Navien very hot at first

[Chrom Artist]

We have a Navien NB-240 with the Heating set at 82C and DHW set at 45C. When we run the hot water, the first minute or two comes out scalding hot. After that, it goes to the set temperature. Even when we lower the DHW temp the first minute is still the same. Any ideas? Thanks!

 

[Dana]

The fact that the primary heat exchanger is at 82C is the almost certainly the problem. It takes a liter or two of water to bring the temperature down to the domestic hot water temperature.

Why is it set to 82C? At that temperature there is ZERO condensing efficiency, running only ~85-87% efficiency. It's minimum fire input is about 18,000 BTU/hr-in, which at 50C-out would be condensing temperatures would deliver 17,000 BTU/hr out but at mid-90s efficiency. At the lower temperature it takes more radiation to emit at that heat rate and avoid short-cycling.

How much radiation, of what type do you have on the system (broken down zone by zone, if mult-zoned)??

 

[Sponsor]

 

[Chrom Artist]

Thanks for your reply. Our rads are slant fin and the lengths are:
Upper #1 16 ft total
Upper #2 17 ft total
Main #16 ft total
Main #2 18 ft total
Lower 9 ft
the house is 2400 sq ft (1000 up, 1000 main, 400 down)

I thought the two systems are separate. It uses water at the heating temp first?

 

[Dana]

At Vancouver's 99% outside design temperature of 24F/-6C most 2x4 framed 2000' insulated houses with 400' of mostly below-grade (?) basement would have a heat load of less than 25,000 BTU/hr, and if it's reasonably air tight and the basement is insulated it'll probably come in under 20,000 BTU/hr, barely above (or possibly less than) the minimum modulated output of the NCB240, which means it never really modulates. If it's cycling, it's important to keep the total number of cycles bounded, which is hard to do at condensing temperatures (or even 82C) with your current zoning scheme.

The very beginning of condensing is~125F (~52C) entering water temperature (EWT) at the boiler, 130F (54C) average water temp (AWT) it's running 88- 90% efficiency. At 130 AWT SlantFin baseboard is putting out about 250BTU/hr per running foot. To pull into the low to mid-90s with an EWT of ~115F (~46C) , AWT 120F (~49C) it's putting out about 200 BTU/hr per running foot.

At 200 BTU/ft-hr a 16'-18' zone is then only delivering about 3200 BTU/hr, a bit less than 20% of the minimum firing rate, which is going to short-cycle like crazy. If the Upper zones 1 & 2 were combined it would still be less than half the min-fire output of the NCB240, and cycling quite a bit. Similarly with Main zones 1 & 2. But if all Upper & Main zones were operated as a single zone they would be emitting 13.4KBTU/hr at an AWT of 120F and 16.75K % 130F AWT, and the cycling will be a LOT less, and in the condensing temperature range.

As it's zoned currently even at 82C/180F output, 170F AWT a single 16' zone is only emitting half the min-fire output (which at that temp is only 16KBTU/hr, since there is no condensing), and cycling quite a bit, but would balance boiler output to radiation pretty well if the zones were combined by floor. Both Upper and Main zones combined it could balance perfectly just above the real condensing zone, but dropping it a few degrees and letting it cycle a little bit to get it into condensing efficiency wouldn't abuse the boiler.

Right now I'm guessing with just a single 16' zone calling for heat even at 82C output the burn times are almost certainly under 2 minutes, maybe even under 1 minute, which takes quite a toll on efficiency (and longevity). That can verified by turning all the thermostats down, turning one way up so that it continuously calls for heat, and timing the burns and observing the duty cycle.

See the service manual, turn to p.70. The heating and hot water sides are isolated with a small water-to-water heat exchanger, but it's only one burner, and one set of primary + secondary burner-to-water heat exchangers putting burner heat into the water. So when only the heating system is running the volume of water in the primary + secondary heat exchangers is at an averag temp of ~75C, and the non-isolated side of the water-to-water heat exchanger is nearly that hot. It takes a few liters of water through the water-to-water heat exchanger to bring store head in all heat exchangers down to the domestic hot water temperature.

 

[Ladiesman217]

We have a Navien NB-240 with the Heating set at 82C and DHW set at 45C. When we run the hot water, the first minute or two comes out scalding hot. After that, it goes to the set temperature. Even when we lower the DHW temp the first minute is still the same. Any ideas? Thanks!

Do you have a thermostatic tempering valve installed on the d hot water?

https://terrylove.com/forums/index....c-mixing-valves-water-heater-tempering.45651/

 

[Dana]

A thermostatic mixing valve would certainly prevent the hot-flash issue, even though it might create a secondary "cold water sandwich" when the initial flow isn't sufficient for firing up the burner, inserting a flue-purge delay. But that delay might already be happening, even with the super-heated initial slug of water. As a moderately priced solution, it works.

The napkin math analysis on the zoning pretty much says that this (like most combi boilers) is a pretty lousy fit for this house with the existing radiation & zoning scheme. Whether tweaking the system design to get some condensing efficiency or reduce cycling losse would be "worth it" isn't clear. But when the thing eventually needs replacing, installing another similar low mass combi boiler would be a mistake. Right now it's only condensing when serving up domestic hot water, but the majority of the fuel use is for space heating.

 

[Chrom Artist]

Thanks so much for your detailed answer, Dana and your suggestion, Ladiesman217. I really appreciate the time it would have taken to make the calculations.

If I understand you, you're saying that the most efficient use of the boiler would be to join all the zones so we can keep the boiler running longer burn times with a lower water temp. This would also help with the hot water issue.

The reason we made the five zones when we set the system up (30 years ago with a standard type of boiler) was so we didn't have to heat the whole house when we were only using one or a few areas and this saved us money. Do you think we'd be better off to do this even if we ended up heating the whole house all the time? I can see it would be using the boiler more efficiently but would it not overall use more energy? There is only myself and my husband in the house these days and heating the whole thing all the time seems counterintuitive.

I will try what you suggested and turn off all zones but one, see what the cycling time is and I will let you know what I find. It may take me a while to get to this as we are in the middle of remodelling our kitchen and redoing the flooring on the whole main floor so everything is pretty upside down right now. It will probably have to wait till we get a little more sanity in the house. In the meantime, I will turn down the boiler temp and see how the temp in the house responds.

Thanks again for your thoughtful answer. I will definitely be back in touch.

 

[Jadnashua]

High temperatures can help with a large setup recovery time, but you may find it more efficient if you ran the boiler at a much lower temperature and kept the temperatures more consistent. Rather than cycling up/down, having a lower, constant heat is more comfortable, and the boiler more reliable and efficient. Ideally the heating system would modulate and provide just the amount of heat to maintain the set temperature in the dwelling. That doesn't happen, but the closer you can get to it, the more efficient and comfort. It gets worse and worse when the heating source is oversized more and more. Even with modulation, most end up oversized.

 

[Ladiesman217]

A thermostatic mixing valve would certainly prevent the hot-flash issue, even though it might create a secondary "cold water sandwich" when the initial flow isn't sufficient for firing up the burner, inserting a flue-purge delay. But that delay might already be happening, even with the super-heated initial slug of water. As a moderately priced solution, it works.

The installation of a tempering mixing valve on a tankless water heater is required by code in my area (MA). Even my parents recently replaced 1960's vintage gas boiler with the obsolete tankless dhw coil had a tempering valve installed in the dhw system.

If the posters system originally had a tempering mixing valve installed, perhaps it is defective now and needs to be replaced.

 

[Dana]

Unless the zone to zone air temperature differences are more than 5C when not heating some zones, there isn't going to be much energy savings. There could even be NEGATIVE savings if running just one or two zones results in short-cycling the boiler at non-condensing temperatures.

 

[Chrom Artist]

The installation of a tempering mixing valve on a tankless water heater is required by code in my area (MA). Even my parents recently replaced 1960's vintage gas boiler with the obsolete tankless dhw coil had a tempering valve installed in the dhw system.

If the posters system originally had a tempering mixing valve installed, perhaps it is defective now and needs to be replaced.

Interesting, Ladiesman. It's not required in BC, though a friend had also suggested it. We are also looking at the other advice of joining all the zones and lowering the temp so it may not be an issue after we do that. Thanks for your input.

 

[Ladiesman217]

Interesting, Ladiesman. It's not required in BC, though a friend had also suggested it. We are also looking at the other advice of joining all the zones and lowering the temp so it may not be an issue after we do that. Thanks for your input.

It does not take much work or any big money to install that thermostatic tempering valve.

The BC plumbing code requires a maximum temperature of 49° C at the tub and shower outlet. That means the water temperature can not be scalding hot for a minute or two.

Section 2.2.10.7

http://www.bcplumbingofficials.com/2012plumbing.pdf