How do I ensure low return temps in my mod/con P/S piping?

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Old 10-27-13, 12:38 PM
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How do I ensure low return temps in my mod/con P/S piping?

Hi folks,

I've finally replaced my 300,000 BTU cast iron beast with a low-mass mod/con combi (Navien CH-240). I worked with a few local plumbing and hydronic experts to design and install my system, and while I'm keeping my house warm, I'm definitely running into some short-cycling issues, and the related inefficiencies.

As far as I can tell, the Navien CH-240 boiler circulator runs at 5GPM. I have 8 zones in my house, with a total heat loss of 80,000 BTU. 5 of those zones are controlled by one pump, and the other 3 zones are controlled by a separate pump (current Grundfos 1/25HP UPS15-58FRC).

5 of my zones have a design-day heatloss of less than 10k. In fact, the smallest three are 5.86k, 6.52k, and 6.75k. Small zones, especially when my boiler minimum is 20k, and especially during the shoulder seasons...

When just one of those small zones turns on, I would ideally want a low GPM through the system loop, in order to get a nice 20-degree delta between system supply and return. The problem is, if my system loops is less than 5GPM, my boiler supply will get fed directly back to boiler return (due to P/S piping). I have witnessed this happening.

In fact, I bought a Taco Bumblebee in an effort to ensure a 20-degree delta between system supply and return, but I realize that will probably make my situation even worse when those small zones run. If my system is pumping only 1GPM, and the boiler loops is going at 5PM, 4GPM of my boiler supply will go directly back to my boiler return.

I assume this problem isn't just unique to my CH-240, but would apply to most low-mass mod cons with a fixed-speed boiler circulator piped in P/S. That gives me some hope that there is a solution!

Some options I've considered:
  • Tie some zones together
  • Add a buffer tank

Seems to me like the buffer tank is a last-ditch effort, at least according to the Navien tech I talked to. He pretty much berated me for even considering it, and said in his 30 years of doing boilers he's never needed a buffer tank. Has always been able to solve this by sizing the system circulators properly.

Any suggestions would be greatly appreciated!


P.S.
For the record, I was leaning toward a mod/con boiler with an indirect DHW tanks (Lochinvar Knight) but was convinced by one of the experts to consider the CH-240 (or another tankless combi). I have, of course, second-guessed my decision, but I don't want to dig too deep into that now, since I've already bought and installed the Navien.
 
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Old 10-27-13, 04:09 PM
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The joys of micro-zoning! The buffer tank will work and the larger the tank volume the less effect the micro-zoning will have on the boiler. Unfortunately it will also negate the fast pick up of the Navien boiler in supplying heat quickly to the system. If at all possible combine zones to make this system work properly.
 
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Old 10-27-13, 06:55 PM
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Thanks for the help, Furd.

I have two Taco zone panels...can I combine zones by just connecting the thermostats wires from one area of the house to a few zones on the zone panel? That way two or three zones would be open when that one thermostat is calling for heat. Good idea, bad idea?

If I ended up going with the buffer tank, should I just adjust the speed of my system loops to make sure they don't dip below 5GPM (to eliminate boiler supply from going straight to boiler return)?

Appreciate your insight.
 
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Old 10-27-13, 08:31 PM
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I am not familiar with any of the Taco control panels. Being a controls engineer I always used discrete components to control the various pumps and burner. Trooper is the guy you want for Taco controls.

With a buffer tank I would have a dedicated boiler pump and have that flowing whatever is required by the boiler. The heat exchange between the boiler and the various zones is made in the buffer tank. With this arrangement the boiler pump cycles with the boiler burner and the zone pumps are completely separate control via the thermostats. An aquastat in the buffer tank in conjunction with the zone pump controls would control the boiler/pump.
 
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Old 10-27-13, 08:43 PM
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can I combine zones by just connecting the thermostats wires from one area of the house to a few zones on the zone panel?
You mean paralleling the thermostat hookups on the panel and running several with one thermostat?

I'm not sure if that's a good idea. Darn Taco for not making the schematics of those panels available, I've been wanting to 'reverse engineer' one for years so I can answer this kind of question.

Here is what I know will work though... IF you have Honeywell valves with the endswitch... I don't think it will work if you have Taco valves though... and it will take a little bit of rewiring...

Run your thermostat to the first zone you want to combine. Leave the zone valve wires on 1 & 2 on the panel.

REMOVE the ENDSWITCH wires from 3 & 4 and wire that endswitch to the thermostat input of the second zone you want to control.

If you want to do more than two together, do the same for the second zone valve.

What this will do is set up a 'domino chain'. Thermostat will call first valve to open. When that valve opens, it will call the second valve to open... when that valve opens, it will call the third valve to open... until finally when they are all open, the final zone valve calls the boiler to fire up.
 
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Old 10-27-13, 08:45 PM
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I like to think of a buffer tank as a 'battery' of sorts.

Boiler 'recharges' the battery when the aquastat on the buffer tank tells it to.

Zones run the battery down.
 
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Old 10-28-13, 01:55 AM
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Trooper likes this picture. It is the granddaddy of expansion/buffer tanks. The white tank on the right side a bit above center. When I worked in this facility it had 98,000,000 (98 million) BTUs/hour worth of boilers connected, about 5,000 gallons in the tank at operating conditions and in excess of 80,000 gallons in the entire system. Design operating temperature of 315 degrees F. at 100+ psi.

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Old 10-28-13, 11:23 AM
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Good to see you here, Trooper!

Thanks for helpful explanation on the buffer tank...I finally get it. I was thinking it should be installed "in-line" in my system loop, but now I see how using it as my P/S interface will give me more mass AND full hydraulic separation. The battery analogy was great. I like the domino chain idea for tying zones together. I have Legend zone valves with endswitches, so this should work for me.

Furd, thanks for the buffer tank details. If I understand you right, I think my system would work like this:

When there was a call for heat (the first time), my boiler would heat up the buffer tank (should take 20 minutes or longer, depending on my tank size), then stop firing. As long as one of the zone thermostat was calling for heating, the system circulator(s) would run, regardless of what the boiler was doing. When the temperature in my buffer tank dropped below the low set point, the aquastat in the buffer tank (in conjuction with the zone pump control) would signal the boiler to fire again.

I like the fact that I could then use my variable-speed ECM pumps on the system size, to make sure I get that 20-degree delta between system supply and return. Plus, I still get the luxury of small zones (and precise heat for those zones).

The one issue I see here is how to use my outdoor reset. The Navien CH-240 has the outdoor reset built into the boiler, and I just plug the outdoor component into a plug on the pcb board. This would mean that my buffer tank temperature (and thus my system temperature) would not be controlled by the ODR, if I see that right.

Any ideas on how to set this up so I get the ODR benefits, and still get to use the buffer tank?

Also, what you guys prefer, if this was your house: go with the buffer tank, or tie the zones together?
 
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Old 10-28-13, 04:02 PM
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Yep, that's the way it would work. the interface between the boiler control and the zone pumps is to prevent the boiler from firing just because the buffer temperature drops, it only allows firing to commence IF the zone pump is also running. The boiler would continue to run however after the zone pump stopped to bring the buffer to full temperature. Obviously the larger the buffer volume (greater mass of water) the longer the boiler will run to achieve this temperature.

On many calls for heat, especially from the smaller zones, the stored heat in the buffer would supply the demand without the boiler firing. Of course for this to work best you need a large differential temperature between boiler firing and not firing as well as a large mass of water. This kind of defeats the purpose of having a condensing boiler and also ODR but it does give increased comfort from the micro-zoning as well as reducing the short cycling of the boiler. It also means a longer house warm-up time due to having to heat that large mass of water. There is also the increased capital cost of the additional tank as well as the space requirements. The buffer tank needs to be highly insulated or it becomes a big radiator. The system IS a good one if you have a large home, say in excess of 5,000 square feet and you need to run different areas at different temperatures but for most people it is way too much and the return is simply not there. I only mention it to show what CAN be done.

Now on a somewhat smaller scale, say using an electric water heater for the buffer tank, you could use the ODR and quite possibly keep that boiler in modulating mode for a fair amount of the time. It wouldn't be quite as responsive to short bursts of heat needed as a result of micro-zoning but it would still give you the necessary hydraulic separation from the boiler circuit and the heating zones.
 
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Old 10-31-13, 12:51 PM
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I am trying to learn about these situations as I am a soon to be owner of a new mod/con boiler. One question I have is
If your total heat loss for the whole house is 80K BTU's, why did you get a boiler rated for 200K?

Is there something else I am missing? You say you have issues with short cycles, would a smaller boiler eliminate or reduce that?

Still a Newbie at this so if its obvious, please tell me anyway!!
 
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Old 10-31-13, 04:06 PM
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If your total heat loss for the whole house is 80K BTU's, why did you get a boiler rated for 200K?
In this case, the 'boiler' is actually a 'water heater', an instantaneous one... in order to provide the temperature rise needed, the BTU has to be higher because the water needs to be heated faster.

I think that's the basic explanation.
 
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Old 11-12-13, 02:35 PM
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Yes, NJ, thanks for answering NYresq's question.

This boiler can modulate up to 200k BTU for my domestic water needs, but can modulate down to 20k as well.

There's got to be a downside to that 10:1 ratio...

For the moment, I am going in the direction of connecting the zones together vs installing a buffer tank. Since the temperatures have dropped in Michigan, and I've kept my space-heating water temp at 140, I have not had short-cycling issues lately.
 
 

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