Post Purge Questions +- Mixing valve. Boiler is short cycling.

okayfine

Long time lurker. First time home owner. My current set up (picture attached):

1. Oil fired boiler
2. Indirect Water Heater
3. Taco SR504 switching relay
4. 4 zones. 1 baseboard, 2 in-floor radiant, and Indirect water heater on priority. The baseboard and the in-floor radiant are very long runs, and they only turn on when the outside temperature is below 15 degrees because I also have a air-to-air heat-pumps x 2.

My manifold had a spare zone with a pump so I added a 5th zone as part of bathroom remodel. This bathroom had no heat whatsoever. This 5th zone is now in-floor radiant heat with 1/2 pex pipes. 

Since I didn't have extra zones on Taco relay, I disconnected one of the upstairs baseboard zone and wired the new zone to that. (The upstairs baseboard rarely gets used since I also have a separate heatpump). The oil heat only runs when the outside temp is below 15 degrees. 

All was running well, but I noticed that the supply temp from boiler was exceeding 200 degrees with the aquastat set to 180 and 140. 200 is at the limit of pex specs so I dialed the aquastat down to 160 and 120, and now the supply temp goes to about 180 after boiler is done firing up.

The problem is that the boiler is short cycling. I noticed some gas smell intermittently in garage where the oil boiler is. The is also some tiny amount of soot that is being ejected from flue. 

So I figured I'd add a new expandable relay (SR503EXP) that has a post purge function so that, after the boiler aquastat is satisfied, the relay will cycle on the pump for the indirect water heater, to dump the excess heat into the water tank. With this, I was hoping to set the aquastat back to 180 as well. Adding the new relay will also let me re-connect the disconnected upstairs zone. 

Speaking to Taco tech support, they say that the purge function only works on the last priority zone. Meaning, when the indirect calls for heat, the relay will run the pump for a set duration after the boiler is done cycling. It will NOT post-purge on the indirect zone when OTHER zones call for heat.

In summary, my problems are:

1. Boiler is short cycling.
2. I would supply temps on the pex pipes to stay below 180.

My questions are:

1. Are there other post purge controllers that will let me dump excess heat into the water heater?
2.  I could also add a mixing valve to lower the supply temps to something lower. The existing zones are really long runs and seem to have been working fine with high temperatures (I bought the house about a year ago). So do I add the valve just for the new zone (see 2nd picture)? The new zone is the one with the orange pex. Or should I add the mixing valve for the entire manifold, which would include the indirect?

hot_rod

I wonder about the boiler size to the heat load of the home? Performing a heat load calc would tell you that.

Those small, what we call micro zones, will not get along well with a boiler like that. If the radiant is infloor or below floor you do need to mix down from the 180.

UltraFin is about the only product that could run that hot.

There are some work arounds like post purging the boiler, maybe into the indirect. Combine some zones? Or electronic fixes like a forced time delay between when the boiler can fire. A buffer tank is a more expensive fix.

All of these would be custom built solutions, with hydronic or time delay relays.

Rule of thumb is at least a 10 minute run cycle every time the boiler fires

As often as we see this issue, @EdTheHeaterMan should diagram up a relay solution

jesmed1

I'm just a homeowner/engineer, but I will second what @hot_rod implied about that boiler probably being way oversized for your application.

From the depth dimension of the boiler in your photo, it appears to be the same size that we have (Weil McLain WGO-5). That boiler is able to heat our entire 4-unit condo building of 4800 sq ft in the Boston suburbs.

Given that your house is (1) probably smaller than 4800 sq ft, (2) also has heat pumps doing most of the heating, and now (3) has micro-zones with low flow and low radiation, that boiler is probably running massively inefficiently.

If it were me, my first order of business would be doing the heat load calculation as hot rod suggested, and replacing that monster with something more appropriate. Even Weil McLain's smallest WGO-2 is probably oversized since you also use heat pumps, but it would be a step in the right direction.

EdTheHeaterMan

The first thing that JUMPS out at me is the defective Barometric Draft control and how it is not properly installed.

Get a new 7" RC Barometric Draft Control and a new 7" 26 or 24 gauge adjustable 90° elbow and install it properly.

Now what do you want for the control system? some type of post purge for any residual heat from heating a space heating zone to be dumped into the indirect tank? Tha may cause the indirect tank to get over heated at some point. Why don't you want the residual heat to be put into the last zone that was calling? I can see that only that one might over heat by a degree or two. and if another zone calls before the purge is over, then that post purge will stop and the calling zone will use any residual heat. That makes more sense to me.

Also holding back on the Micro-Zone for 10 minutes in hopes that a second zone may also call for heat.

okayfine

@EdTheHeaterMan I don't know anything about Barometric Draft Controls. I guess another thing for me to learn! I'll look into this now.

Ed, you said, "Why don't you want the residual heat to be put into the last zone that was calling? I can see that only that one might over heat by a degree or two. and if another zone calls before the purge is over, then that post purge will stop and the calling zone will use any residual heat. That makes more sense to me."

The reason is, most of the time, the only zone that's calling for heat is the new "micro-zone" which is the pex radiant for the 1st floor bathroom and the Indirect water heater. The thermostats are programmed to turn off the heatpumps when outside temperatures are below 15 degrees. At that point, the house is only heated by the boiler, but this happens only about 20 days of the year. Rest of the time, heatpumps are heating the house, and the boiler is called upon only for the bathroom.

I haven't done heat load calculations, but I suspect the boiler is appropriately sized for heating the whole home when it is the only heat-source (heat-pumps off), because when the thermostat is set to above 70 degrees, the house temp doesn't rise above 70. We have it set at 68-69, and the house is very comfortable at that temperature, and the boiler cycles normally.

This bathroom microzone did not have any heating whatsoever. The original house was built in 1976, and has a crawlspace which is heated by other zones, but the bathroom (about 170 square feet), is located in an Addition which was built around 2000, and it had baseboards. Oil boiler was the only heating source until the previous owner added heat-pumps 2 years ago. We bought the house about a year ago. Strangely, the baseboards were connected to the in-floor radiant heat for the Master-Bedroom that's directly above this bathroom. So the only way to heat this bathroom was to turn on the Master-Bedroom infloor heat, but since the Master was already warm from heat-pumps, the thermostat never called for heat. If we jack up the oil-heat thermostat, then the Master would get really hot. So this bathroom was always cold all last winter.

So when we remodeled bathroom, it seemed like a good idea to use the unused manifold to run radiant heat. But as @jesmed1 and @hot_rod said, it's a micro-zone, and the boiler is massively over-sized for it. The problem with replacing it with a smaller boiler is that the current boiler is appropriately sized when it's called upon to heat the whole house.

So, is there any reason why I shouldn't try adding a mixing valve to this micro-zone???

techforlife

I would repair that broken draft regulator. May have busted the stop with the burner running rough causing odors.

hot_rod

you could also measure the amount of fin tube and see how it matches the boiler size

Figure 500 btu/ft as a realistic number for a boiler at 180

I agree with adding a mix valve for the radiant. How warm does the floor get now?

EdTheHeaterMan

When the Microzone (bathroom) with the mixing valve is satisfied, you should want the residual heat left in the boiler to be put into that zone. It is the only thing that makes sense. if you are putting it into the DHW tank, you may over temperature the tank more than you actually want. You will also want a limit on that zone so the burner stops at 150° or 145° so as not to have as much residual heat to purge into that zone. You may also find that you can set that thermostat lower in that zone than the rest of the house because the purge function may cause an overrun of a few more degrees than a normal larger zone. this is all going to be trial and error to get it dialed in.

What if I tried to do a purge into the previous zone and see how that works in the mild weather on the microzone and the DHW. On the other 20 or so colder days, you can see that it won't make much difference since there are usually other zones operating when it is that cold outside.

As far as the Barometric draft control is concerned, there is one that is defective and installed in the wrong location on your boiler. If you look at the photo and the photo that I altered, you can see the difference between the right way and the wrong way to install the draft control.

Here is the information from the instruction manual that the original installer did not follow.  The text portion is on page 12 and the diagram is on page 13. if you have a WGO Series 1 manual

okayfine

https://forum.heatinghelp.com/discussion/comment/1840063#Comment_1840063

@EdTheHeaterMan I'll fix the barometric control. Thank you for the information!!

I see your point now about purging into the microzone, after the mixing valve is satisfied. Once the mixing valve is there, hopefully the valve would be regulating the supply temperature, and I don't have to worry about temps getting above the limit of pex. Is it possible though, if the zone is so small, the return temps are barely below the supply temps, so the mixing valve ends up doing nothing much at all … ?

https://forum.heatinghelp.com/discussion/comment/1840027#Comment_1840027

@hot_rod ,

Manifold 1: Indirect. On priority.

Manifold 2: Master Bedroom, 2nd floor. Approximately 200 feet of Pex in the joist space between the bedroom and the garage below.

Manifold 3: Baseboard, rest of 2nd floor. 60 feet, across 4 rooms. This zone is currently disconnected on Taco to make room for the microzone.

Manifold 4: "Microzone" 1st floor bathroom. Without the mixing valve, the floor gets nicely warm, but not too warm. My wife and I would be happy with this temperature. I think it would work fine though when run at lower temp.

Manifold 5: First floor, original house. Approximately 500-600 feet of Pex, in the joist space below the floors and the crawl space which is about 4 feet tall. The entire crawl space is encapsulated by closed cell foam.

Side note, all the original pex runs are with Pex C! If it hasn't split yet, I guess it's fine? I can't read the markings on the pex to see if it is oxygen barrier variety.

Getting back to adding the mixing valve, do I add it between the manifold supply and the taco pump, like in this picture? Asking because most examples of mixing valve installation I found were on the main supply and the main return, instead of an individual zone supply/return.

Thanks so much everyone for your replies.

hot_rod

maybe the purge idea will not work when you have such a small micro zone to purge into. Or overheating the DHW to 160 or more?

Manifold 2,4 & 5 all seem to be micro loads.

The question is how much purge and how often?

It seems the EK systems get away with purging into the DHW? Maybe less boiler mass to cool down?

Perhaps the time out option is better, as long as the rooms don’t drop too much on cold or design days. At least you are not putting heat where it is not needed, wanted, or desireable.

So you might be looking at over-heating, or under-heating?

Right sized boiler or a buffer might be a more accurate solution.

Mix valves like this

IMG_1021.png

EdTheHeaterMan

@hot_rod Bob is trying to tell you the proper orientation for the mixing valve. By your diagram, I can see that you may not be completely seeing what Bob is actually illustrating when it is on your system. The Hot for that zone must go directly to the Hot in on the mixing valve. The Cold from the return must go the the Cold in on the mixing valve with a tee to the system return. The MIX from the mixing valve then goes to the circulator inlet. Caleffi actually has a valve made just for your situation as illustrated in the photograph below.

Zoom in to the mixing valvearea to se the detail.

Otherwise you will need a lot of 90° elbows to make it work like in this illustration:

hot_rod

In this suggested piping, remove the plugs at the end of the headers. The H hot port of the mix valve screws into the upper manifold. The C cold port of the mix valve gets a tee. The radiant return comes into the top of the tee, and the run of the tee connects into the lower return manifold. It just seems like the quickest, cleanest piping.