My boiler temperature sometimes overshoots -- today I saw that the thermometer in the hot water supply line (to the baseboard heaters) was reading 230 deg! I spit on my finger and touched the pipe and it sizzled like a hot iron, so it was above boiling. The pressure gauge read about 20 psi, I think the recharge regulator is set for 15 psi. The burner was off, and the temp came down as heating water continued to circulate. A few months ago I was watching the temp when the burner was running and when it got to 220 deg I got nervous and tapped on the aquastat with a screwdriver handle. The burner went off and when I have checked since then, it seems to shut off within a few degrees of the aquastat high setting (190 deg). Until today, that is.

Should I replace the aquastat? How urgent is this?

The boiler is a Buderus and the aquastat is a Honeywell L8124C. There are three circulators on the system, and one of these for the original part of the house is powered from the aquastat. This circuit has three zones each with a thermostat and zone valve. Each zone valve closes a contact to the thermostat input of the L8124C to tell it to turn on the circulator. I don't think this system is part of the problem, but what do I know?

The other two circulators are controlled from relay boxes and thermostats and are not connected to the boiler aquastat.

I have priced the L8124C on the net and found $200 and $160. I haven't checked locally yet.

 

I don't think I'd wait too long to diagnose and repair... since you have a 'triple' aquastat, is it safe to assume that your boiler is also producing domestic hot water? If so, that's a pretty dangerous situation there... expecting 'normal' hot water temperature to come out of the tap and getting blasted with 200 degree hot water... time for a 911 call... the other possibility is that the burner will continue to run and start making steam... and the relief valve on the boiler opens up, system starts to dump it's water, and the burner keeps going... I can envision fairly catastrophic outcome to something like that happening.

It does sound like the aquastat is 'hanging' based on your description.

 

Thanks NJ Trooper. Here's another question. Currently the circulator control on the aquastat is hooked to one of the house heating zones (actually 3 zones with valves on one circulator). So when any of these thermostats call for heat, the circulator starts *and* the oil burner fires. I guess this is to keep the boiler water from going down to the low limit due to the heating load.

It seems to me that the domestic hot water circulator should be hooked here instead, since variations in hot water temp are more noticable than variations in radiator temp. The domestic hw uses an "Aquabank" which is the size and shape of the boiler and serves as the base of the boiler. This is full of boiler water and has a pump to circulate from the boiler. A domestic water coil is inside this. There is a thermostat with a bulb in a well in the aquabank and a relay (I think its a Honeywell R89) to control the circulator. I could connect the domestic hw thermostat and the 120 vac for the circulator to the boiler aquastat (L8124C) and hook up the heating circulator and the zone thermostats to the R89 relay.

My question is, should the domestic hot water be controlled by the L8124C boiler aquastat instead of the heating zone?

By the way, I checked the local price for the L8124C and it is $235 plus tax. So I ordered one from PEX for $160 + $12 shipping. I like to buy local but $65 extra is a bit steep.

 

Actually, this is so that the water is hot enough to heat the home... if the burner didn't fire, where would the heat come from?

Do you have any more info on this 'aquabank'? (manufacturer, model #, etc). You say this tank is full of 'boiler water'? and the domestic coil is inside this tank? Are you sure that the coil in the tank is not where the boiler water circulates and the tank is full of domestic water?

When the aquastat on the 'aquabank' calls for heat, that also fires the burner, is that correct?

So, when these two zones call for heat, they have to wait for the main zone to fire up in order to get any heat. In other words, they are 'piggy backing' the main zone.

One change that may be beneficial would be to set these other two zones up to ALSO call the burner to fire up. You would need different relays and some additional wiring.

I need to understand your system a little better before making any other suggestions... can you take some pictures? If so, you can set up free account at Image hosting, free photo sharing & video sharing at Photobucket and upload there. Drop a link here for us to view the pics.

 

OK, NJ Trooper, to answer some of your questions, not in order.

I looked at the specs and diagram for another aquastat (I think it was an 8148) which is not set up for domestic water heating, and it does not have a "low limit" setting, so if the thermostat is not calling for heat, the boiler eventually cools down to room temperature, so it is important for the burner to fire whenever there is call for heat.

Since the L8124C aquastat is used where there is domestic water supplied, it has to hold the boiler at temperature all the time, even when the house is not calling for heat. Hence there are low limit contacts that turn on the burner when the boiler temp drops below the low setting (this also opens power to the circulator so it doesn't continue to draw heat from the boiler when it is too cold.) When the boiler temperature rises above the low setting plus differential the burner turns off unless the house thermostat is calling for heat. In that case the burner continues to run as long as heat is called for until the high limit is hit. This turns off the burner but the circulator continues to run as long as the thermostat is calling for heat. If the boiler temp falls below the high setting minus the "high" differential, then the burner turns back on if heat is still called for. On the 8124, the high differential is fixed at 10 deg F and the low differential is adjustable from 10 to 25 deg F.

This may be more than you want to know. I can tell you, it took me quite a while to figure this out by reading the Honeywell Installation Instructions over and over.

On my system, the high limit is set for 180, the low limit for 140 and the low diff. for 10 deg. The boiler is a Buderus G115/28.

The Aquabank is a Buderus LT-160 (42 gal). You were right, the tank is full of domestic water and the boiler water runs through a coil heat exchanger inside. The circulator for the heat exchanger is controlled by thermostat with a sensor bulb in the outside of the tank, inside the insulation. This thermostat does not control the burner, so domestic water temperature depends on the boiler temp being maintained by the aquastat. Even if the boiler overheats, the domestic water doesn't since the Aquabank temperature is controlled by the thermostat running the circulator.

As I mentioned, one of the heating circulators is controlled through the 8124 aquastat, so when heat is called for by any of the three zones fed by this circulator, the burner fires and heats the boiler up to the high limit (and sometimes over when it hangs up).

The other circulator feeds only one zone and is controlled by a thermostat through a RA89 relay. This does not fire the boiler but depends on the low limit control to keep the boiler hot enough. This works fine, 140 degree minimum seems to be enough. The application diagrams in the 8124 instructions show how to hook up other circulators so they also fire the burner when they call for heat using R845 relays. I don't think I'll do this since I have no problem with that part of my system function.

My new L8124C aquastat is due to be delivered Monday and I plan to look it over carefully before trying to install it. I can handle the electrical with no problem.

Are there likely to be complications in getting the old one out of the immersion well and installing the new one? I don't want to be without heat with the temp going down near zero around here at night!

Sorry to be so long winded.

 

After you loosen a screw clamp on the bottom of the aquastat, the sensing bulb should slide right out of the immersion well. If not, don't be afraid to use a little force. There's a heat transfer grease (sort of a never-sieze) that should be applied, but some ppl don't, not reading the instructions or knowing what it is.
I've never seen corrosion in the well, as we're talking a brass well, and copper bulb.
With the heat transfer grease, you may be able to lower the lo limit somewhat as it will sense the actual temps better...
You should have a tempering valve on the DHW coil output, right?
Tom Beer 4U2

 

Hardhack, yes, those triple a'stats are somewhat confusing to understand... but I do understand them...

What I don't understand is why you are running a system with a nice indirect water heater and NOT running it 'cold start'. There's no reason whatever to keep the boiler warm 24/7... the indirect should be controlled and wired just as any other heating zone. When the domestic water cools, the aquastat on the indirect should call for heat from the boiler and fire it up to heat the water.

Your system SHOULD be using the 8148 and the appropriate controls to fire the boiler when ANY of the zones call for heat.

It's a waste of energy to run that boiler as a warm start... totally negates the efficiency gains of having the indirect water heater.

Tom, there's no coil... the LT is an indirect...

 

HVAC Mech,
Thanks for the info, I'm sure the installation will go fine.

There is no tempering valve on the domestic hw, but I don't think one is needed since the aquabank is held at a pretty even temp by the thermostat controlling the circulation of boiler water through the heat exchanger. Remember this system is not a tankless coil right in the boiler. The hot water doesn't seem to vary much, and never gets hotter than you can hold your hand in for a few seconds.

 

NJ Trooper,
I just read through the Buderus instruction manual and sure enough it shows a wiring diagram with an L8148A aquastat and three circulators controlled with R845 relays. These are all for heating zones though.

Another diagram is "SP34D providing domestic priority w/ Buderus tanks and boilers". This has a pump for DHW and one for heating, with a tank sensor and a heating t-stat and a L8148 and a SP34D. The circuit shows that the heating and DHW circulators can't run at the same time, and I guess that DHW gets priority. They also indicate that this can be wired for multiple zones.

So, if I were to convert to cold start then I would need an SP34D or some other priority control. I'll have to look into it, since it seems a waste to keep the boiler hot, especially in the summer. A project for the future.

Thanks for your help

TomD

 

The SP34D is simply a setpoint control. It would install in the well of the indirect much the same way as the aquastat that's already in there.

What would probably be your best solution would be to switch to an 8148, and add a Taco (or equivalent) Switching relay ... wire the two thermostats that currently are running the two 'wild' zone circs, and the aquastat for the indirect to it. Wire the two wild circs to the outputs for them... MOVE the wires that are currently going to the boiler aquastat from the zone valves to another 'channel' on the switching relay panel. Wire the 'endswitch' on the SR panel to the boiler aquastat. MOVE the circ that is currently wired to the aquastat C1 and C2 terminals to the same 'channel' as the zone valves are now wired to...

Yeah, a diagram would help... I believe this would work... indirect priority, zone valves would start circ/fire boiler, other two thermostats would start circ/fire boiler... run boiler cold start.