Tankless. How Many Cycles?

[Robenco15]

I have a tankless water heater set up to run as efficiently as possible with the help of users in these forums. I heat my home with hot water baseboard heating.

I was wondering how often my thermostat should cycle per hour though. It takes hours to heat my downstairs. I’d say it takes 3 hours to go from 60 to 66.

I believe it is set to cycle 3 times an hour. Would 5 an hour be better in terms of heating my home more quickly? Would 5 an hour go through propane more quickly and therefore cost me more money?

Now yes my first floor could be better well insulated and I could blow through propane by putting my Navien from 125f to 180f. But the former isn’t possible for a few years and the latter is irresponsible financially. Ignoring all of that, I’m simply asking if 3 cycles an hour is what I should have it set at or is 5 what it should/could be set at.

Thank you!

Edit: For what it’s worth, my thermostat will be set at 70 but the most it’ll get to is 66-68. I figure that is a result of not great insulation, but my thinking is also less cycles an hour means less opportunities an hour to replenish that heat that is being lost.

 

[Dana]

How long are the burn times per cycle during your 3 hour recovery ramp with 125F water? The water temperature makes more of a difference than the number of cycles. Limiting the number of cycles shouldn't be necessary to prevent short-cycling if the system was designed correctly.

If it's a condensing tankless (or is it a combi-boiler?) just keeping the thermostats at 68-70F (no setbacks) and running the lowest possible water temperature that keeps up would be more efficient than setting back to 60F. With setbacks you're trading away combustion efficiency of higher than necessary water temps in exchange for lower heat loss when the room actually drops to 60F. In most cases that's a bad trade.

 

[Sponsor]

 

[Robenco15]

How long are the burn times per cycle during your 3 hour recovery ramp with 125F water? The water temperature makes more of a difference than the number of cycles. Limiting the number of cycles shouldn't be necessary to prevent short-cycling if the system was designed correctly.

If it's a condensing tankless (or is it a combi-boiler?) just keeping the thermostats at 68-70F (no setbacks) and running the lowest possible water temperature that keeps up would be more efficient than setting back to 60F. With setbacks you're trading away combustion efficiency of higher than necessary water temps in exchange for lower heat loss when the room actually drops to 60F. In most cases that's a bad trade.

Not sure how long the burn times are. Sorry.

It’s a Navien tankless condensing heater. You helped me out with it this past March. Since then we’ve had the basement insulated! So you’re saying we should set the thermostats at 68f and never adjust them? I typically turn the thermostats down to 62 when we got upstairs to bed and then up to 68-70 when I come down and go to work the next morning (my wife comes down 2 or 3 hours later with our newborn son so by then it is warmer).

 

[Dana]

Yes turn the thermostats to 68F and just leave them, and do NOT limit the number of calls for heat, but measure the burn times. Then start bumping back the water temperature 5F at a time until the thing starts short-cycling (burn times less than 3 minutes, way more than 5 burns per hour), or until it isn't able to keep the place warm.


With 125F out, 115F back the tankless is hitting the low 90s for combustion efficiency. At 115F out/105F back it'll be in the mid to high 90s. Run it at 100F during milder weather if it's still keeping up and not short-cycling, and only bump it up as-needed as the season grows colder:

How much radiation is there on each zone, and which model Navien?

 

[Robenco15]

This is my thread from awhile ago. I think it answers your questions. https://terrylove.com/forums/index.php?threads/home-heating-tips-to-save-money.74555/

How do I go about measuring the burn times? Sit in the basement and watch the heater?

You say to not limit the calls for heat. The thermostats are set at 3 cycles an hour. You’re saying I should adjust that or just let it be. This is all confusing to me. Sorry it takes multiple explanations for it to make sense for me.

 

[Dana]

The CH 210 is a combi boiler, not a tankless.

To measure the burn times, yes turn the thermostats down on all zones except the one your testing and stand in front of the boiler with a watch and a notebook to mark the times (to the nearest 5 seconds, if you can.)

Without re-reading the whole 3 pages thread it looks like you have (or had) four zones:

Zone 1 - 30.25'
Zone 2 - 23.5'
Zone 3 - 40'
Zone 4 - 50.5'

Is that still the case? Did you combing any of them? Which zone(s) is the first floor?

At an average water temp of 110F (115F out, 105F return) you get about 150 BTU/hr per running foot of length out of the baseboard. So that 23 footer (zone 2) would only emit ~3500 BTU/hr while the boiler is putting it's minimum output of ~19,000 BTU/hr into the zone. That's about 15,000BTU/hr or (/60= ) 250 BTU per minute of extra heat going in that isn't being emitted. There's probably something like 15-20 lbs of water (~2-2.5 gallons) in that loop + boiler (could be less, could be more, but YOU should figure out the total water volume in your zone plumbing + baseboards to run this analysis). Let's be optimistic and say it's 20lbs. With 250BTU/minute divided by 20lbs the temperature of the water in the zone rises by 12.5 degrees per minute. Assuming the boiler is set up to turn off at 10F above the setpoint and not restart until it's 10F below the setpoint you have 20 degrees of window to work with, which would be a burn time of 20/12.5= 1.6 minutes, or about 95-100 seconds. That's a pretty short cycle.

If the heat loss of that zone at design temperature is say 5000 BTU/hr the boiler would operate at a 5000/19,000= ~25% duty cycle, so out of the 3600 seconds in an hour you'd have 900 seconds of burn, which would be 9 cycles, which is a lot of cycles. When the heat load is lower than that it would be fewer cycles.

At twice the length of baseboard it's twice the heat emitted and probably twice the thermal mass, so you'd have only 12,000 BTU/hr (200 BTU/minute) going into 40 lbs of water and the burn times would grow to more than 3 minutes, and with twice the heat load the duty cycle would be 50%, not 25%. That would still be about 9 burns per hour but much longer burns.

Taking some burn time measurements on each zone separately (give it at least 2-3 burns per zone, with that T-stat cranked way up, and the others turned way down) would be enough to tell how well behaved the system is going to be when set to 115F (or how well behaved it currently is a 125F.)

Ideally with 19,000 BTU/hr minimum fire the smallest zone would have about 100' of baseboard, or at least 75'. With 75'-100' of baseboard the amount of excess is lower, the burns much longer, duty-cycle is higher, and the likelihood of calls for heat from the zones overlapping goes way up.

Limiting the cycles/call for heat with the thermostat doesn't save you from 1.5 minute burns, it just means it takes a higher water temperature and longer time to satisfy the thermostat. It will still take the same number of burn cycles before the thermostat is satisfied- limiting it at the thermostat just interrupts it so that it takes longer.

If you're spending $3K /year on propane it's probably going to be cost effective to install a cold climate ductless mini-split heat pump to heat a major zone of the house. A 3/4 ton Fujitsu -9RLS3H could probably heat your whole house during the shoulder seasons, and most of the house during cold weather at less than half the typical price of propane. In my area that mini-split would be installed for about $3500 sometimes less.

What is your current propane prices & electricity rates?

 

[Robenco15]

$1.94 a gallon. Got a fixed rate. Hopefully between that and adjusting the temperature to 125F that should cut the cost down to $2k a year.

 

[Dana]

Skimming your prior thread, it looks like you had an implied heat load of ~45,000 BTU/hr, and with the available radiation would need a water temp of 140F at design condition. Depending on the individual zone load/radiation ratio it might still need to be that high or higher when it's really cold out, even if it can be heated with 120F water most of the season.

But the simple answer to the question in this thread is DON'T limit the cycles per hour using the thermostat- it buys you nothing.

And if it doesn't short-cycle at lower water temperatures don't bother with setbacks- lower the water temperature for higher combustion efficiency.

Setting up the outdoor temperature sensor and programming it to automatically raise and lower the temperature in response to outdoor conditions is a cheap investment in time & money that is well "worth it". But run the burn timing tests on the smaller zones first. Short cycling the boiler can severely cut into efficiency (and longevity).

Combining zones to suppress short cycling to enable operation low temperatures may also be "worth it".

Spending the money on a cold climate ductless mini-split (or two), may also be "worth it".

In a southern New England location a Fujitsu 9RLS3H will average its nameplate HSPF 14.0, which means it delivers 14,000BTU per kwh of power use. So normalizing to a million BTUs (MMBTU) it takes 1,000,000/14,000 = 71 kwh/MMBTU. At CT's state average 21 cents/kwh (yours might be higher or lower) that costs ~$15/MMBTU.

Assuming you can run it cool enough to get the 91% AFUE out of the combi boiler, every gallon of propane puts 0.91 x 91,600= 83,356 BTU into the heating system. Normalizing to MMBTU that's 1,000,000/83,356= 12 gallons of propane. At the recent CT price average of ~$3/gallon that's costing you $36/MMBTU.

That is more than twice the cost of heat supplied by the mini-split.

The capacity of a Fujitsu -9RLS3H at +5F (probably cooler than your 99% outside design temperature) is 15,000 BTU/hr, which is about a third of the whole heat load. At +17F (cooler than your wintertime average temp) it's 16,000 BTU/hr. At +47F it's good for 22,000 BTU/hr, which is more than the whole house heat load at +47F. (Using the analysis in response #8 from last year your load is about 14-15,000 BTU/hr @ 47F.) Depending on where you put it single 9RLS3 could cut your propane use by more than half, and your total heating bill (electricity + propane) by about a third. Feel free to crank it up to put some heat into adjacent zones too. Most people won't complain about 75F heat in one room to achieve 68-70F in another. The more heat that gets supplied by the mini-split, the lower the overall heating cost.

 

[Dana]

$1.94 a gallon. Got a fixed rate. Hopefully between that and adjusting the temperature to 125F that should cut the cost down to $2k a year.

Looks like we posted at the same time.

At $2/gallon the cost of heating with the boiler is ~$24/MMBTU, which is still substantially more than heating with a better-class mini-split, but not 2x "only" 1.6x.

If you buy the outdoor temperature sensor, try setting the curve at K=3 as your first shot, under return water temperature control. See the discussion/instructions starting on page 47. If it's keeping up and not short-cycling too much, drop it to K=2.5.

Do you have the NR-10P remote installed (necessary for setting the K-factors)?