Post purge with closed zone valves.

keyote

I have a problem I noticed then realized its possibly tied with other issues, and i see i'm not the first.
https://forum.heatinghelp.com/discussion/145319/post-purge-of-system-using-zone-valves

So I noticed my KHN 085 runs a post purge which is great on the DHW calls because it dumps the near boiler heat where i want it. I just noticed it also runs it after heating calls,The first problem is this dead heads the pump because my configuration is S/R headers with zone valves, and the valve closes when call is met but before purge has finished. It was suggested that I needn't worry for a short duration and in fact I think i can reduce the time of purge perhaps even to zero. KHN basically said I piped it wrong because the closest configuration in their manual has a DB connecting the Supply and return headers which i did not include.If i recall that was deliberate after consulting with you guys and siggys book, If i recall the reason is that if i used a pressure sensing pump like the alpha or viridian it wasn't needed and Im guessing it would also confuse the pump, if I recall that was also true about using them on the radiant manifolds.

It turns out this is causing another problem. KHN includes a SWT sensor and recommends it be installed on the system piping, but my piping is very short and frankly i doubt that the temp after a run anywhere withing the 6 feet of supply pipe before the zvalves varies by more than a degree or three. However I noticed a subsequent heat call would not begin until until the sensor had cooled off. I could trick it by pulling it from the well for 30 seconds otherwise it takes about 25 minutes. Again KHN said i did that wrong too, that i should have placed the well farther away from the boiler, its true i put the well right after the boiler, but im confident moving it to just before the first zvalve (3') would make no difference. What i think would make a difference is if the post purge were able to dump the heat in the near boiler piping. I thought perhaps I could just remove the sensor from the well because KHN made the comment that with it so close i might "just as well let the boilers internal SWT sensor do the job." This doesnt work Im not clear which sensors the boiler uses for what yet but basically it seems to allow the boiler to get to hot. I would go ahead and move it but as i said 2-3 feet is not going to be any different if the pump cant dissipate the heat in near boiler.

So i was sure this was just me stupidly wiring the zone valve wrong but it turns out it may actually be existing zone controls (im using the taco zone control and taco zone sentry valves) are not actually designed for a closed header system. Interestingly there is also a pre purge but of course the valve being controlled by tstat through zone board has already opened, and it wont prepurge until the temp has fallen.

Interesting after a DHW call when water is even hotter even with a post purge the boiler isnt fussy and will answer immediately an incoming call or waiting heat call.Its only between two heat calls, with a single zone this wouldn't be a problem of course.

Am I wrong is there a simple way through controls to keep valves open?

Am I wrong would a DBP not interfere with a pressure differential pump?

Any other ideas?

I had one idea though its got issue too. instead of a DBP i could pipe a small open zone off the headers that whichever zone opens would also get pumped, not the most efficient idea.

hot_rod

You could add a pressure bypass valve so there is a path for flow when zones are closed

Power the pump from the relay board on a heat call, no post purge.

How long does it post purge?

I wonder what a delta P circ would do. Zone valves close, circ ramps down to no flow condition, post purge runs down the clock with no dead heading.

keyote

Bob Thanks for taking a look at this. I am pretty sure since i can adjust post purge time i could adjust it down to zero or one second so the dead heading now only last i think 30 seconds and as you said isnt to big a deal. But after you said that I realized its connected to the other issue which is the boiler wont answer another call until the system cools down a bit, which the post purge would do great if it were actually circulating the water, in that case i would probably be lengthening the purge to cool it asap for next call.
yes a DBP could be added between the Supply and Return headers, but as i said pretty sure i thought about and discarded the idea because wanting to use a differential pressure pump with my zone valves opening and closing.

Now you say power the pump from the relay board. I need the purge to cool the system prior to a new call. I need a zone valve delay, im sure i could find one at a sky hook shop or maybe when the apprentice is at the fitters shanty getting me a bucket of steam he could get one from them

keyote

I though instead of a PBP that might confuse a pressure pump i could simply have a always open loop/ zone with no valve that gets water whenever the pump is feeding any zone or after the zone closes and the purge continues. If the heat water were always hotter than the dhw it would be neat to use the tank for that but with radiant maybe not always the case.

NY_Rob

When I installed my HTP mod-con it would also dead-head the CH pump (Grundfos Alpha set in constant pressure mode) for 1min after the zone valves closed. It didn't damage the Alpha pump, and I don't have a DP Bypass valve installed either. Last Jan I replaced the boiler controller board (under warranty) and the new board with the newest firmware now had an option to turn off CH post purge.... no more dead-heading.

Are you sure the KHN doesn't have an option to turn off CH PP? It's basically the Cadillac of boilers and has adjustable settings for everything.

That's odd behavior concerning your SWT sensor.
Most mod-cons actually have them built right into the boiler housing supply water piping, so it's not a problem having it close to the source.
As soon as a call for heat starts and the CH pumps starts moving water the SWT will drop down to almost the RWT because the HX in mod-cons hold very little water, new incoming water from the cold CH loops will cool the HX (and the supply water) very quickly.

Maybe you have the "Anti-Cycling" feature enabled and set to 25min- and that's the real reason you're not getting immediate fire on calls for heat?

keyote

Thanks Rob, yes I'm told i can adjust the purge time, so imagine I could set it to 0 or 1 second, but it turns out i actually need it to cool off the system water after a call before another cal can start. I too thought it might be timer between call i could adjust but found that by pulling the sensor out of its well it will cool in about 15 seconds and the call will be answered then not 25 minutes later. I think this is to prevent high limit lockouts, but as you say if it would just be patient and let the water flow a minute it would get cooler return water from the zone. It does have an internal swt senor as well this one comes in a bag and is called the system sensor. I guess with larger P/S loops its a bigger deal, I am piped direct, I tried just leaving it out of the well but that makes it get too hot, Im not totally clear on what it uses the two sensors for but it seems to need them both.

Leon82

You can unplug the system sensor and it will use the outlet instead. The manual has a section about it

keyote

yes but it would still be too hot to start another cycle,

Leon82

It may be because your max setpoint is lower than 180 so it's blocking the heat call

Boon

it would still be too hot to start another cycle,

It won't answer the next call because of the SH offset & differential. You can change them but watch for short cycling after you do. You probably need to take a bigger picture look at your strategy.

keyote

yes it will say setpoint met while its cooling, Im trying to remember which temp it had to get below before it would start the call. It confuses me because i would think it would understand its a different zone calling, Its has a short cycle blocking of 60 seconds before that, I havnt had a short cycling issue Im actually worried the heat loss on tat top floor was way of and when i add the other zones it may be too small a boiler, but youre right Ill have to get neck deep in the whole thing again last year as we got some design day spells i kept upping the swt and i seem to have overidden the outdoor reset bewcause it seems like it ignores the outside temp and just uses the max temp i used when it was really cold.? youre saying its what? sh and diff, if i recall that was an overshoot max temp to avoid a lockout?

Jamie Hall

Might I mention that one purpose of a post purge -- and a pre purge -- if this is on the burner is to make sure the combustion chamber is clear of unburnt fuel when the burner lights off. The objective of this is to avoid unpleasant pressure spikes when the burner lights (aka "boom"). Might want to consider that...

keyote

Thanks Jamie, I guess you mean that in addition to running the water pump its running the combustion fan to clear out the combustion chamber for a precise air gas ignition, I think i did read that happens i doubt if i can change purge duration that purge would be effected i would think only the water pumps run time is effected for safety. everyone seems to ignore that i dont want to eliminate the water purge i want to make it so zone valve is open to accept the purge which seems to not be possible because everyone else uses bypass valves and non pressure pumps or something.

keyote

So could anyone say if connecting the headers with a pressure bypass valve will confuse a differential pressure pump ? or if theres a type of control to keep the valve open till purge is over?

Boon

@NY_Rob

Are you sure the KHN doesn't have an option to turn off CH PP?

It isn't part of the post-purge sequence but you can change the amount of time the system circ runs after the call for heat ends. The minimum is 1 second up to 40 minutes. I have mine set to 30 minutes so that it delivers the heat in the buffer tank after the boiler stops firing. You can also set the delay for a boiler pump and DHW recirc pump.

hot_rod

keyote said:

So could anyone say if connecting the headers with a pressure bypass valve will confuse a differential pressure pump ? or if theres a type of control to keep the valve open till purge is over?

why would you use a PAB with the ∆P circulator?

Leon82

I'm not sure but I think the boiler has terminals for the zone valves.

The taco box doesn't really communicate with the boiler.
I have the Sr box and only the primary pump runs after heat call. The zone pumps shut down once the thermostat is pleased

keyote

@ leon
first i dont have primary secondary im one pump direct to headers. I cant remember if boiler has zv terminalks ill have to look and see what they do, i know the taco zone control was a bit redundant but i had 4 main zones and DHW and was thinking of a couple other possible zones or even actuators so thought it was best to go with i did connect pump to boiler not zone control or maybe its more like pump gets boiler to power zone control box then pump i know boiler doesnt start pump with zone valve. You bring another possibility i cant remember why but i used the boilers "boiler pump " terminal not sytem pump terminal for the single pump i wonder if the other would work better damn it i knew all this last year but am going senile w memrory

keyote

@ hot rod,
why would you use a PAB with the ∆P circulator?

Assuming PAB is what Ive been calling a PBP pressure bypass valve, the answer is I didnt because it seemed redundant, but when i explained the dead heading to KHN they said well thatt cause you piped it wrong" well i said thats not true and looked and saw in the configuration with zone valves on headers and a DHW on its own pump they did show a dbp connecting the supp and ret headers. and i remembered oh yeah i didnt do that on purpose because what you said. but then i thought i probably knew nothing about this post purge and this system temp sensor issue.
the thing with putting that in is I dont think it really solves my issue of needing to dissipate the heat in the near boiler pipes circulating it around 8 feet of pipe isnt going to help it need to make a circuit around the zone cant believe im the only guy in america thats piped direct

keyote

hey boon do you know off hand, i am using my single pump system wired to the 'boiler pump' not 'system pump' terminal.
I forget why i think it comes on first and is always on but am wondering if the system pump stays on for post purge and if so what else would change if i switched terminals.

Boon

>>> am wondering if the system pump stays on for post purge

If the system pump is set to CH (as opposed to WWSD) then it does stay on “post purge,” as you say, depending on the parameters you choose on the Circulating Pumps installer menu. Minimum is 1 second, maximum is 40 minutes. I think the default is 2 minutes.

KHN does not have zone valve terminals.

IIRC, Hatterasguy egged you on to pipe it direct with a ∆T pump.