system flood protection with Heat-Timer vents?

Binnacle

Heat-Timer Varivalve vents seem an attractive option where adjustable venting is desired--but they lack a float and offer no protection against water damage from system flooding.



Do any reasonable options for protecting against flooding at the boiler or mains exist that would enable worry-free use of Heat-Timer?



House is two-family; boiler has a manual make-up valve.  My concern is that one discounts the lack of sense in the population at-large only at severe peril to hardwood flooring.  One business trip or vacation is all it takes.



Have read that an automatic make-up system, if it were installed, is perhaps less trustworthy than a handyman, house sitter or tenant.



Hoffman 1As might do, but they are noisy and of lower capacity than Heat-Timers.  Vent-Rite #1s are definitely too small.  Replacing existing Dole 1As--these are noisy, lack precision, and cannot be adjusted low enough on the boiler+sun-side of the house.



Any ideas much appreciated!



Thanks

BobC

Vents

The varivents have huge capacity that is more suitable to mains than radiators in most instances. Usually you want to vent the steam mains fast and the radiators slowly - how slowly depends on the system layout.



What setting are you using on them? If they are almost wide open (0.66cfm) something is wrong with the system. The Gortons and Maid O Mist's go up to 0.33cfm at the high end of their range and you can get one version of the Maid O Mist that comes with 5 different orifices so you can dial it in easily. Both of these have floats that should seal if there is water in them.



i consider automatic fillers to be the work of the devil, avoid them if you can. If you can't avoid them, make sure the seats of the valve are checked for "leaking by" regularly. Also if one is going to be used get one that has a water meter on it so you will know if it's letting water in.



Bob

Binnacle

Heat-Timer attractive

Have the system mostly balanced with temporary Maid-O-Mist vents and a Gorton #2 at the end of a 38 foot main.  The reason I like the Heat-Timer is that most of the radiators on the first floor are closed as the tenant likes it cool (I like it warm), but if he leaves the next tenant may like it warm.  Then I would be faced with having to replace and/or rotatable fixed Gorton vents to restore balance.



With adjustable vents a rebalance would be quick and easy.



The Heat-Timers have a continuous range adjustment that goes all the way down to where Gorton #4s are, and one troublesome radiator at the end of the 38' main and a twisty 20' riser can use the high venting capacity.  So like I said--it has attractive features.



I suppose I could use Hoffman 1As and hang a Gorton #2 off a big pre-valve tee on the problem radiator (no exposed riser pipe), but the Varivent fits the overall situation better aside from the lack of flood protection.  Have Dole bullet vents now and do not like the clicking and hissing, so I probably won't like Hoffmans.

Binnacle

Maid-O-Mist

Didn't know that a Maid-O-Mist with a full compliment of orifices was available--that could solve it.  But this vent is not as durable (plastic float) as Gortons.



Nevertheless Varivents would be ideal if some form of flood-protection could be applied at the main.

nicholas bonham-carter

The great flood

If you have very generous main venting, and slow radiator vents, the excess water in steam is less likely to end up in the radiators, and spray out of your oversize vents.

Dirty water in the boiler can also cause a lot of water to go up into the radiators. Unfortunately, the oily dirt is floating on the surface of the water, and must be skimmed off. Draining and refilling cannot clean the boiler.--NBC

Binnacle

Murphy's Law

What I'm worried about is someone cluelessly leaving the fill valve open while I'm away on a trip. Or perhaps me doing it when I'm old and senile. Only has to happen once in 30 years to ruin the party with $30k or more worth of damage.



Probably I'll go with Gorton vents but, like I said, the Heat-Timer vent has some nice attributes.

Binnacle

mix 'em up

Or perhaps I'll use Vent-Rite on most of the radiators and put Gorton C's and D's on the long runs. Or put a Gorton #2 on the problem riser. Still have some system improvement to complete before I'll know if that riser will continue to vex.

MDNLansing

Overfill Protection

If the person you rely on to add water isn't observant enough to shut the valve after 3 mins of topping off the water, you might want to find a new person to assist you. If it really is that much of a concern, installing an over fill protection device would be the way to go. Something like a LWCO doing the opposite of it's function. I know that overfill protection is required on some large installations in some states so I'm sure there are off the shelf devices that do this. I've never looked for them, but I would think a Google search would turn something up.

Binnacle

people

Which person?  The point is that people, in general, are unreliable.  Over a long enough span of time who knows what can happen?  Maybe ten years out the house-sitter's dumb-**** teenager is asked to cover (against explicit instructions), leaves the valve slightly cracked and is gone in a mad rush.  A day later and every floor in the house is ruined along with half the furniture and the first-floor ceiling has collapsed in three places.  I get the phone call in the middle of the first vacation I've taken in three years, just as I was starting to relax.



A float valve may not entirely prevent water leaking, but goes a long way toward preventing the worst.



The Heat-Timer only works if the system can withstand the fill-valve being left wide open for a day or two.

JStar

Water

You can also install a self-closing shower valve on the water feed. It needs to be held open in order to pass water, and will spring close when let go.

Binnacle

self-closing valve

Thank you Jstar!



 I like this idea, especially as it does not violate the KISS principal.



Now I'm curious if anyone can shoot down the combination of a self-closing fill-valve and Heat-Timer vents as I scared myself with the doomsday scenario.

MDNLansing

Float Vents

Any float style vent should help at holding water back. As for the varivents, I know a lot of pros here say to avoid them. They tend to vent too fast, even on the lowest setting, for a typical steam system. Gortons are probably the most recommended, but they are bi-metallic vents that won't do much to retain water. Overall though, if you're system was filled and charged with city pressure, you might have bigger problems than the vents. Most original steam systems were assembled without thread tape or sealant and will likely leak under 30 - 40 psi. These cast iron systems just aren't designed for that much pressure. The vents are obviously the path of least resistance and the float style can help with that. If the varivents work on your system, you should have that covered.



JStar has a great idea with the fill valve. That will certainly prevent someone from overfilling by mistake. The combination of the two is probably the best you can do to avoid this scenario.

Binnacle

will test it first

I'll purchase one Heat-Timer and try it on the just-above-the-boiler radiator to see if it's ok.  The data sheet shows the lowest Varivalve setting corresponding to a Gorton #4, so it will be interesting to see if the literature is accurate in that regard.



I find the continuous adjustment appealing since it theoretically should allow tweaking the system into near perfect balance.



Probably I'll pick up a Vent-Rite to test out as well.  Hard to tell from the literature but it looks like the adjustment might be continuous rather than discrete.



Appears to be an excellent idea to install a self-closing feed valve whichever way I go with the vents.



Will update the thread in awhile after experimenting.

nicholas bonham-carter

Testing this, waiting for that

What about testing some Gorton 2's on your mains first, and if I were you, I would just get a modern vaporstat, and use that. There is no sense in putting off these corrections to your system which our experience has shown to be so important in the proper operation of these wonderful old systems. Go to it, and report back here with the results!--NBC

ChrisJ

Varivents

I had 4 varivents but I gave them to my neighbor to use as main vents.

In your situation I would recommend putting together some Danfoss TRVs with Gorton vents.  I only have two of these in use in 2nd floor bedrooms but I already want to install more.  They would allow more even heating all of the time and allow tenants to adjust the heat as they want it without replacing anything.  I'm considering installing two in our kitchen to keep steam from going there when its already hot due to the oven and or stove.



https://lh3.googleusercontent.com/-X8diRnYwBYU/Up1QfeBtBEI/AAAAAAAAB_o/i--m83wGEEg/w997-h748-no/DSC04064.JPG



https://lh4.googleusercontent.com/-WmFrGjaZqyE/Up1Qc538VqI/AAAAAAAAB_Q/JpxQvQvzjfk/w997-h748-no/DSC04065.JPG



https://lh4.googleusercontent.com/-UiagT5tWGzc/UVo5jt-oQgI/AAAAAAAAB7A/plQHyGhTWNA/w997-h748-no/DSC04144.JPG



I have also installed a vent on one of the risers to make the TRV more sensitive.  The Gorton 6 vent gets heat to the 2nd floor fast while the Gorton 4 on the TRV vents the radiator nice and slow giving the TRV time to react.

https://lh6.googleusercontent.com/-tTciYYhnVLA/UVo5i8bBUiI/AAAAAAAABOA/l7tfwlWap2I/w997-h748-no/DSC04142A.jpg

Binnacle

WIP

Per my earlier post in this thread (perhaps worth a read), have the first Gorton #2 in place and the system mostly balanced with a hodge-podge of temporary MoMs and ancient Doles. Pressuretrol cranked down to minimum.



It's at that 85% point where the house is comfy and the gas bill significantly improved.



So this is about the longer-term not-in-a-hurry plan. As I've oft-repeated, the Heat-Timer vents are appealing for a couple of reasons and so I want to try them out. The big worry was flood protection and the solution for that been graciously provided by JStar.



Better pipe insulation, Vaporstat, additonal main venting, etc. are in that plan as well along with vent selection.



Part of the fun with Steam is that substantial room to improvise, customize and do it one's own way comes with the territory.



Ok?

ChrisJ

capacity

Be warned according to the Balancing steam PDF avaliable on this site the Varivalve is closer to a Gorton #5 all the way down.



Absolutely part of the fun is experimenting. I owned 12 Hoffman 1As and replaced them all with Gortons and then kept switching Gortons. I have a ton of extra vents but I keep refining and tuning.

Binnacle

TRV idea

Thank you ChrisJ for your post--I see your experience matches the direction I'm thinking in and I'll consider it carefully.



TRVs are a bit expensive, but the rave review cannot be ignored.



Thanks!

Binnacle

minor epiphany

Ok, so for the 20th time I've run around the house with my smart-phone stop-watch following steam (highly recommended) and comprehension has arrived.  A useful trick is to put one's finger on the bare pipe (if the insulation is not yet 100%) at the first main tee and click off when steam arrives there (not a fuzzy transition--ouch).  Then run down to the end and repeat just before the main vent.  Then click off when the vent closes.  Then head upstairs and time steam arrivals and vent closings.  Most illuminating.



"vent mains fast, radiators slow"



Playing with the worst-case vent adjustment really drives it home.  Past something like a Gorton #5 or #6 setting faster radiator venting gives at best no speed-up of heat more likely retards the rate at which the radiator warms.  Can hear the point where the rate of condensation breaks high and water back-flow disrupts the arrival of steam.  The steam and condensation fighting each other sounds much like a kettle simmering, though much louder.



So now I see why the Heat-Timer gets no respect.  If the minimum setting is roughly equivalent to a Gorton #5 the adjustable range is useless for practical purposes.



My fear of the low-range CFM scale on the Vent-Rite is gone.   That the Vent-Rite maxes out at around a Gorton #5 makes perfect sense.  The middle-room rad with a MoM #5 exhibits sounds of steam and condensate fighting each other, so it can clearly go down.  The rad with a #4 is perfect.



So Vent-Rite it will be, even at $40 a pop.



Plus a Hoffman 75 added to the Gorton #2 on the main since the Hoffman closes at 180 degrees, 50 degrees hotter than the Gorton #2.



The problem-riser gets a Gorton #2.



And the boiler fill valve gets replaced with a self-closing type.  Cheap flood insurance.

ChrisJ

Depends

I have found the size vent you use depends on the radiator, the length and size of piping to it and what pressure you typically run at. The higher the pressure the slower the vent.



Here is a diagram showing my current venting lay out. You'll notice I'm running Gorton Cs on two radiators but mostly 5s. My system seems to typically see around 1 oz or less so fast vents still perform kind of slow, if that makes sense. The venting I am using is the result of constant tweaking for 3 heating seasons now and I'm very happy with the results. Though I have a feeling I will be adding a #6 vent to the 10' runout to my downstairs bathroom radiator. I want heat in there faster and that pipe is raining on my parade. The main problem is most likely that radiator is simply too small for the room and the room is still drafty because I haven't fixed that yet. But the end result is I want more heat in there faster.



Another issue I have is my short main comes off of the header first so those radiators tend to vent quicker which is why two of them have #4s and the main only has a single Gorton #1



I know it seems contradicting that I have Gorton Cs on two radiators but don't like varivents but they just don't seem to work well. They are small so they like to spit and they vent very fast which encourages the spitting. I have also been told by a few guys that they don't seem to last very long.



https://lh3.googleusercontent.com/-04hozo2VJTQ/UpuDzX2_7ZI/AAAAAAAAB-I/X_4P3mmG6L8/w1078-h673-no/ventsGortons.jpg

mcsteamy

In limited defense

A Varivent is essentially a very small thermostatic trap, if you look at Heat-Timer's literature.  Unless it breaks, you can't complain that it doesn't work as advertised.  Properly used, they are handy.



My favorite adjustable vents are the Dole 1A vents, which have a good, linear adjustment range, and a broader ranger over the Vent-Rite.  However, I can't find anyone selling them at a fair price right now.  Cheapest source for an adjustable vent is $17 each for Legend T-60A at Patriot Supply, which is a rebadged Hoffman 1A from the looks of it.  The only problems it that a Hoffman 1A is, has always been, and always will be a piece of junk.  Oh--it adjusts, but the design is awful and the adjustment is a real hassle.  I would probably buy Maid-O-Mist kits before I ever bought another Hoffman.  Except the Hoffman's look a lot better...

ChrisJ

Hoffman 1A

Besides the annoying way they adjust which could be completely avoided I had problems with my Hoffman 1As getting a tiny drop of water in the venting orifice. This caused the radiator to stop heating until I cleared it with a can of air and a straw.





Now in their defense I have had this happen with the Gorton vent, but its very very rare and seems to clear it self after one cycle. The Hoffman 1As it happened a lot and would not clear it self.

Binnacle

minimizing burner time

My understanding of optimum system operation has evolved to this:



* goal is to minimize fuel consumption while providing even heating and a quiet system



* low pressure is key to this goal



* which leads to



   "vent main fast, radiators slow"



Takes my boiler 4 to 8 minutes to start generating steam, depending on how long it has been off.



Then it takes something like 12 to 15 minutes to transfer enough heat to the radiators to complete the cycle, though at present it tends to overshoot due to the not-yet-replaced pressuretrol.



So once the vaporstat is in place and the mains and risers adequately vented, the objective is to have the radiators generate enough back-pressure to allow the vaporstat to shut off the burners as early as possible and let thermal momentum do the work.  I'm thinking that reducing the 12 to 15 minutes for heat transfer is not possible, but for most of that interval the burners can be off.  Right now the burners are active for 100% of the heat cycle.  Perhaps a 35% cost savings can be found here.



For this purpose slow radiator vents are best.  Faster vents do not produce faster heating of the radiators, they only cause the pressure in the system to remain too low to satisfy the pressure control.



Inherited ancient Dole 1A everywhere and the are way too fast in light of the above.  Have the radiator vent that's 60 pipe-feet from the boiler set to 1 and it's about right, and this is only the case after replacing the Dole 1As closer to the boiler with MoM #4s and #5s.



It seems to me that if one can put in a Gorton #2 on a riser instead using a #C or #D on a radiator, it would produce a better result.  Of course that can get a bit ridiculous, which perhaps is why the #C and #D have a place in the world.  Perhaps my max pipe-run is not long enough for the C/D settings to of use.

ChrisJ

Negative

I think you missed something there.

You technically never want to "satisfy the pressurtrol".



My system took 1 hour 20 minutes to build 1.5PSI and cycle the burner from pressure during a 9 degree recovery. It never sees that kind of run time unless something happened like power outage. I typically see around 1oz.



The lower the pressure the better *always* as long as you can maintain even heating at all times. Faster vents when done correctly allow steam to get to (via the runouts) and to fill the radiators faster, to an extent. But too fast is probably just as bad if not worse then too slow. The faster you fill the radiators, the faster your thermostat shuts off.



I have found if I vent a radiator too fast, the steam likes to take a short cut across the bottom and shut the vent. This seems to fill the radiator far less than a slower vent which allows the steam to go across the top and somewhat fill the sections first. Again, the pressure the system runs at, boiler size, piping and radiator size all seem to play a roll on this. I installed Gorton Cs on 3 of my neighbors radiators and all 3 cut across the bottom while the two Cs I'm using on smaller radiators do not. Most likely because my boiler is a lot better matched and allows much lower pressure at all times.

Binnacle

-

Right, but for now I'm stuck with the pressuretrol as the control that turns off the burners.  The thermostat is out by that 60' distant radiator and does not cut-out till after the pressure control.



Once a vaporstat is in place, I'll want it to cut-out on a few ounces of pressure and that requires a little bit of back-pressure from the radiators once the main vents close.  Not planning to move the thermostat--it turns off after about a one degree rise and that's fine.



The boiler is perhaps a bit large for the house, a 200k Weil McLain E-9 from, so all that cast-iron stores ups plenty of energy to keep the water boiling for three more minutes after the burner is off.

Binnacle

-

>You technically never want to "satisfy the pressurtrol".



I disagree and you won't change my mind.  A little pressure helps move steam around and there's nothing wrong with letting the pressure control fulfill it's design purpose.  That said, the "less is more" principle advocated on HH is certainly correct.



>I have found if I vent a radiator too fast, the steam likes to take a short cut across the bottom and shut the vent.



Yes.  Have two radiators with this tendency, though the rest are not susceptible.  Anything over a MoM #5 or #6 triggers it here.



So for my house I see no use for anything above a #6 (as I am willing to install riser vents) and adjustable Vent-Rites should do nicely.

ChrisJ

ok

ok

Binnacle

bi-metal vs diaphragm vents

Just replaced the last Dole 1A bullet vent with a Maid-O-Mist #6 on the furthest-out radiator.



I noticed this earlier, but on the last vent it made a large difference:



The bullet style of vent operates in a binary fashion.  Until the alcohol in the float hits180 degrees and boils, the vent is open.  Then *click* and it's closed.



The bi-metal disk type vents such as Gorton and Maid-O-Mist are variable and close gradually.  So the orifice size / CFM rating primarily matters only until steam starts to reach the vent and it begins to close down.  This last vent started closing a few minutes sooner than the Dole closed, and after about three minutes closed most of the way, at approximately the time the Dole would normally close.



This results in a smoother heating of the radiator and much reduced sounds of steam and condensate back-flow fighting each other.



Also can see the boiler pressure gauge registering a few ounces of pressure sooner than with the bullet vents.  Once the pressuretrol is replaced with a vaporstat this will result in the burners cutting out sooner--a big positive.



So I'll have to try one Vent-Rite to see how it behaves before committing to using them everywhere.  Vent-Rite appears to be a bullet type vent with an alcohol-filled diaphragm capsule inside and so will operate in a binary fashion.  On the other hand the CFM scale of Vent-Rite is much lower and this may result in an outcome similar to what bi-metal vents produce.  Will include a Gorton in the test as well to see how it behaves since Maid-O-Mists are somewhat differently constructed.



If the Gorton bi-metal design results in better system operation than Vent-Rite, I'll have to just put up with swapping valves around if and when the first floor apartment is occupied by someone who wants all the radiators active.

Binnacle

found a nice-looking self-closing valve

To prevent absent-minded folks from flooding the system and water-damaging the house:



"Brass Ball Valve with Spring-Close Lever Handle"



http://www.mcmaster.com/#4088t11/=pwb830



comes in many sizes



http://www.mcmaster.com/#catalog/119/428/=pwbcww

Binnacle

blown radiator valve hiding downstairs

I left working on the first floor apartment till last and got quite a surprise.  Turns out the vent on the bathroom radiator was a junk model from Asia and was busted in the open position.  Was blowing steam like mad.  Tenant seemed to think that was normal--go figure :-)



So fixing that did wonders for the problem riser & radiator on the second floor.  Also causes pressure to build faster and the Pressuretrol to cut out about one minute sooner.  L608A Vaporstat (good one with  a mercury switch) will arrive soon and so it will be interesting to see how it all runs on low pressure.



Also realized that all the risers are a) uninsulated [bad] and b) exposed [good, so I can insulate them].



Might go for insulating this season and open up the three-of-five closed radiators downstairs as the risers will no longer be heating the floor.  Then everything can be balanced now and the desire for adjustable vents reduced to the point where perhaps fixed-sized Gortons will do fine.

Binnacle

or perhaps the classic Ez-Flo style valve

Anyone who has ever showered at a campground will recognize this classic and be reminded of much-too-cold water :-)



search on "Ez-Flo Self-Closing Stop Valve Brass"



Is compact and very hard to jimmy in the open position.



Suspect this is what JStar had in mind.

Hap_Hazzard

Adjustable vents in a rental unit? That's crazy talk!

Just in case you're still considering some kind of adjustable vent in this application, don't.



Tenants are going to fiddle with anything adjustable. If you tell them not to it only means they're not going to admit it. It won't slow them down a bit.

Binnacle

-

1) You post is rather off the thread topic.



2) If you read the thread (which it seems you did not), you would have observed that it's a owner-occupied two-family where there is not much to keep track of and the point made, in addition to being off-topic, is not relevant.

Hap_Hazzard

Well then

I'd just disregard it if I were you.

Binnacle

installed Vaporstat, added Hoffman 75 to main

Managed to obtain an older L608A mercury bulb Vaporstat.  Very nice control.



Started out at 3.5oz cut-in and 7oz out-out (per the Wika 20oz gauge).  This immediately eliminated temperature overshoot and the water-hammer in the further-out radiator.  Overall much better system operation and more comfortable.



However I realized that the furthest radiator (40' of main plus a 20' riser) was a bit short on steam as a result of the low pressure, especially as there is no pipe insulation on the risers and the main is temporarily insulated with pipe wrap.  Also the gas burners were turning off and on four or five times per heat cycle and this could reduce the life of the already ancient millivolt gas valve.  Finally the lower pressure and smaller room temperature rise resulted in a 30 minute reduction in the length of the heat cycle, which despite the reduced burner time still appears to increase fuel consumption somewhat.



So for now I've raised the Vaporstat to 5oz cut-in 11.5 oz cut-out.  This seems to be a reasonable compromise.  The pressure control cuts-out and cuts-in only once per heat cycle and the problem radiator gets a good fill of steam.  Total burner time is about the same and time-per-cycle is back to the original interval.  Of course this results in a modest temperature overshoot, but I'd take that in exchange for a lower gas bill.



Once the pipes are properly insulated I'll have to revisit the pressure settings and vent settings.  Hopefully insulated pipes will result in a much reduced interval between when the boiler starts producing steam and radiators receive meaningful quantities of vapor.  If it's quick enough I may be able to opt for something like a one-hour system cycle with a narrower living-space temperature range for the same cost as the current two-hour cycle.  The five-or-so minutes it currently takes to heat the pipes each cycle is costly.



Was hoping the Hoffman 75 would cause the main to vent a bit longer and reach a higher temperature faster (due to the 50 degree higher close temperature versus the Gorton #2) but it does not seem to have had any real effect.  Timed how long it takes steam to travel from the first tee by the boiler to the end with the pipe open and, at least when cold, the open pipe only vents about ten seconds faster (at one minute thirty seconds) than with the Gorton #2.  If the second vent provides a benefit, it probably will only be when the main is fully insulated and the pipe remains hot between cycles.

Binnacle

Heat-Timer Varivalve--great when the situation calls for it

Per earlier comments above, have a problem radiator at the end of 40 feet of poorly insulated main and another 20 feet of convoluted and uninsualated run-out and riser.



Wanted get it working acceptably in spite of a deferral on the effort to correct pipe insulation.



With the vent orifice on a Maid-o-Mist vent completely removed (i.e. beyond D setting) the radiator would pant sharply as steam advanced and retreated and not heat quickly enough w/r/t the remaining rads.



Swapped in a Varivalve set wide-open and it solved the problem; works beautifully having the rough equivalent of a Hoffman 75 attached to this radiator.  The problem-child now fills about as fast as the others and adding this vent significantly slowed the arrival of steam to the rest and so now they are decently in balance.  The Varivalve operates quietly.



-----



Also tried swapping a Gorton #4 for one of the MoM #4s to see what they are like.  The Maid-o-Mist was quieter than the Gorton.  No doubt that Gortons are of superior quality and durability, but allowing that the Jacobus vents may require replacing sooner, they work just fine.  The difference is not sufficient to compel changing out of the remaining vents before they fail.

BobC

I'm happy with them

I replaced some of my Hoffman 1a's with the MOM's last year and have been happy with them. I like the fact you can buy them as a kit with 5 orifices, it makes experimenting a lot easier.



If you insulate the feed for that one radiator with the VariVent on it you might have to close it down some.



good luck with your system,



Bob

Binnacle

where to buy?

Could you post a link for purchasing those kits?  I looked once but could not find them anywhere.

BobC

Sure

Amazon carries them -



http://www.amazon.com/Maid-O-Mist-0220-5L-Steam-Angle-Radiator/dp/B003DV3AGE/ref=sr_1_1?ie=UTF8&qid=1390180475&sr=8-1&keywords=maid+o+mist



Bob

Binnacle

revised pressure

Had to settle on 5.5 cut-in and 8.5 cut-out as anything higher causes the furthest radiator to experience water-hammer.  Can't get the old L608A to diff less than 3oz, possibly because some of these units have a 3.5oz differential design minimum.