Unzoning hot water heating system

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Mhmmofro

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I have a boiler controlled by a 4 zone taco relay box. Zones are 1st fl, 2nd fl, basement and domestic hot. I have been home sick this week and its been cold out so i have been able to observe the boiler working under its hardest conditions.

I am constantly hearing the boiler fire for one the zones shut and then only a few minutes later have to fire for a another of the zones. This is a single stage 85% efficient boiler.

I am thinking on days like this week when people are home all day it may make sense to have the boiler operate all of the zones off of one thermostat.

My plan is as follows:

Splice the neutrals for all of the thermostats together.

Take the hot from the main 1st floor thermostat and take 3 pigtails off of it.

The first pig tail will re attach to the hot size of the 1st floor relay box.

The second pig tail will hit a switch then a diode (to prevent back feeding the first floor zone when we want more heat in the 2nd floor) then splice it into the hot coming from the basement thermostat and head for the relay box.

The third pig tail will hit a switch then a diode (to prevent back feeding the first floor zone when we want more heat in the basement) then splice it into the hot coming from the 2nd floor thermostat and head for the relay box. .

My questions are as follows:
1) Will this work to achieve my heating control desired?
2) What size diode do i need? I will probably use a led so it can double as an indicator light.
3) Any idea if this will improve efficiency? I am thinking that at times like when we are on vacation during the winter it deff will.

TIA
Mark
 

DonL

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You are working with AC not DC.

So using a Diode, will not work in your application.


Good Luck on your project.
 

Dana

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The heat loss characteristics of a basement are nothing like those of an attic, and running it all as a single zone will not result in higher comfort or efficiency. With one thermostat on the first floor odds are you'll over-heat the basement & upstairs, but you can give it a shot.

For multi-zoned systems that don't have enough radiation to balance with the output of the boiler it's often better to let all of the radiation sip from a central insulated buffer tank, and slave the boiler to an aquastat on the buffer:


Buffer.jpg


The thermal mass of the tank and the temperature swing of the buffer's aquastat controls defines a minimum burn time for the boiler. The boiler is agnostic of the state of the zone thermostats. The buffer introduces some standby loss, but the standby loss is usually a lot less than short-cycling losses, and WAY less wear & tear on the boiler.

From the boiler control you'd then have two zones- your indirect hot water heater, and the buffer tank. The space heating zone controls would then only conrol the radiation circulation pump(s) & valves.

If you're not sure how the control scheme works with your existing controller, mid-winter isn't the right time to just start hacking at it.

You may be able to tame the beast with a $200 retrofit economizer control like the Intellicon 3250HW+ (you can find them cheaper elsewhere if you search) which purges heat from the boiler down to a programmed low temp on any new calls for heat before firing, and anticipates the end of a call for heat based on the behavior of recent burns. The exercises whatever thermal mass you have on the zone & boiler over a wider temperature swing to lengthen the burns, and reduce the number of burns. How well it works depends a bit on your zone radiation, the thermal mass of the boiler, and the degree of oversizing you have between boiler input and radiation output on each zone.
 

Tom Sawyer

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Every time I see a low mass boiler coupled to a high mass storage tank I kinda scratch my head and wonder why they didn't just leave the old high mass boiler there in the first place.
 

Dana

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Every time I see a low mass boiler coupled to a high mass storage tank I kinda scratch my head and wonder why they didn't just leave the old high mass boiler there in the first place.

There are a few reasons for that.

It sounds like the system here isn't a mod-con if it's an 85% efficiency burner, and it probably IS "... the old high mass boiler ...", and yet it's short cycling on a 3 zone + indirect heating system. (There's an outside chance that it's a finned water tube type 85% efficiency boiler.) That means that the "...high mass..." isn't nearly massive enough, or the high/low differential hysteresis it's being operated at is too small.

Typical cast iron boilers that are right-sized for the load from a burner size point of view don't have sufficient thermal mass to avoid short cycling on multi-zoned systems with insufficient low-mass radiation on the zones to deliver the output of the boiler.

If you micro-zone the hell out of it the problem is only made worse, since the radiation per-zone is now even smaller.

HTPs self-buffering modulating-condensing boilers & combi-systems are the only truly high-mass boilers I'm aware of being sold in the US. The Energy Kinetics System 2000 non-modulating steel boilers purge heat into buffer tanks to limit short-cycling losses, but they too can be set up to short-cycle.

Adding heat-purging economizer such as a 3250 HW+ onto an "... old high mass boiler ..." system that's short cycling will probably tame the problem, but would be woefully inadequate for managing those systems where every room is it's own zone. For multi/micro-zoned systems with low-mass radiation it's usually cheaper (especially in system retrofits) to add a buffering thermal mass in the form of a tank rather than adding sufficient radiation to each zone to quell the short-cycling problem.

Scratch the simple math model of the system on a napkin, not your head- it's easier to read later! :) But the old high mass boilers are more often that not insufficiently massive for multi-zoned low mass radiation.

Typical design practices of the past sized the radiation for the whole house to match the boiler's output at 180F, which means once it's split into two zones it's guaranteed to cycle, and the zone-by zone napkin-math will tell you just how much. If you're averaging 5+ minutes per burn and fewer than 5 burns per hours it's probably not worth doing more than adding an economizer. If all burns are shorter than 3 minutes and it's averaging 10+ burns per hour it's an efficiency and wear & tear disaster in progress. It may or may not be treatable with a heat purging control, but it's still worth a shot before reconfiguring the system with massive buffers.
 
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