Electric Boiler Short Cycling

cormierd

Hi All,

I have a Dettson Hydra III 20kW electric boiler. My house has 6 zones with a zone valve setup with a Taco 0011-f4-2ifc. The house is new, it was completed in 2021. It has 2000 sqft of living space and the garage has 1200 sqft.

Inside the house, I have 1 large zone with 5 loops and 4 small bedroom zones. For the attached garage, I have another large zone with 6 loops.

The way I'm setup is a have my thermostats with floor sensors and I keep the floor temperature constant so it's comfortable under our feet. And I have a heatpump with an air handler with a thermostat that monitors the room air temperature and makes up the difference. So it's a pretty nice setup.

However, the problem I have is the boiler short cycles when only the small zones are calling. When it kicks on, all 4 elements kick on one by one with 2-3 seconds delay each, and it then overshoots the setpoint very quickly and then they all shut off completely. The pump keeps on going while this happens. Then the water cools down and then it kicks back on with all 4 elements and overshoots quickly and so on. It does this cycle every 3-4 minutes roughly, until the thermostat is satisfied.

This doesn't seem like it's working at peak efficiency. The way I see it, I feel the boiler should recognize when the internal temp gets close to the setpoint and modulate down accordingly before it overshoots and settle with 1 or 2 elements to keep heating constantly instead of short cycling like this.

But is this a boiler issue, or is it an install issue? What do you guys think?

hot_rod

It seems like the modulating logic is not working correctly that it would keep stepping up under low load. A good question for the factory tech support. Does it modulate just on time delay, or by measuring temperature rise?

Ideally on low load it would stay on stage one and not overshoot. So it could be a control glitch, or the time period between steps is too short.
A 20K boiler with 4 steps would be 5K per element, or 17,000 btu per hour.

Any idea what the heatload is for the home? 20K is 68,000 btu/hr. Seems high for the house load. Without the garage load it could be quite a bit oversized.

It would be nice to have the low firing rate match your smallest load to allow for long run cycles. In some cases buffer tanks are added to lengthen cycles.

cormierd

I'm not sure how to calculate the heatload, but I'm in New Brunswick, Canada and in January and February the average outdoor temp hovers around 16*F. This time of year, the outdoor temps hover around 32*F. My house is new, it's 1 year old.

I actually called the manufactuer this morning, and the guy I talked to told me it was perfectly normal. I explained thoroughly what it was doing, and his explanation was that it's a small zone so the boiler senses cold water so it ramps up all the elements quickly but it reaches the setpoint faster than the elements can modulate down. So I talked to him a while and explained everything and questioned him a bunch, but he assured me that it was normal behaviour for that machine. So either that's how it works, or the guy I was talking to didn't know what he was talking about.

Could it be that the way the boiler works, just can't handle a small zone very well? I had a guy come over and check things out and he said a possible solution would be to install a bypass valve between the supply and return lines. I'm not sure the details of that, he just mentioned it quickly, but he said it would let some hot water go to the return line and the boiler should run more smoothly. Does that make any sense?

I thought maybe it could be a flow issue. Would changing the circulator be an option? Or is the design/install just not the best?

I'm just not sure what to do with all this, I've got a nice system, but it just doesn't seem to be working like it should. I mean it works, the floors are warm and right at the temp I got it set at, but it doesn't seem to be doing it very efficiently.

Hot_water_fan

Looking at the manual, it seems like a bad control logic. Ideally, it should let you stay in stage 1 for a customizable time, then step up to 2, etc. You should be able to lock out some stages entirely, but it doesn't seem able to do that. 

Also, the minimum flow rate is very high at almost 7gpm. 

The boiler is clearly very oversized at 20kw with another heating source. 

That said, beyond the overshooting temperature, this doesn't really impact efficiency. 

GGross

@Hot_water_fan

Technically electric boilers are always 100% efficient! No flue attached to them


From the manual
"The control activates the electrical elements. There is
a delay before turning on (2 minutes max.) or off (30
seconds max.) each individual element. When the set
point is reached, the number of activated elements will
be adjusted in order to maintain the water temperature,
while minimizing the on/off cycling. This allows for a
better longevity of all components and energy savings."


Now that being said if you are very oversized for the load of the smallest zone this won't function as well as it does in writing. I would try to verify that the control is behaving in this way, work with tech support to make sure there is nothing defective.

I would have to say that off-hand it sounds like you may have a micro-load zone, and your best bet may be a buffer tank. If each element is equal that puts your lowest modulation at 17,000 BTUH, what is the square foot of your smallest heating zone?

cormierd

Thanks for the insight guys. I really appreciate the help.

The smallest zone is an ensuite bathroom and it is 124 sqft.

The boiler does modulate. Like when the larger zones kick on, it stabilizes and modulates to whatever it needs, but the timing of the modulation might be off is what you're saying I think.

Is there a disadvantage to have an oversized boiler? I've always felt that because it's electric, whatever energy goes into heating the water gets distributed to the house regardless of boiler inefficiency or short cycling or whatever, so even if I'd have a perfectly dialed-in system, the amount of kW used wouldn't change. Is that a correct assumption?

There's a setting on this boiler to set it at 50% capacity, so that never more than 2 elements will come on. Do you think that could be an option?

What about that bypass valve I mentioned in an earlier comment above? Would there be merit in looking into that option?

hot_rod

Short cycling is both annoying and shortens the life of the relays. Try the 50% setting. You could also lower the operating temperature to lengthen the run time

If it has outdoor reset option in the control, use that also 

Hot_water_fan

Is there a disadvantage to have an oversized boiler? I've always felt that because it's electric, whatever energy goes into heating the water gets distributed to the house regardless of boiler inefficiency or short cycling or whatever, so even if I'd have a perfectly dialed-in system, the amount of kW used wouldn't change. Is that a correct assumption?

The boiler efficiency isn’t the main issue - it’s that if the boiler overshoots the floor temp, then the heat pump supplies less of the heat. So you’re paying 3x as much for that overshoot, even if the boiler efficiency hasn’t changed. 

What about that bypass valve I mentioned in an earlier comment above? Would there be merit in looking into that option?

This is more likely related to the minimum flow requirement of the boiler - when small zones are on, the boiler is probably not getting the minimum flow it needs. That smallest zone probably uses 5-10% of the minimum flow needed. That might be causing it to cycle as well. 

Since the floor heating is more of a supplement, you might consider just combining all the zones in the house. 

pecmsg

Tie the smaller zone T-stats 2, 3 or 4 in series creating a larger zone. 

cormierd

Hot_water_fan said:

The boiler efficiency isn’t the main issue - it’s that if the boiler overshoots the floor temp, then the heat pump supplies less of the heat. So you’re paying 3x as much for that overshoot, even if the boiler efficiency hasn’t changed. 

It's not the floor temp that overshoots, it's the boiler setpoint. I have floor sensors and the floor temp is pretty much bang on.

I had thought of it before but as was mentioned above, it really does look like there's not enough flow going through the boiler in those conditions.

I understand the idea of linking all zones together, but the ensuite zone I like it warmer than the rest so the ceramic tiles are always nice and warm under the feet, so I'd like to figure out something to get that to work properly without combining it with the rest.

I can try the 50% setting to see how it works but I'm afraid there won't be enough power to maintain the 2 large zones. But then again, maybe 10kW is plenty to supply the entire house and the garage from what you guys are saying here. What do you think?

So if it's a flow issue, is there something that can be done about that? Would a variable speed circulator make any difference in my case? I'm no expert, so this might not make any sense. I don't really understand the flow/circulator/zones thing. If the circulator always runs at a constant speed, wouldn't the flow stay the same regardless of which zones are on or off? Can someone enlighten me on how that works?

Hot_water_fan

So if it's a flow issue, is there something that can be done about that? Would a variable speed circulator make any difference in my case? I'm no expert, so this might not make any sense. I don't really understand the flow/circulator/zones thing. If the circulator always runs at a constant speed, wouldn't the flow stay the same regardless of which zones are on or off? Can someone enlighten me on how that works?

Nope, as the zones close, you’re trying to force more flow through a tube that’s not changing in diameter. The resistance might be high enough that the circulator can’t reach the minimum flow. That’s what the bypass is for - it basically keeps the minimum flow by creating an additional loop. But that doesn’t solve the oversizing - so it should still short cycle.  

cormierd

Hot_water_fan said:

Nope, as the zones close, you’re trying to force more flow through a tube that’s not changing in diameter. The resistance might be high enough that the circulator can’t reach the minimum flow. That’s what the bypass is for - it basically keeps the minimum flow by creating an additional loop. But that doesn’t solve the oversizing - so it should still short cycle.  

Yup, it all clicked. It all makes sense now, thanks.

Ok, so if I understand correctly, with a bypass, the flow through the boiler should improve drastically, right? If the system can maintain the boiler's minimum required flow, wouldn't that help with the short cycling?

The way I see it, if the flow is where it should be, the heat that's produced by the boiler should be able to "flow out" closer to the speed that it was designed to "flow out", so the internal temp shouldn't increase so quickly and it might be able to modulate properly. Am I on the right track here?

ratio

The minimum flow has to do with carrying the heat produced away to "make room" for more heat. A bypass will carry that warmed water back around & run it though again—so the heat hasn't actually left. You'll still short cycle.

hot_rod

At the  the 50% setting, it should modulate down to 5k, one element firing, that should cover lo load and not require much gpm?

cormierd

@ratio I see what you're saying. So what is usually done for a micro zone (124 sqft) like in my case?

@hot_rod I tried a few things this afternoon. I set the boiler at 50% and it still short cycles the micro zone even with just one element firing. I then shut that zone off and turned on the garage only to see what it would do, and at 50% it fires 2 elements, but it won't bring the water up to temp. The setpoint with the ODR today was 96* (it's mild out today) and the internal temp hovered around 85*. So I put it back to 100% and after a few minutes it went up and settled and was hovering around the 96* like it should with 3 elements firing. This zone is my garage and it's 1200 sqft.

So from what I gather, I'll need to run it at 100% for the 2 larger zones to work properly.

What do you think is the solution to get my system fine-tuned and operating as smooth as possible?

cormierd

Here's the zoning drawing for my house in case that can help anything. The ensuite zone is the one at the far left.

cormierd

The boiler is located in a mechanical room in the upper left corner of the garage where Manifold 7 is.

ratio

If it were me, I'd swap out the contactor on stage 1 for an SCR & use a modulating controller on it for partial load conditions. You'd be able to add just the right amount of heat to the water, which would help even out the heat compared to on/off operation.

Hot_water_fan

@hot_rod I tried a few things this afternoon. I set the boiler at 50% and it still short cycles the micro zone even with just one element firing. I then shut that zone off and turned on the garage only to see what it would do, and at 50% it fires 2 elements, but it won't bring the water up to temp. The setpoint with the ODR today was 96* (it's mild out today) and the internal temp hovered around 85*. So I put it back to 100% and after a few minutes it went up and settled and was hovering around the 96* like it should with 3 elements firing. This zone is my garage and it's 1200 sqft.

Try 50% again - it should run a long time, that's what you want here - it should get to 95 eventually. With the garage and large house zone running for longer, the odds of the smallest zones overlapping with these zones increases.

cormierd

Hot_water_fan said:

Try 50% again - it should run a long time, that's what you want here - it should get to 95 eventually. With the garage and large house zone running for longer, the odds of the smallest zones overlapping with these zones increases.

Ok, so how should I set my thermostats for this to run like it should? They're Rehau Dual-Sensing Thermostats. I have them set to sense the floor only. But I can adjust the cycle time and the proportional band. Right now I get the proportional band set at the tightest it lets me at 1.8*F. Is that correct? But what would be the ideal cycle time? And should all the zones be set the same? The large zones are on "instant zone valves" (not sure the term for those), but the other small zones are on thermal actuators.

Dave Carpentier

Im trying a thing this year with my 2 very short zones.
Disconnected the short zone end-switch wires so that the short zones cannot instigate a heat call on their own. The room tstats still command the actuators to open, but they have to wait until any other zone opens and turns it's end switch on to start the circulator.. and then heat flows into both (or more).
Those 2 small rooms wont overheat, because the tstats will close the valves.
But they might underheat while they wait for any other zone call.
So far, no underheating. The thermal mass of the slab holds enough heat, and with 5 other connected zones cycling.. it seems to work nicely.

hot_rod

A cold garage slab will take all that boiler can supply, maybe for days to catch up from a cold start or even a 5 degree increase will take all the boiler input.

Here is the math involved in concrete mass. If you measure the size of the garage calculate cubic' and see that to raise 1 cubic foot °

If your slab is 4" thick you have 378 cubic feet

You would be best to maintain the garage, of course that will cost $$. But turning it up even a few degrees will hammer that 20K boiler.

At some point either get a true modulating type electric boiler that varies instead of steps. Modulating boilers use triac relays that vary output. Or add a buffer if you want to get rid of short cycling.

Can you hear the relays click in an out? The older electric boilers that used contacters could be very noisy. That and light dimming were the biggest complaints.

Dave Carpentier

Have you checked what kind of relays it uses, and if they're commonly available ?
If the only risk is potential relay lifespan, and they're cheap and easily replaceable, maybe just live with it ?