Thermostat not turning boiler on but running hot water does

Mj93_

I noticed today that my baseboards were cold so I went downstairs to my boiler and decided to look around. I tried purging the air out of the lines and got a little air out but baseboards were still cold. When I went upstairs and turned the thermostat all the way up boiler did not turn on. When you run hot water in sink, shower etc it does turn on though. It has one groundfos circulator and one taco and the groundfos feels extremely hot to the touch but both feel like they are running. There is a box mounted on the side of the boiler which I thought was the transformer but it appears to be wired into the taco circulator and then there’s another box mounted to the domestic coil. Any ideas what could be going on?

109A_5

Hello Mj93_,

Check for loose or broken wires. If I had to guess from here the solder joint(s) that connect the actual relay inside either unit to the circuit board (aquastat (Orange arrow) or the relay box (Red arrow)) have failed. However I would verify that the thermostat is working correctly first since that is probably the easiest to verify with a jumper wire. Since the relay box looks older and crustier than the aquastat I would pick on that if the thermostat is fine. Actual aquastat and relay box part numbers may help to analyze the circuit.

EdTheHeaterMan

You may have an air problem that is not letting the operating circulator pumps from moving the heated water to the radiators.

You may have an electrical control problem that is not powering the circulator pump(s)

You may have defective circulator pumps

To determine which one is the issue, you need a multi meter for the control diagnosis and you need to read the water pressure and temperature to diagnosis the air/water flow problem.

• The first step is to shut off the boiler for a few hours and let the boiler get cold.
• Next. turn all the thermostats down below the room temperature. if you can't do that then disconnect the thermostat wire from the control and the relay
• Then turn on the boiler and the burner should operate until it reaches the low limit temperature. once the burner stops, there should be no heat in any of the pipes past the flow valves (they stop the hot water from leaving the boiler in the summer)
• If there is no heat past the flow valves then set the thermostat for the aquastat control onlt (not the relay control) to call for heat (or reconnect that wire) and feel the pipe past the flow valve. If that gets hot but does not circulate thru the system, then you have an air problem. If that does not get hot then you have a electric problem or air problem.

This is the only test that I can offer you from this wheelchair. If I were onsite then I could do it much faster and move on the the next step. A professional would cost more but it appears that you want to try to DIY this to save $$$

Examples of a Flow Control Valve that is usually located on the supply pipe just above the boiler. There needs to be one for each zone.

The valve on the far right is inside the pump and you will not see it, however the pump should have a model number that ends in IFC it there is one inside the pump. In this case if the circulators are on the supply side of the boiler, the test will work as described above. If the pumps are on the return side, then you need to feel the pipe between the boiler and the circulator ot see if it gets colder once the pump starts. If it does and you don't get heat to return eventually, then you have the air problem.

Mj93_

Thanks for your response. I am going to try what you suggested tonight when I get home. I did see those valves you mentioned on the line too

Lance

Circulators do fail. Flow checks add resistance to flow. Pumps can fail and still run. Also flow checks can stick. Also how the manifolds are piped make a difference. A boiler maintaining temperature will not run unless there is a temperature drop. Your DHW coil does this, your circulators do not? Its unusual to have both circulators fail. But a check at each pump for power when a call for heat will point to the pumps or an air or flow check problem or relay control issue. I am starting to see a fair amount of pumps undersized for flow checks. Does anyone ever check flow and head pressures against the pump curves, or do they size by lowest price?

I can't tell by the pic but many flow problems I have seen were corrected with air elimination auto vents at the drops leading to the pumps. A partial blockage can prevent flow checks from operating with enough flow.

You do not have to have a cold boiler to check this. Actually I prefer a working hot boiler when checking supply vs return temp drops. Volts, amps check at each pump. Document the readings. Low amps means low or little work. Check the pump curve against the head calculation for each circuit. and I hate to say it, as I have seen it wrong, make sure the pumps are in the right direction same as the flow checks.

Jamie Hall

Ah… @Lance , there are an awful lot of folks who have no idea what a pump curve is, nor any idea how to figure a system demand curve…

Lance

Oh but it so important when flow checks are used. A lot of old systems had 1725 rpm B&G 100. Could handle a lot. The new tiny stuff 3450 rpm, makes noise in a properly sized pipe system, but most pipes are oversized so no big deal here, but a flow check can add 20-50-80 ft of head by itself. Mfg. publishes this. All we have to do is look it up. The last diagnostic engineering review I did for a client showed his existing circulator was just not capable of meeting the flow design of his system. The flow check put him over the capacity of his pump. Any degrading or wear on the pump, or shall I say the flow check, which told me it may not feel like opening now no matter how long this pump wants me to. Answered the real cause of the problem. Get the right pump, or remove the flow check, but oops that was not an option. This was a primary secondary system. So he lives with it working when it feels like it. Not enough flow to take the heat off the boiler to keep it from short cycling.

hot_rod

Are you confusing EL, equivalent length with feet of head for Flo control valves?

The numbers here are from B&G for EL of their Flo control valves , the 3/4 is equal to 83’ of 3/4 tube, not 83’ of head

Grundfos pump curves used to show a curve with and without the integral spring check, very little difference in the curves

IMG_9022.png

IMG_9023.png

Lance

Well yeah Hot Rod you got me there. Thanks for catching my mistake. My goal of head calculation simplification gets in the way of my saying I need those EL's to calculate Head.

I also learned that I am fallible and should check my work more often then I do; and if a system is close to failing, it takes very little to cross that line. Especially when allowances are never included. What I do need to know if a check valve is present and what condition it is in. It can skew the diagnosis.

And even if I calculate an existing system perfectly, it still may not work if it is not perfect. Restrictions, contaminates, damages can still be unknown. But at least I'll know the engineering was right. Hence the need for performance testing.