Burnham Revolution boiler, Honeywell L8148e aquastat not working on generator power

241comp

We have a Burnham Revolution RV5 boiler, baseboard + radiant floor heating system which has a Honeywell L8148e aquastat. Our house has a 20kw Generac standby generator and automatic transfer switch.

We recently had a power outage for about 2hrs during single-digit weather. When the power went out, the boiler shut down. When the generator came on about 1m later, the boiler did not start up again. I narrowed it down to the switching relay in the aquastat (I think). I measured ~119VAC on the line input (L1, L2) and ~24VAC on the thermostat connection (T, TV) in the aquastat, however the switching relay did not close. Advisable or not, I manually operated the relay with an insulated tool handle and the entire system did power on and operate normally. When I removed the manual assistance from the relay, the system shut off again.

As I was diagnosing this, utility power came back on and the transfer switch automatically switched back. At that time, the aquastat began behaving normally again and the switching relay operated without assistance. Of course, I would really like my boiler to operate on generator power without any manual intervention.

1. Should I expect a Honeywell L8148e to operate normally from modified sine wave 120VAC from a standby generator?
2. Do I possibly have a marginal or bad relay, or is there another possible explanation?
3. Should I replace the aquastat with a new Honeywell or possibly switch to a different brand such as Beckett Aquasmart?

HVACNUT

If the thermostat is set above room temperature, and R-W is closed in the thermostat, then it should read 0 volts at T - TV.
Is the thermostat 24 volt or battery powered?
Try jumping T - TV.

241comp

You're absolutely right, I wrote that wrong. I meant that measured across the 24V circuit (I think B2 is what the manual said to use, but can't recall exactly), both T and TV had 24v - meaning there was 24V into the thermostat and out of the thermostat (thermostat was closed).

HVACNUT

241comp said:

You're absolutely right, I wrote that wrong. I meant that measured across the 24V circuit (I think B2 is what the manual said to use, but can't recall exactly), both T and TV had 24v - meaning there was 24V into the thermostat and out of the thermostat (thermostat was closed).

You should be able to read off of B2 for Common, but W is common where the thermostat wires connect.

Are these wires coming from a thermostat or a zone panel?

If you jump T-TV, does the boiler start?

241comp

I have 5 zones with 5 thermostats, but no zone panel. The thermostat wires in the aquastat connect to a relay that closes when any of the 5 thermostats call for heat.

I'll give jumping T-TV a try when the temps rise a bit (hopefully tomorrow). I don't want to shut things down by changing over to generator power at the moment as it is single-digits here. When I switch over to generator power, are there any other diagnostic steps I should take?

offdutytech

I had a customer with a similar issue when on generator power. We cleaned up the zone valve wires by adding a zone control panel. This reduced the load from the 24VAC aquastat on the boiler. It isolated the loads of the boiler and zone valves. It may would help determine if it's an issue with the boiler controls or zone valves. For the marginal cost of a taco zone control panel it makes trouble shooting much easier. 

241comp

Thanks for the suggestion, I might explore that if my other investigations don't turn up an answer.  The 5 zone valves (and 5 thermostats) are running on 2 externally mounted, phase matched 80VA transformers.  I think this should mean that the load is already isolated from the 24VAC transformer in the Aquastat itself.

241comp

With the warmer weather today and a storm on the way, I decided this is the day to diagnose this problem. I shut off power, allowed the generator to kick in and went to diagnose the boiler. It is powered on and operating properly, under generator power. So I don't appear to be able to reproduce the problem.  Thinking ahead to the next power outage when the problem will inevitably arise again, what can I do to be prepared?  Is there a way to bypass the main relay in the aquastat?  Should I have a spare aquastat on hand so I can swap it out?

pecmsg

If there are any computers on that system, then an Inverter Generator is required for its clean sine wave.

A UPS power supply might clean up that bad power.

WMno57

Inverter Generators don't make a pure or true Sine wave. It's impossible. Inverter generators make a weird polyphase alternating current. Then rectify to direct current. Then invert the DC to the 120/240v 60hz AC your home needs.
Good inverters make an approximation of a true sine wave by having many steps, but it's not a true sine wave. It is a multi stepped square wave.
Your 20KW Generac makes a true sine wave. Your issue may be coming from other devices that are back feeding total harmonic distortion. Try turning off everything else in the house. Does the boiler run now? Be suspicious of florescent lights and computers.
Another thing to have your electrician check is bonding of neutral to ground. I have read anecdotal reports of furnaces and boilers not working when there are bonding and/or ground issues. Neutral should be bonded to ground in one and ONLY ONE place. Without a transfer switch this is done in the main panel. Guessing with a transfer switch, that one and only one place is now the transfer switch, but that is a question for your electrician.
** edit ** If whole house than single bond should probably still be in main panel. Ask your Sparky.
Small portable jobsite generators usually come wired with neutral bonded to ground at the generator. I have read this can cause an issue when there is also a bond in the main panel. There are switches and plugs to eliminate the generator bond. Just remember to reconnect the neutral-ground bond on the generator when camping or tailgating.

241comp

I performed some other maintenance on my boiler in the interim. This included replacing a broken hose clamp on the VS3000 controller's temperature sensor and relocating it to the position specified in the manual, which probably did not have any effect on the generator-power situation. I also used CRC electronics cleaner to clean up all the contacts in the aquastat relay. It no longer buzzes when turned on and this may also be allowing it to operate properly on generator power (just a guess). I just wanted to add that detail here in case anyone comes across this thread in the future, with a similar challenge.

Regarding generator power, my generator is a Guardian 20KW which Generac advertises as a Pure Sine Wave generator with <6% THD, so I don't think that is my problem. None of my electronics have issues with the generator power, including the UPSes on my network and computers. I don't think anything short of an online (double conversion) UPS would actually clean up the power, and even then it may not be any better than what I'm getting direct from my generator. That said, if this happens again, I will try removing all the other loads and re-introducing them one at a time (via the circuit panel) to see if that affects my boiler.

Thanks, everyone, for all the input and suggestions.

EdTheHeaterMan

What model/brand thermostats are you using?
Is it possible that the thermostat(s) or the zone valve transformer(s) were on a circuit that is not fed by the generator? Are the thermostats Smart or Wifi Thermostats?

Since you described a test that we all normally do to see if there is power to the control (manually push the relay clapper in) and the burners and circulator started. Then when you released the clapper the burner stopped. That is an indication that there was no call for heat from the zone valves to T and TV on the aquastat.

You also mentioned that you have zone valves. There is the possibility that the zone valves opened when the power went out and the end switch did not close to power on the T and TV terminals for some reason. I do not believe that the L8124 aquastat relay is your problem. I believe this may be a problem with the built in time delay of the zone valve operation.

How long was the power out?
How long before the generator kicks in?
What thermostats are you using? (some thermostats have a 5 minute anti short cycle feature)

Since you did that test and everything worked as it should, I would let it go until you have a problem again. Then have a jumper wire nearby to place between T and TV to make the circuit. Then you can open zone valves manually if needed in an emergency.

And we can help you at that time if it ever happens again.