Can Reverse Indirect PSI be different than Boiler PSI?

D107

Recent Install, boiler and autofill gauges PSI used to be about 15. Pressure Reducing Valve is set to about 16. Went from warm start to cold start a few weeks ago--removed low limit from Hydrostat. Just noticed that boiler and autofeed gauge PSI now go from about 18-22 on a call for DHW. I know 22 is not that high, perhaps the cold start increases the pressure swing?

But the Turbomax gauge at top of tank reads 29-30psi--right at the limit of the pressure relief valve, though there's been no discharge or weeping from it. Is that gauge possibly defective--can pressure vary that much between boiler and tank? (There are check valves on both heating and indirect returns if that means anything.) The Caleffi 553 autofeed/prv instructions say:
"When the system is filled, the shut-off valve can be closed. In order to restore the automatic topping-off condition, merely re-open the valve. The pressure in the system will gradually return to the set pressure.”
The Autofeed/PRV is currently open; would closing then re-opening it restore the correct pressure?

Hard to believe the new Turbomax Amtrol Thermxtrol ST-5 (for DHW I believe) or Boiler Amtrol EX-30 expansion tanks are faulty. There's no air in the radiators. The auto air vent on the Turbomax is a little unusual in that it is ordinarily kept closed tight compared to the boiler's Caleffi Discal Air Separator. If tank was airbound would that explain the high reading or is this all just normal for a 50 gallon reverse return tank?

ScottSecor

I have a Turbomax in my own house. Gauge is not the most reliable. In my case the thermometer section of the gauge has been pretty accurate since installation two years ago, however the pressure section of the gauge has been not so accurate.

When I filled the system for the first time it went to almost exactly 12 psi on the gauge (and the boiler gauge) as expected. A few days later the pressure gauge on the Turbomax read zero, yet the boiler still read about 12. Month later the Turbomax reads 12 psi again and has been fine ever since. I cannot explain it, but it works.

One thing to keep in mind is that regardless of what type of heat emitters you have (HWBB, radiators, radiant, etc) you will need a much larger than normal expansion tank for the heating system. In my case I used a Extrol 60 due to the 'enormous' capacity of the Turbomax as compared to the rest of my system. If it were not for the reverse indirect, I could have possibly got away with a Extrol 30 or perhaps a Extrol 15 if there was no cold start ups.

hot_rod

Are you reading static pressure with no circs running? Dynamic pressure, which indicated fluid in motion, would be fill pressure plus whatever the circulator ∆P added.

As Scott mentioned the hydronic expansion tank needs to be sized with the volume of the TurboMax included, it could be you need an additional or larger tank on the boiler side? Amtrol has sizing charts if you can guesstimate the volume of the system, boiler, piping, radiators, indirect.

That is a pretty wide pressure scale on the gauge to read low pressure accurately, a 30 or 60 psi gauge might be a good upgrade

The cap on that Caleffi air vent is a hydroscopic, it has fiber discs that will give you a second shut off seal should something get stuck in the needle valve, a nice option in critical applications, tanks, boilers, etc. It should be tightened down all the way.

Jamie Hall

And on the pressure swing -- yes, a cold start will increase the pressure swing.

D107

@ScottSecor Thanks. Note that this is being used only as indirect not buffer, but I think your comments hold true regardless.

Do you happen to know how long the Turbomax gauge probe is? The Ranco Aquastat probe is situated relatively low in the tank. After trial and error I ended up setting the cut out temp supposedly at 135 (when tank gauge shows about 155) and cut in differential at 15 = probe temp supposedly of 120 when tank gauge shows about 145.

I would feel more secure in these settings if I knew the tank gauge probe was 12 inches or so, so that it wasn't just reading the hottest to part of the tank.

We set dhw mixing valve around 110 and hot water's been fine. So even if aquastat is not calibrated perfectly, we're getting what we need. Seems like with this setup hot water output at tank top will always be above 140--though I think less danger of Legionnaires due to the dhw being only in the coil, not the tank.

@hot_rod I check both dynamic and static pressure--at rest and when there's call for DHW and VR1816 is running. I notice more change in pressure from the temperature increase than from the circ kicking in. During DHW call pressure goes from about 18-22. With 48 gallon turbomax, plus 5 gallon Peerless boiler plus 25? gallons in 10 CI rads and 1 baseboard zone (small 38Kbtu heat loss), piping = about 70-80 gallons total volume not counting expansion tank itself.

So while based on all your advice after I check the sizing charts I'll probably find I need a larger exp tank, it does look like the tank gauge must be inaccurate--that physics-wise it shouldn't be possible for the tank psi to vary that much from boiler to tank. I'm guessing that during the heating season --heat's off now--the pressure swing could be even greater.

ScottSecor

@D107 I did remove mine to inspect it when it gave me funny readings, I am not certain but I seem to recall the probe it is four or five inches long.

In my case, I'm using the Turbomax as a buffer and an reverse indirect with a wall hung condensing boiler. The lower I can keep the tank temperature the more the boiler will condense. With the exception of days (or nights) that the the outside temperature drops below ten or fifteen degrees, my return water temperature stays below 130 - year round.

With regard to heating system expansion tank sizing, it looks like your temperature swings may not be much higher than my system. Just keep in mind that the smaller the temp swing the smaller the expansion tank (within reason). My boiler goes from ambient (70) to about 160 worst case, however my indirect stays between about 125 and 150 year round. I'm guessing your boiler goes from ambient (60?) to 180 on design days, however, your tank temps are probably similar to mine.

D107

@ScottSecor Yes, boiler from 70 to 160. (We'll see if 160 holds up in worst case given piping is all series--old converted gravity.) Indirect swing as I stated above.
So I assume turning autofeed off and on will not correctly reset this, and also that if the tank temp temporarily goes below 125 (temp at probe) Legionnaires is not an issue with a reverse indirect, or less of an issue. If so, I can try to increase the differential between cut out and cut in.

Right now with no demand --and as yet uninsulated piping--dhw will call every 4-6 hours. Total gas use hits about a therm a day which is quite a bit more than i used with just a stand-alone 40kbtu hwh. But I should save quite a bit on heating with a correctly sized boiler. Perhaps expanding dhw differential a little more may help. Should have installed a Turbomax 23 not 45 for my small boiler, but, so be it.

hot_rod

Amtrol shows AX 15 V an 8 gallon capacity tank, a # 30 mag be too small
Use the commercial sizer at Amtrol, it lets you input volume, temperatures, fluid and pressure swing allowed. I used 12- 25psi

D107

At the Amtrol site, I input the numbers for the hydronic exp tank and got the results below in photo, but it's a little tricky since heating water volume is about 30 gallons (boiler + piping) with temps ranging from 80-160--cold start. But DHW water volume is 50 in turbomax plus boiler 5 = 55, temp range 145-155. So i got the same recommendation whether I put in total volume 30 or the whole system volume of 80--for boiler, turbomax and piping.

Since we use DHW priority, heating and DHW won't be on at the same time, so not sure which numbers to use. (Note this is atmospheric boiler) So obviously these commercial units are WAY too costly; I'd have to find the equivalent in the residential series.

ScottSecor

I installed an EX-60 and notice almost no change in pressure under normal circumstances. In my system it may go from a cold start with 12 psi initial fill pressure to 14 or 15 psi once system warms up completely. I realize you old gravity piping and cast iron boiler are much larger than my HWBB loops, radiant loops and condensing boiler, but I suspect you could get away with about the same size (Extrol EX-60 or even an EX-90). If needed, you could always add a second expansion tank. Tons of systems have more than one expansion tank.

Closing off the fill valve does not reduce system pressure. As a matter of fact system pressure can and often will still raise with feed-water closed as the system warms up.

In most cases we are faced with sizing expansion tanks based on worst case scenario. In your case under normal operating conditions boiler water only changes about seventy degrees, maybe only forty degrees during the summer. However, what happens if by chance the pump or flame sensor fails on the coldest day of the year, boiler and system temp drops to 50 degrees overnight and boiler heats to 170 degrees to overcome the load. In my second worst cast scenario, the relief valve would likely start to leak due to the extreme change in temperature of the boiler water.

hot_rod

I’ve seen jobs with 3 tanks headered together, add another or multiple until you get the capacity you need, it doesn’t need to be an expensive commercial tank