at say 140, but if I turn the aquastat low up to 160 the circulation cuts off when the temp goes below 160,etc.

So since the temp won't get much above 160 with the wood, I have been letting it circulate at the lower temps as that does heat the house some, just not all that well because of the lower water temp. Then at night, I just turn on the oil part to run whenever it dips below 160 and keep the wood going as well for some of the night, but the wood just doesn't seem to be helping keep the temps up very much and the oil burner runs alot.... but as I say that does get the temps up where they should be and then the house is heated just fine.

Are the heat exchangers connected to the tankless coil part of it (the part the does the domestic hot water)? I ask because that part of the Tarm was leaking or not working correctly over the summer so we stopped using it for hotwater and turn that water flow off and rerouted it to an on demand propane hot water system that a plumber installed for us. I was told that the bad coil would not affect the heating part of it, but maybe somehow it is?

 

[Dave T]

OK..Are you using a glove and opening the door in the middle of the Tarm and changing the damper inside the unit if I remember correctly all the way up for wood to encourage the flame to go down (it has a little kickstand it sets on, to be set for the wood side the internal steel door needs to be all the way up)..For the oil side the damper should be all the way down (in this setting it would be easy to mistake it as a piece that doesn't move, it will look like a straight run all the way to the chimney but that is a door all the way down)..Do you have a chimney temp gauge??Dave
I am throwing a pic up of the front of my old Tarm the middle door is just visible in this shot..

 

[sfjohnson]

Thanks, you have a good memory... Yes, I have that part all the way up. The directions talk about changing it to halfway or all the way down for oil as you mention or for bad draft days, so I've tried adjusting it from time to time to see if it made any differece with the wood burning, but it did not, so I've just left it up as you mention.

I don't have a chimney temp gauge. Can I get one at a hardware store? If the chimney temp is low, what does that tell me?

The diaphragm pressure was 14psi today.

Thanks again for the help...

 

[webbie]

A mixing valve or a mixing loop (return loop?) might help greatly in a situation like this. What happens is the "all or nothing" effect of very hot water leaving the boiler and very cold water coming back in. We used to liken this to a car where the gas pedal is either all the way to the floor or all the way up. But a 4-way automatic mixing valve is more like cruise control, where water of various temperature is constantly circulated to the house and some of it right back to the boiler, with the result being a more even supply and demand of heat. Such a value should first be installed on your main zone - the one most used.

https://www.hearth.com/econtent/index.php/wiki/4_Way_Mixing_Valve/

 

[Dave T]

I would think the BTU's from the wood burning has to be going somewhere,possibly up the stack??the temp gauges can be bought at any hardware..Lets lay out a couple specifics of your system,how many pumps what size?how big is the house your heating,how bout insulation value? is the unit temp sensor the one you are using,do you have any others in the system?is the supply line hot, is the return line cool?is it possible the unit is under sized and struggling to keep up?Glad to see you jump in here WEB, is there heat exchangers in this unit??..Dave

 

[webbie]

The entire unit is a heat exchanger - wet leg and wet base and wet all around, etc. - the DHW coil is a heat exchanger, but no used in this case.

A boiler and the resulting heat transfer do not function as well with colder return water, and wood burning does not take well to instant on. That is the reason for the mix valve. Yes, heat is lost in a number of ways because when the samson opens all the way, too much goes up the stack, when it closes all the way, not enough for complete combustion.

The 4-way provides moderation of these forces and can end up making a big difference.
Some plumbers do it by adding a manually adjusted bypass, which can be changed at various times of year.

But I like the continuous circulation of a mixing valve for many reasons. For one thing, it always keeps the water in the baseboards at warm...so you don't get that cold water coming back. It also adds the piping and baseboards to the storage capability of the system, and we all know how important storage is to wood burning hydronics.