Differential pressure bypass valve in a variable speed system

Harold

I am rebuilding my heating system and I have a question about using a bypass valve. As currently piped, the secondary loop has a bypass valve between source and return. And a dumb circulator pump way oversized. I understand why this bypass would be used. But does it make sense to include this when the circulation pump in the secondary loop is a smart pump that adjusts pressure and flow.

In this situation I can see some sort of high pressure overload relief valve. But not a reaction to a range of normal variable pressure conditions. It could be a bypass valve with sufficient range. But any relief pressure across the secondary less than the maximum possible generated system head presented, would circumvent the intelligent operation of the pump. Allowing flow through the bypass at various pressures could confuse the pump and cause it to run at high pressure when it should not.

Does it make sense to put in a bypass valve in this circumstance. I have not yet looked for a bypass with the "proper" range. Or is there an alternative? I am also not precisely sure if the pump will deal with everything without a bypass. The pump is an AquaMotion AM55.

An explanation of how things should work in this situation would be greatly appreciated.

Ironman

A bypass is not necessary with a delta P circulator.

Harold

Thanks Ironman,
I am still a bit confused.
I did some further researching after I posted here. The pump vendor said there should be a bypass. The boiler manufacturer requires one for a system that has zone valves.

The secondary circulation pump is, I believe, not going to operate as a delta P. Which I believe you assumed to be Delta P. I should have provided more information initially. The pump is a AquaMotion AM55. It has a set of fixed speed settings and a set of variable speed settings. I intend to use the variable speed mode at one of the available settings.

If I understand correctly, in the variable speed mode, the pump will follow an efficiency optimized curve based on demand. Not attempting to maintain a fixed pressure.

I am seeing some mutually exclusive requirements. Again, if I understand. I believe that the bypass is to be a relief for over pressure. To be functional, I believe I need to put a bypass valve at the end of the secondary loop, that can be set to some value a bit above the highest pressure the pump can provide.

Any bypass setting not greater than the peak pump pressure would interfere with the variable speed pressure output control. And the bypass should be a bi-stable device. Off for anything up to just above max pump pressure, open for greater than that pressure. Thus safety is provided with no impact on operation.

Have I gotten this right?

hot_rod

If the boiler is the type that requires a certain min. flow rate, primary secondary or a loss loss header is a better solution compared to a PAB, or bypass pipe.

A PAB is intended to "shed" excessive pump output as zone valves close, for example. You set the "threshold" start to open mode, in the PAB, generally with all ZVs open, no bypass. As valves start closing down the valve opens and starts bypassing a % back to the return.

I don't think a PAB and delta P circulator would get along well?

Jamie Hall

As always with a control system, you need to step back and start with what, exactly, are you trying to do?

As @hot rod says, if the boiler requires a certain minimum flow, that is non-negotiable. You have to provide that -- but that is not pressure dependent, nor can it be achieved easily never mind efficiently with an on/off valve or pump arrangement. Again, as he said, the best way to do that is primary/secondary or low loss header, and a dedicated pump sized correctly for the boiler flow. If the boiler can modulate, that pump needs to be controlled -- probably by temperature -- to manage that.

It should not be necessary to provide a pressure relief bypass if the pump is selected even remotely correctly. It shouldn't be capable of providing enough delta head to break anything. However, some pumps don't like being operated at zero or low flow, since they depend on the moving water for cooling and lubrication. There are several approaches to this, but providing a bypass -- though it works -- set at some pressure below the shutoff head of the pump is not particularly efficient.

Figure out what you are trying to do first.

hot_rod

One rule of thumb is to use a PAB with 4 or more zone valves on a single circulator. Or a real flat curve circulator, the old B&G 100 were ideal for zone valves.

The PAB keeps the circ happy, but also prevents over flowing a zone, which could lead to excessive velocity, noise and wear.

A PAB basically tries to make the pump curve flat.

If you have a long, large or uninsulated header, connect the PAB right after the circ, to eliminate always flowing to the end of the header, if only the first few ZVs open.

Harold

I just can not seem to explain this properly. My question is (to the best of my understanding) unrelated to anything except the secondary loop. That is the only portion of the system I am addressing.

There is a pump for the boiler and the primary loop. A pump for DHW. A pump for feeding the secondary loop via an 80 gallon buffer tank. None of these are going to get a bypass. And then the pump I am discussing: the secondary loop circulator. To the best of my understanding, this is the only pump involved with a bypass whose intent is to protect from all the zone valves closing together. I believe this is where the pump manufacturer wants a bypass. The boiler manufacturer is not completely clear, but I do not think they are asking for a bypass anywhere other than the secondary loop. They do specifically reference zone valves. But in this configuration, they should not be involved anywhere other than the secondary loop. I could be wrong on this one.

I posted the above while another contributor submitted information. I am attaching the curves for the secondary pump. The buffer tank pump is the same model pump. The operating mode for these pumps will be variable speed.

I think the responses so far are not considering the function of the adaptive flow pump. What is being said about bypass does not seem to me to be consistent with the system design and selected components.

hot_rod

But if all zone valves close, the circulator should stop. The PAB type bypass is to keep the pump from over pumping when single or small zones are open only. Nothing more, nothing less.

A simple pipe between supply and return would also do this but in an un-predictiable and maybe not ideal way.

A delta P circulator properly adjusted, takes away all these issues and gives you at least a 50% reduction in energy consumption.

Harold

I agree. If the zones are all closed, the distributed intelligence in the boiler and assorted parts should simply turn off the secondary pump. That has always been my understanding when designing the system. My understanding is that the bypass is a safety element. It should not be involved in modulating flow in the designed system.

There are other places in the system (other than the secondary loop) that could cause strange results if the associated pumps do not turn off when necessary. But there does not seem to be any similar considerations for these other pumps. At least partially, I believe, they just do not get the opportunity to hurt anything.

Jamie Hall

True. The bypass would be a safety element. So is the pressure relief valve. However... If we look at it that way, what are you trying to protect against? The pump which it is protecting should be selected to match the system; it really should not be able to produce a pressure high enough to damage the system. So... what you are protecting against is damage to the pump if it is operating under a no flow situation (dead headed). Your electrical controls should prevent that...

hot_rod

A primary loop pump, a boiler loop pump, or an indirect pump would have no way to dead head, they are all loops.

The zone valves are on/ off switches for flow. Either the pump needs to shut down, have a bypass, have ∆P intelligence, or dead head when all zone valves are closed.

As you mentioned, end switches on the ZV, a zone relay board or the boiler itself have logic to stop the cir when all heat calls go away.

Which boiler manufacturer is suggesting a bypass for zone valve piping?

Ironman

What's controlling the speed of the pump? Delta P? Delta T? The modulation of the boiler control?

Again, as stated, the end switches of the zone valves should be wired in so that if no zone is calling, the boiler and pumps stop. Unless the the pump is delta P, then that's not necessary.

Harold

The posts while I am typing have made some things confusing. The stuff below was written before the last couple of posts.

The secondary circuit pump can generate 21 ft of head. Using the variable speed curves; about 14 ft. Look at the attachment above.

I fully agree that the electronics at multiple levels should not allow anything to damage the system. I view the issue of bypass to be unnecessary.

The manufacturer of the secondary loop pump recommends the bypass. The manufacturer of the boiler says I must use a bypass if zones are involved. On the boiler requirement, I don't believe that the requirement is actually related to a primary/secondary installation. But I don't know. I intend to simply ignore any reference to bypass until I get to the secondary loop.

In the secondary loop, the pump should not be able to fail and cause a problem. If it went to full speed, it should not be able to damage the system. And the assorted electronics in the boiler, Tekmar, and pumps should defeat any attempt by the system to blow up the house. My opinion is that the bypass is a useless vestige of old technology. And has no place in a modern system. At best is seems like a CYA.

But I am trying to resolve the multiple issues. Default in my mind is no bypass. It just feels like a useless/stupid idea.

Re: the post above mine. It is an existing system. Tekmar deals with the valves. There are no end switches. The valves are all simple 24 VAC from the Tekmar system. It has been working fine for 10 years. With the exception of multiple boiler failures. I am not sure how much the existing boiler has been involved in anything except making water hot. Nor am I actually sure it has been modulating. I am currently having an issue with the soon to be replaced boiler complaining of overheating and stopping. After 10 years. I will be happy to see it go and start over. And boilers are much smarter now. Unfortunately I have a 12 year old Tekmar system managing the system. They should be smarter now as well. Unfortunately they changed the number of wires to the thermostats. Otherwise I might update the whole system. May or may not be an issue.