IBC SFC125 combi 2023 install, poor DHW production;

wcs5050

No experience with the brand. Sounds like it never produced DHW properly.

I'm the third guy. Prior outfit said need new boiler, charged for no solution. Second outfit charged more and said shower valves faulty. When hot water arrives at a faucet I measure a raging peak at 160d that quickly dies down, and settles at 106d.

No AM101 or similar mixing valve present. Guy has a wye strainer on inlet cold. Primary pump does shut down when DHW flow detected. Swap of DHW temperatur sensor no luck. Noticed the guy connected supply/return of system backwards to close tee manifold, but poor DHW happens even with heating zones shut down so hydronic siphon through unlikely(?). I have only one call into tech support, and they seem stretched thin.

hot_rod

does the flow rate drop off also, or just temperature?

Most often there is a small stainer in the unit somewhere that plugs. Maybe in multiple locations.

Ironman

Has the heat exchanger been de-scaled?

GGross

Sounds to me like the boiler is set up for high temp space heating? If so you need to have a mixing valve on the domestic side. The water in the units heat exchanger is already hot enough that it overtemps the DHW on a faucet opening. I would get that part done correctly first

wcs5050

Flow rate doesn’t drop, just temperature.

Installed 2023 and never made DHW correctly. I doubt it’s scaled up but worth a cleanse anyway.

Most of their piping diagrams show a mixing valve on the DHW. I’m thinking that initial shot at hydronic temp level is needed for the inertia of regulated tankless DHW production through the heat exchanger. If regulated through a mixing valve it should sustain at 120 in theory. Hate to do the valve and have it not matter but it’s looking like that might be the problem if it relies on that regulation to pick up heat through the heat exchanger.

hot_rod

the unit needs a minimum amount of flow to fire and keep running. A small lav faucet may not be adequate, especially if the aerator is partially plugged. Do it do this with a tub filler running?

kcopp

I have A lot of those out there including a similar version in my own home.

Hot water has been decent.

Have you called tech support?

Where is this install?

You prob need to go into the installer parameters and make dure the settings are proper. Its possible they got switched at some point.

bjohnhy

Make sure there is not a crossover somewhere else in the domestic water that's inadvertently mixing down the temp for DHW.

Does the DHW temp drop to 106 as it comes out of combi boiler? or is it just at the faucet? ie, is the temp coming out of the combi 120(or whatever its set to) and then it gets mixed down by the time it hits the faucet?

wcs5050

As per tech support suggest, I’m test running off the hot side service tap with laundry hose into a bucket with cooking thermometer. Still behaving poorly. Flares at hydronic level 160ish, settles to 106

Framingham, ma.
I’m new to the control, so just getting acquainted. I did get in to change to comfort mode. What else should I be looking at?

Good one bjohnny, but I’m test running dedicated out of the unit bottom.

kcopp

What is the flowrate? 4 gpm is a bout the max.

If the ground water is super cold that will work against you for sure.

Have you gone into the Installer menu or the Advanced menu to look at the settings?

wcs5050

Not sure of the exact gpm measure flow rate but I’m simulating about 1.5 gpm into a bucket off of the service outlet. Measuring 46dF incoming cold, so yes it has a lot of ground to cover.

Next move is looking at installer and advanced menu settings to see if they are default or not. Just getting acquainted with control manual on those.

bjohnhy

https://forum.heatinghelp.com/discussion/comment/1837754#Comment_1837754

That "second outfit" was worried about crossover in the shower valve, presumably.

Unless that service tap isolates the house during your test, it could still be cross over, in theory, through back-flow from the house.

When you valve off the DHW at the combi, and turn on a hot water faucet, does it stop completely or do you get persistent slow flow.

What is the temperature setting for DHW?

kcopp

One different set up with this boiler is to not set the DHW to 140+ and mix it down.

Set the DHW and let the boiler determine the outlet temp. I usually set it at 122F… my own house 120F.

I have the Reps number for that area. I will send it to you if you like.

Teemok

The SFC has a fast acting cold mix down bypass valve intended to prevent the high temperature burst. The valve mixes down output until the output temperature can be managed by the flame rate. The valve closes down and lets the burner control temperature. I'm guessing the valve is stuck in a mid position.

hot_rod

the manual also mentions water pressure below 40 psi can cause scalding?

IMG_9542.png

kcopp

https://forum.heatinghelp.com/discussion/comment/1837835#Comment_1837835

That bypass valve is only on the 199. The 125 does not have that.

Teemok

@kcopp Can't say I've ever seen an sfc125. Thanks for the info. At 125K input flow would need to be limited to say 2.5-2.7 to get a consistent 70 degree rise. Very cold inlet water and or scale could make the max flow much lower to get near 125F supply. A functional mixing valve would be a must to handle the hot HX block heat burst. I hope someone didn't sell this unit as a great DHW generator. A storage tank would make it a better performer but the small combi is usually the result of economy of funds and space choices.

wcs5050

Very true bjohnny… ghost flow through a crossover could be a possibility. That’s an easy test for sure. It was at 140dF, I moved to 120dF and it presents same behavior.

My sales guy offered rep contact but I’ll take it anyway to confirm.

Need to check pressure HR.. we have pressure redux valves from yesteryear on all mains and they can definite fail.

kcopp is correct… no bypass valve visible that I can tell on cold inlet, just flow propeller body that sensor clips to/around.

Good insights Teemok. When I saw max rate of 125k I was a bit surprised. It would perform far better with a storage tank off of these connections as the manual options. House flip, so I’d say budgetary comments are correct. I’d love to put a mixing valve on there to see if that burst can be turned into a choked heat mass than can keep up with the draw.

Teemok

You need a mixing valve set to the desired temperature and most importantly a flow limiter on the hot supply output (post mixing valve if you use one). Maybe the mixing valve alone will limit hot side flow enough. Not sure about that. @hot_rod will know. Tell me your coldest incoming water temp. and I can get give a close flow rate for a desired supply. A low as possible would be best 120F is as low as I'd go.

Edit: The mixing valve alone will not do it. A ball valve set at the right flow rate measured by supply temp with the handle then removed would do it.

wcs5050

I measured cold inlet at 46dF the other day.

Teemok

So 74F degree temperature rise and at a conservative100k btu transfer the flow rate is 2.7gpm 110-115K might be closer to reality but 2.7 or so rate will give some room for colder inlet water with out falling below 120F.

hot_rod

we had a 110 Nobel combi, 102k output, at our last home. It provided plenty of DHW for two of us. Well water running around 52F. Solar pre-heat was added to lessen the load.