Lochinvar KHN-199 DHW to SH Transisition

acruxksa

Greetings,
I've recently purchased a home that came with a new Lochinvar KHN-199 boiler. The entire house is radiant floor heat with an additional manually switched zone to heat the outdoor walkway. 5900 sq. ft. 8 zones (counting the heated walkway). The boiler was installed 2 weeks before we purchased the home because the existing Munchkin Boiler (installed when the house was built in 2007) was unservicable and the previous homeowner had to replace it as part of the purchase deal. In any case, the installers seem to have done a good job with installation but only the basics. I had to purchase and install the outdoor temp sensor myself.

Enough of the background. I've had some short cycling issues. 1362 cycles in 2205hrs of runtime over 3.5 months. They were much worse before the installation of the outdoor temp sensor, even with that addition, they are still happening. I've managed to read some threads on here and have eliminated one major cause. Basically when the boiler would get a call for DHW it would shoot up to 180f and wouldn't modulate down fast enough to keep the temps from going over the 185f offset. I solved this problem by lowering the temp to 175f and increasing the offset to 10f. It now modulates sufficiently to finish with DHW and switch back to SH.

TL;DR:
The issue I have no solution for and need help with is that when the boiler switches from DHW back to SH, it essentially dumps all this hot water into the SH manifold and if there aren't enough zones running, it trips the offset, shuts down (blocked) and and goes into anti-cycle mode for 2 minutes ( Increased it from 1 minute early in this adventure). DHW is set to priority and is connected to the Lochinvar via a honeywell aquastat. Indirect water heater is a Superstor Ultra SSU-80 which says its heat absorbtion rate is "less than 200,000 BTU/HR. If there are 2 or 3 of the 7 zones running, temps seem to normalize fast enough to keep it from tripping, but if no zones or only 1 zone is running it frequently short cycles. To some extent this is zone and temperature dependent, but I feel like it shouldn't be happening at all.

I should also add that I think to some extent, this problem got worse with the addition of the temp sensor and activation of outdoor reset. I've got 0f at 120f set for the low and 60f at 80f set for the high. We've been seeing 5f to 15f temps lately so the boiler has been using 108f to 113f (ballpark) system temps and when the switch from DHW to SH happens system temps rise to the high 120's almost instantly. The house is very tight and has no problem maintaining temps at these settings, but they seem too low to handle the transition.......

Prior to outdoor reset, the installer had simply set the system temp to 125f (probably correct for the design temp of -10f or so) but when I asked him about outdoor reset, he said it was too much trouble to run the sensor to the North or West side of the house........took me about an hour to get it to the West side

Would moving the SH offset from the default of 10f up to 15f be a reasonable solution to this issue?

ced48

You can move it up to 20, the max, and see what happens-

Zman

Some details on how the boiler is piped would help.

It is not your imagination that the cycling got worse when you lowered the temps. Lower temps=less heat output=more boiler cycling.

The settings you are adjusting are band aids. The only way to shorten cycles is to either adjust boiler on off differentials (with settings you are adjusting) or add mass to the systems (buffer tank). The boiler is putting more energy in than radiant is taking out.

NY_Rob

If you're concerned with the inefficient start/stop/start clumsy changeover from DHW mode to spaceheating mode... it happens on other boilers too.
The HTP UFT (mod-con) boilers exhibit exactly the same behavior on switching from 180F DHW mode to 120F spaceheating mode.

There's not really much you can do about it because according to the ODR curve you only need 120F SWT for spaceheating and the SWT is at 180F because of the call for DHW... so the control board shuts down the gas. As soon as the cooler water from your spaceheating loops hit the HX and lower it's temp below the ODR CH setpoint differential the boiler fires up again. It's inefficient and adds needless wear and tear on the hardware.

The manufacturer can correct this with better control software, but that doesn't seem to be a priority with them.

You can extend the DHW "post purge" time to a couple of min or more to try and lower the HX temp and SWT a few deg after the flame is out... but that's about all you can do at this point.
Keep in mind the longer the DHW priority call and the longer the DHW post purge time... the longer you have no spaceheating water circulating.

acruxksa

Here is a video taken a couple days ago after a blower motor replacement. It was before I installed the outdoor temp sensor but should give everyone a good idea on my systems configuration.

https://youtu.be/zRlGuvSSWtE

acruxksa

Thanks for the advice as well. I did go ahead and increase the SH1 offset to 15degrees from 10 and will see how that affects things.

I'm hesitant to increase the post purge time mostly because as mentioned, it also extends the time SH isn't being supplied.

I see that there is an option to change DHW from priority to a zone, does this require additional wiring or configuration? It doesn't sound like a solution because all it would do it force the boiler to bring the entire system up to 140 deg to supply water to the indirect water heater. Is this correct?

Zman

I would get the controls dialed in the best you can and evaluate from there. I suspect you will either need to put in a buffer tank or live with some short cycling.
Looks like a clean installation. The man likes his pumps.....

Brewbeer

What is the diameter of the piping between the boiler and the indirect? Hard to tell form the video.

acruxksa

Brewbeer said:

What is the diameter of the piping between the boiler and the indirect? Hard to tell form the video.

OD is 1.125" so 1" I think?

Zman said:

Looks like a clean installation. The man likes his pumps.....

That was my thought as well when I first stepped foot into the boiler room. In fairness, one of the zone circulators is for a zone that never got hooked up (leak in the line somewhere). It was meant to heat the wall in the master shower. Essentially it's a spare taco pump in case one goes out on a weekend or holiday.

Going to start researching buffer tanks now.

acruxksa

*update* approx 1 month later.
I still have the same issue with hot DHW water dumping into the system and causing the offset to trip which then puts the boiler into anti-cycle. I tried a couple different post purge settings but it really doesn't help, especially since requested SWT's are now in the low 100f to high 90f range with the warmer (20f-30f) winter days. I've come to believe it will be impossible to prevent the system from doing this except on the coldest winter days due to the massive temperature difference between DHW and SWT temps during the spring, summer and fall.

However, I've managed to dial in my outdoor reset further and short cycling seems to be a non issue now. I've taken a couple photo's of the boiler's run time screen and used them to run the numbers. Here's what I got over the past 29 days.

Feb 13th approx. 0900 to Mar 14th approx 0930 (using 29 days)

SH Runtime 2878 - 2296 = 582hrs / 29 days = 20.06hrs / day
SH Cycles 1659 - 1462 = 197 / 29 days = 6.79 cycles / day
AVG Cycle time 20.06 hrs / 6.79 cycles = 2.95hrs per Cycle
DHW Runtime 91 - 32 = 59hrs / 29 days = 2.03hrs / day
DHW Cycles 423 - 267 = 156 / 29 days = 5.38 cycles / day
AVG DHW Cycle Time 2.03 hrs / 5.38 cycles = 22.64 Min per Cycle

Outdoor reset curve is set to 120f water at -15f outdoor temp and 80f water at 60f outdoor temp. Going to push it down to 120f water at -18f since we've had zero issues heating the house and design day here is -18f (although we never got below -10f this winter).

NY_Rob

Great supply temps!

What is the minimum time you can set anti-cycle to? Can you disable it completely?

acruxksa

I can try to disable anti-cycling. It doesn't seem to really be needed anymore for SH since I installed the outdoor reset. It's set to 3 minutes now.

**edit** Looks like there isn't a way to disable anti-cycling. Lowest I can set it to is 1min. Don't see a point in that, so I'm just going to keep it at 3min.

tim smith

I would lean to a buffer tank also as I would guess you have some fairly small zones in house and the lowest firing rate on that boiler is I think around 20,000 btu. 10:1 turndown.

ced48

Why not try limiting the DHW call to a max temp of 160 degrees? It will take longer to compete the cycle, but who cares? It might make the transition without stopping the firing. To avoid the same problem, I limit my own boiler this way.

The boiler will just modulate down, not stop firing-

acruxksa

I am leaning towards a buffer tank eventually. There are a lot of zones, but it is a large house, still, I'm thinking a buffer tank may be the answer. It is a 10/1 turn down and frequently modulates down to %10-%12 (not sure why it shows %11 or %12 but it frequently does). I've run some very rough heat loss calcs that actually show the 199K btu boiler as being about the right size (large house with lots of windows) Honestly though, given how little I've seen the boiler work even at -10f, I'm pretty sure this house could get by with a 155K btu boiler. Just doesn't seem cost effective to swap given it's a nearly new install. Which also helps with the buffer tank argument.

I mentioned a buffer tank to the guy who installed the boiler 5 months ago and he really didn't seem to have anything to say about it. Just kind of went hmmmmm and continued replacing the taco pump that went out 3 weeks ago on the DHW loop.

With respect to the buffer tank, any idea on sizing? Since I wasn't the original owner of the house, I have no idea what each zone consists of in terms of length of runs and head loss etc. and couldn't begin to guess on the size of buffer tank needed.

In the mean time, I may try to lower DHW to 160f. Only issue is that I have 3 teenage kids and a wife as well as 4 showers....... It's rare for more than two to be going at once, but it does happen. I'll lower it tonight and see if I get any complaints in the morning. Although, even at 160f, it's a huge drop to SH temps which are currently at 95f (36 degrees outside)

NY_Rob

Give the 1.5gpm shower heads a try if you haven't already... it could help. I've been using them for years now and we're all very happy with them.

If your kids take 20min showers going from 2.5gpm shower head to 1.5gpm shower head would save you 20gal per shower.

https://www.amazon.com/Niagara-Earth-Massage-Handheld-1-5gpm/dp/B002KTQFXK/ref=sr_1_3?ie=UTF8&qid=1521124888&sr=8-3&keywords=niagara+showerhead