Thermal Stress

Tim_9

I just installed a new Westinghouse WBRU-80W and its working great but I do have one concern. The system is a high mass slab on grade with rwt in the mid 70's. The issue that concerns me is during a DHW call the boiler of course ramps up to 180F. Now if there is a CH call at the same time, once the DHW call is satisfied the CH circ kicks on and 180F water is dumped into the slab and mid 70F water is dumped into the boiler. I can hear the pipes and boiler groaning when this happens, I mean we are talking about a delta T of over 100F. This can't be good for the boiler in the long term, right?

There is a software setting for DHW post purge, but when there is a CH call this setting seems to be ignored. @NY_Rob, I know you also have this boiler. Think this is an issue? any solutions you know of? Thanks in advance...

the_donut

Sounds dangerous. No primary/secondary? Bypass valve closed?

Tim_9

Its piped direct per HTP specs, no bypass valve. It does sound concerning doesn't it? Maybe mix the CH supply? I hate to cut back into the system after I just installed it.

SuperTech

It sounds like it needs either a bypass valve or primary secondary piping, but I would like to hear from @NY_Rob since he has the same boiler.

Zman

Your arrangement is very common in mod con systems.
My Triangle Tube has run that way for 10+ years.
You might check with Westinghouse and see if they have had any issues.

EBEBRATT-Ed

Put a 3 way mixing valve in

Zman

It is probably possible to install a soft start relay on the CH circ.
I don't know a source, but they do exist.

Tim_9

Thanks guys, I appreciate the comments.

@Zman glad to hear you have the same situation without any problems, does help put my mind at ease. Are you a high mass system as well with low return water temps?

@SuperTech, I am also curious to hear what @NY_Rob has to say.

@EBEBRATT-Ed, this was my thought as well if it is a major concern, but if @Zman is not having problems I would rather avoid it.

Tim_9

@Zman, I like this idea since It would be a lot easier than re-piping. The Alpha 2 is already at the lowest setting, but if I could spread the ramp over 5-10 min that would certainly help.

Zman

Yes,
High mass low temp water. No noticeable noise.

DZoro

Are you in need of the full 180 dhw? Or could you get by with turning it down to say 160 or so?
Just a thought, maybe something to try, costs nothing.

Tim_9

Thanks @DZoro, I will give that a try. We rarely have back to back showers and the supply temps only hit 140-150F before the shower ends anyway. It would just take a little longer to bring the indirect back up to temp. Thanks again, going to try it now.

NY_Rob

@Tim... I've seen the same thing on my HTP UFT-80W after a 180F DHW call with a spaceheating call waiting (yes, as you suspected the DHW post purge is ignored if there is a spacheating call waiting). I did lower the DHW temp to 170F to soften the blow of 100F incoming spaceheating water.

I've never heard groaning, etc... coming from the HX during the transition from DHW heating to spacegheating.

My take on it is that after the DHW call the cooler spaceheating water starts flowing through the SS HX- but it's not like we're dumping ice cold water at 25GPM into the HX, it's 70-100F water at 1-5GPM so it's a fairly gentle temp transition. You will notice it actually takes a few min for the SWT to drop below setpoint and fire up the ignition again. The HX has fairly decent mass, I don't think you're shocking it at all.

Maybe you're just hearing the spaceheating pipes expand with the 180F water flowing into them? I do hear that on my system.
You can pull the boiler cover and listen to try better determine if the sounds are actually coming from the HX vs. the near boiler piping.

FWIW- if you do run the boiler w/the cover off for an extended period... you may see a flame sensor error in the error history due to light entering the boiler cabinet. Just clear the error code history at that point if it bothers you.

NY_Rob

BTW- you can enable "step modulation" for DHW too on your boiler.. you may find you satisfy the DHW call before reaching 170-180F. My DHW session most time ends once the SWT hits about 165-168F.

Tim_9

Thanks @NY_Rob, that does put my mind at ease. I knocked the DHW temp down to 165F as you and @DZoro suggested. We will see how it goes. The alpha 2 is only pushing 2.5 GPM so as you said most likely a gentle transition. Appreciate the advice!

NY_Rob

That boiler should give you great service on high mass gradient... what kind of run times are you seeing (when not interrupted by a DHW call)? I'm guessing it runs for hours at a time?

Tim_9

I am in northern California so the temps are pretty mild right now. Daytime highs in the mid 50's or so and mid to low 40's at night. During the day the circs are on pretty much on all the time pushing the heat around the slab from the night before trying to get the delta T down but the boiler never fires as the delta T hangs around 4F (firing delta T set a 8F). At night the boiler fires and runs constantly modulating between 10-20%. Thermostat room temp never varies more than a degree and 80% of the time sits a setpoint (68F) so really pleased with the system so far.

Shoulder season right now, wish i had it in earlier so I could see how it performed at design temp.

NY_Rob

^ sounds like the ideal setup for a mod-con.

Great job!

Tim_9

Yeah...its a polished slab with tile in the bathrooms so pretty ideal in terms of the lowest RWT.

Now for the bad news When I ran the supply/return runs to the supply/return manifold, a 90 degree fitting at the beginning of the run was about 1/8" away from a cold water line. I thought this would be ok since outdoor reset would limit the temp swings and my design SWT was only 83F. Also, one of the holes through the floor joist blocking was a little on the tight side. Again, for the same reason, didn't think much of it.

Of course now that I realize these runs will be seeing over 100F temp swings 4-5 times a day during the winter, I know this is going to be an issue down the road. Its a 35' run and am pretty sure it is expanding more than 1/8" after a DHW call. Same situation with the tight hole, a lot more wear is going on than I assumed. Long story short, it is only a matter of time before I get a leak due to wear. It was a newbie mistake, but I am going to have to tear out some drywall to fix it. Live an learn...