HS Tarm Solo overheating while minus 5 outside

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Bryan Hadlock

New Member
Jan 29, 2021
5
Littleton NH
I grew up using the HS Tarm Solo burning wood since 1980 and install the same model in my new home in 2002, an HS Tarm Solo MKII. My brother lives in my parents home now and continues to uses the original HS Tarm to heat the home. Both have an oil boiler that we use in the warmer months for hot water and back up heat if needed.
We actually keep the oil boiler turned off all winter unless we will be away from the home for a long time.
Each home is just over 5000 SQ FT and I heat my garage and basement with a Modine units.

We live in norther NH where outside temps with winds can go below - 50 degrees. My HS Tarm solo has run perfectly using wood or coal since installed.

Recently the outside temps have gone below zero. HS Tarm water temperature is continuing to increase to 210 degrees when all 6 zone circulators are running as all thermostats are calling for heat.
BEFORE ANY ONE SUGGEST CHECKING THE TARMS IN LINE CIRCULATOR, I'VE ALREADY DONE THIS AND REPLACED WITH A BRAND NEW ONE.
All valves are open and working correctly.
I've also purged the entire system, running new water through each zone, making sure there's no air locks. Plus, there are are air releive vales all over the place too.

The plumbing for the system is set up correctly for a home using both oil and the HS Tarm. The diagram was followed perfectly. when we turn off the oil, it's only the burner, so all other controls are in place and functioning.

What I have found is when all 6 zones are running, the return lines are entering the 2 inch main in front of the hot water coming from the Tarm boiler. The water is merging with the hot water coming from the HS Tarm and being diluted down to 120 degrees.
This is causing all the water being pumped through all zones to only be at 120 degrees!

The circulator pump pushing the water through the Tarm is a Grundfos multo speed, which has at least twice the power as the TACO pumps moving water throughout the house. Again, I've never had this problem in the past.

Attached are pictures of the boilers and plumbing.

Does anyone have any thoughts>

left side.jpg right side.jpg temp.jpeg
 

Bad LP

Minister of Fire
Nov 28, 2014
1,795
Northern Maine
Sounds like a mixing problem in the loading unit or your diverter valve depending on how it was plumbed but I'm just tossing poop at a wall to see if it sticks.

What is the temp of the return water within the boiler?

Have you or can you reverse flow the boiler to back flush and obstruction?
 

3fordasho

Minister of Fire
Jul 20, 2007
1,005
South Central Minnesota
I don't know what your boiler return protection device is but I would look there - if the thermostatic element type perhaps it is stuck in the position where the hot water is all short cycling from boiler supply to return and only lower temp water is making it out to your loads?
 

peakbagger

Minister of Fire
Jul 11, 2008
6,616
Northern NH
I would need to see a detailed piping diagram to make an educated guess. My total guess in line with Bad LPs methodology of seeing what will stick to the wall, is you need a buffer tank or a hydraulic separator. Right now it sounds like your wood boiler had found a way to short circuit your supply header. I think your response will be that it has always worked this way so why do I need to change it?.

Here is another guess The system looks like it been around for awhile. Tarm's usually have a thermostatic low water temperature protection valve. It runs from the outlet of the wood boiler and tees down to the inlet of the boiler. The tee on the outlet or at some point on the bypass has an internal bymetallic or wax type thermostat that moves an internal piston that forces outlet water from the boiler to go to the inlet of the boiler until the inlet temp is over some setpoint like 140 degrees F. This happens at start up to keep water from condensing on the gas pass of the heat exchangers. As the boiler water warms up the valve s supposed to slowly open to supply more hot water to the house and diverts less to the boiler inlet. If this valve ( I think they are also called Thermovars or loading valves) was stuck, it would be circulating your hot boiler water down to your inlet skipping the house. My guess is typical plumber would not put an isolation valve on this device so no one bypasses it, If there is an isolation valve on the bypass, i would close it and see if the circulator temps rise. The folks down in Lyme may have a rebuild kit since they probably sold the system originally. Its pretty rare for bimetallics or Wax bulbs to fail but looking at the boiler my guess is that its been there for more than 20 years possibly 30?

Looks like someone beat me to it with my second guess. BTW I dont have one of these valves on my Burnham wood boiler but I have a cruder temperature control that sort of does the same thing.
 

Bryan Hadlock

New Member
Jan 29, 2021
5
Littleton NH
Thank you for the quick responses.

1. Water Return is 120 degrees, due to the circulators pulling the home heating water into the main.
2. My system is set up 100% as listed by the HS Tarm.
3. yes, I do have the Thermovar, BUT, I turned off the valve years ago, making it a one way flow. It was doing it's job by keeping the water temp going back into the boiler higher, but the negative result when allowing it to be on was not enough hot water running through the main through the oil boiler and out through the house. Short story, when it's on the oil boiler would constantly run unless I turned the internal thermostat down to 130 degrees. This is why I have the burner turned off.
4. Attached is the diagram that appears to be how it's set up, but as stated above, it never worked well so I turned the burner off to the boiler. I've also attached a picture of the pipes behind the Tarm. You will see where I've had to insulate around the flue to prevent the circulator from getting to hot and going bad.

I've shut it all down and blead each area, allowing for any blockage to be moved. nothing but clean clear water, with all air removed.

I have one of our best plumbing and heating specialist stopping by in the morning to take a look. I'll be sure to let everyone know the results.
Who knows, maybe one of you will get to say "I told You so."

Stay warm!..
 

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maple1

Minister of Fire
Sep 15, 2011
10,812
Nova Scotia
Something is way off here. If you have 120 going into the boiler it should not be that hot coming out. Unless there is no flow thru the boiler. Are you sure that temp gauge isn't bad?

Edit: fiberglass around the flu pipe? Pretty sure I wouldn't be doing that.
 

Bryan Hadlock

New Member
Jan 29, 2021
5
Littleton NH
PROBLEM SOLVED!!!!!

Before providing the answer, here's another quirk to the scenario.....
I loaded up the wood boiler at 12:00 Pm last night and also turned on the oil burner, which is set at 170 degrees.
The wood burned off slower, but the TARM water temp never got higher than 190 degrees and helped to heat the house and hot water all night.

SOLVING THE PROBLEM WAS ACTUALLY SIMPLE:

The issue ended up being the C-3 in line Circulator, ( A Grundfos 3-speed ).
Apparently Grundfos is like many other corporations the past 20 years and lowered the abilities of the circulator pump being used.
The short story is even thought it was the same model the new pump I installed this past fall was not able to produce enough pressure to circulate the Tarm boiler hot water through the main, past where all the zones were entering, which was on the cold water intake for the oil boiler.
It was an actual cold water dam.

Ben stated that water will always take the route of lease resistance, which often includes water that's a different temperature. In this case, the cold water dam from the different zones all entering at once was preventing the hot water from going past the cold water return manifold. The zone circulators were for the most part, just circulating the same water.
(This is why when the oil boiler was turned on and the water in the main was heated, the cold water dam disappeared allowing the Tarm hot water to flow.

When we called and spoke to a friend who owns the Local Plumbing and Heating supply company, he said this has become a common issue now for people with indoor and outdoor wood boilers the past few years. He's been selling many of the larger circulators, which has solved this problem.

We installed the new Grundfos Circulator, UPS26-99FC. It's also a 3-speed pump. It worked well on low and medium, but we found that on High speed it worked the best when all 6 zones were calling for heat.

Now that the house and zones are back to to temperature, everything is working correctly with the main staying at 190 degrees.

STAY WARM!
 

peakbagger

Minister of Fire
Jul 11, 2008
6,616
Northern NH
Thanks for the update. Yup that was a weird one. I started an upgrade to wood boiler in Maine pre Covid and put in a grundfos. I matched up the specs to the old unit but I will keep it in mind if I have problems.

My guess is hydraulic separator or a buffer tank would cut down on your circulator pump load. They are pretty well standard on new installs (as is storage, Tarm will only sell boilers with storage these days).
 

maple1

Minister of Fire
Sep 15, 2011
10,812
Nova Scotia
I'm thinking there is something that is plumbed differently in your system, than in the diagram you posted? What you posted for the resolution almost makes sense - when 6 zone circs are all pumping, the boiler circ needs to overcome that to get its flow into the load loop. But in the diagram, there is only one circ that the boiler would need to inject into or against - the primary loop circ. If the loads have 6 circs, the boiler wouldn't see the flow of those, they would be separated by the primary loop via the close Ts. Do you have another circ in your system besides the boiler circ, and the 6 load circs?
 

Bryan Hadlock

New Member
Jan 29, 2021
5
Littleton NH
No other circulators in the loop. The two boilers are close together, so the loop is short, which is part of the problem.
It's been a week and the problem has been solved. The boiler is running better than it has in 15 years.

We'll see how it goes this summer, after installing a home made storage tank made from a large propane tank,
I'll be sure to seek out advise prior to getting started.

Stay warm and Covid Free!
 

peakbagger

Minister of Fire
Jul 11, 2008
6,616
Northern NH
If its running well right now, you will be thrilled with a proper storage tank. Its going to cut down your wood usage big time.
 

maple1

Minister of Fire
Sep 15, 2011
10,812
Nova Scotia
No other circulators in the loop. The two boilers are close together, so the loop is short, which is part of the problem.
It's been a week and the problem has been solved. The boiler is running better than it has in 15 years.

We'll see how it goes this summer, after installing a home made storage tank made from a large propane tank,
I'll be sure to seek out advise prior to getting started.

Stay warm and Covid Free!

It does sound like your system is different somehow from the one in your diagram. It has a boiler circ, then each load or zone would have a circ (you have 6), and the primary loop is in between with its own circ. I am thinking your new upsized circ is an overcompensation to account for a system design flow issue when all zones are heating. But if it is making it work, that is what counts. I know my system gets a little out of bounds if all my zones are flowing when I'm burning but that's a little bit different than here - it ends up sending all flow through loads and none through storage so my boiler temps creep up since the loads can't shed as much heat as my boiler can generate. I alleviated it somewhat by now having my 3 speed loading unit set to speed 2 rather than low speed all the time, that seems to pull some cold water out of bottom of storage. It's all one big balancing act with constantly varying things to balance.