Cadet combi 120 problem with domestic hot water.

kencyr1

I have a customer who has a Cadet Combi 120. It is piped properly, with the kit than can be purchased with the boiler. Low loss header, circulator flanges with ball valves etc. The heating system works fine. When opening the hot side of the kitchen faucet, the boiler gives error codes of n02:fan slowing down because of temperature exceeding 185*F, and n03: fan slowing down because temperature rise exceeding 55*F on heat exchanger.
Could this be a problem with the circulator for the domestic, as the heating works fine? Is it a sensor? When call for domestic, the boiler is surging above 185*F, and the inlet is only 125*F. Could it be the primary pump? It is a B&G nsf-25 3 speed, even though the book calls for a nsf-35.

kencyr1

I don't know why the kit sold with boiler comes with a nsf-25, which is a 1/15 horsepower, when the book calls for a nsf-36, which is a 1/6 horsepower.

hot_rod

there is a flow switch that triggers the DHW hx circ, maybe it is not closing?

How long has it been installed? Could be the HX is getting limed up, it's a fairly small plate hx.

i have this boiler in my house and it performs fine. DHW drops when incoming winter temperature water hits it. i have solar pre-heat and that helps with DHW performance. When the suns out

kencyr1

Been installed 2 years, maybe I will try the flow switch, not sure how to clean the domestic hx

hot_rod

The flow switch triggers the circa pump built into the boiler, I think it's a Grundfos brand. the flow with triggers that and allows the boiler to go to high temperature. sounds like you are goingbto high fire but either not circulating, or not transferring due to a foued HX.

the screen indicates what is going on, it should show DHW call and the temperature rises, fan speeds up.

if that is happening then the flow switch is working, it takes at least .5 gpm to trip that switch. A faucet flowing a trickle may not turn on the whole procedure

Or the DHW pump could be locked up. it's a little tough to get to but finding the wires going to it at the molex plug, use a volt meter to see if it is getting power on a DHW call.

If so, either a such pump, air lock or plugged HX. Just keep narrowing it down.

If it started suddenly, sounds like the pump is not working or air locked.

There is a black manual vent on the boiler, maybe the boiler HX is air locked. Does it make a gurgling sound when running, like air in the pipes?

Cleaning the domestic side of the HX requires circulating a vinegar or mild cleaning acid. Various companies now make kits with a small pump for flushing tankless heaters, Utility is one brand.

you need a couple valves the isolate and get into the HX. Webstone makes a nice isolation valve for flushing.

It'll take some tools and meters to troubleshoot this easily.

kencyr1

Yea, I've purged it completely, no air in the system. I don't have a voltage meter, but I can hear the DHW pump coming on. When I put the sink to half the flow, it can get up to 135*F or so, when on full flow it only gets up to 115, the domestic, that is. The boiler atill surges to 190 or so, which is when the fan slows down and I get n02, fan speed slowing due to temperature exceeding 185.

hot_rod

It should easily keep up with a faucet flow. in fact close to 3 gpm is what it should flow, depending on incoming water temperature. Sounds like a HX de-lime may be required. How hard is your water? How much DHW do you use per day?

When that boiler loop runs 180- 190F it really precipitates minerals out of the water.

Snowmelt

Sounds like the hex like hot rod said.
I do have a question have you called navien tech, there pretty good. They will Guid you threw the whole procedure or should I say sequence of operating for trouble shooting, you must have your volt meter and your gas meter handy.
Where is this job at.

kcopp

Its a Lochinvar...not a Navien.

kencyr1

Im in Massachusetts, my lochinvar dealer sells a cleaning kit for the hx, maybe ill try that. They also said that the boiler pump should not be running on call for hot water, and I think it is, can anyone verify this? Will check that too. Going back there sometime this week to troubleshoot.

hot_rod

That is correct, then boiler or system pump should be wired to the boiler terminal provided for it. If the boiler circ pump is wired via another device, like a relay box, you could in fact be running then heat and DHW pumps at the same time. That would reduce the DHW capacity, no doubt.

This is one of the reasons I have always bugged pump manufacturers to include small indictor lights on the circ pumps, make it easy to see when they are powered. Some models have lights or displays now.

kencyr1

Thank you, hotrod, I'm almost positive the boiler pump is coming on. I'll have to rewire it.

kencyr1

Wasn't the pump. The temperature rise on these boilers is only 70*F, so with incoming water at 45*, had to set it at 115* in order to maintain. Crazy, just doesn't seem practical, especially with only 3 gallons/min.

hot_rod

It's getting narrowed down to a limed HX. This is what they can look like when they lime up.

if you have hard water, from day one you are building up a lime scale. Just like tarter on your teeth

Also a link to a de-lime process and kit from Rectorseal.

https://www.youtube.com/watch?v=VjU6caDOLWk