Tie two programmable thermostats in parallel to tie two heat zones together

erickyung

I had a new Viessmann Vitodens B1HE-199 installed about a month ago. After several weeks of operation, it ran into A.19-50, then F.62-50. With careful monitoring, I have figured out that when the heat call from the sunroom alone, the boiler overshoots its temperature set point even when its modulation is at the minimum of 10%. I don't believe this boiler can go lower.

There are 3 zones: first floor, second floor and sunroom. So I've re-programmed the sunroom's thermostat to match the timing of the first floor's thermostat, and that seems to reduce the chance of the sunroom calling for heat by itself. Nonetheless it can still call for heat out of band due to more heatloss from large windows and patio door.

My next idea is to tie two thermostats (RTH2300B1038) in parallel so that either one can call for heat for both first floor and sunroom. I am thinking of jumping the two W wires together on the relay side of the Taco SR503 - basically adding a short wire from one zone's W terminal to the other zone's W terminal. Does that work?

I probably should call both Honeywell and Taco to make sure it won't fry anything.

https://www.honeywellstore.com/store/products/5-2-day-programmable-thermostat-filter-change-honeywell-rth2300b.htm

I will consult with my contractor for a reasonable permanent solution that can keep the 3 zones separately, but any piping reconfiguration if needed has to wait for after this winter season.

Derheatmeister

Do you have a Low Loss Header(LLH) installed ? Is the Supply sensor installed in the LLH ?

Jamie Hall

You can certainly parallel two thermostats by basically connecting the two white wires. No problem (usually...)

erickyung

@Derheatmeister There is a LLH in the system but there is no supply sensor installed in the LLH. I am not sure if Vitodens 100 B1HE-199 has a connection for a LLH sensor. I will look into the manual and installation guide.

Redbaran

Vitodens 100E does not utilize a Low Loss Header Sensor. The Viessmann LLH is used in several boiler models - the 200E for example has a landing terminal for the LLH Sensor. The well is there, the landing point for the sensor is not (on 100E)

erickyung

@Redbaran Mine is a Vitodens 100-W B1HE and I don't think it has a landing for LLH sensor either. I don't see it on the wire diagram.

I called HoneyWellHome and they confirmed that I could tie 2 RTH2300B1038 thermostats in parallel without causing issues with the thermostats themselves. However when I called Taco support to see if the setup would work with my SR3503-4, the answer I got was 1 zone 1 thermostat.

What if I just use 1 thermostats (first floor) to call for heat for the sunroom as well - meaning the sunroom's thermostat is no longer connected. Can I jump both R and W wires of the living room over to the sunroom's R and W terminals on the relay side of SR503-4?

I am wondering if there is an issue with the relay getting twice the signal if both are calling for heat. Or when one is calling for heat, it does not have enough power to flip both relays.

My last resort, an expensive one, is to extend the sunroom loop in the basement but it has to wait for off season.

Jamie Hall

Somewhere there must have been a crossed wire -- in communication, not your wiring (at least not yet)..

A thermostat -- or at least the red and white wires -- is a switch. Nothing but a switch -- no power is supplied by the thermostat to anything -- it's just turned on or off. It is either open (no call for heat) or closed (call for heat). There is absolutely no reason why two thermostats can't be paralleled to the same relay coil. Now if the thermostats are also powered -- that infamous C wire -- they must be carefully wired to that transformer so that they really are all in parallel. But if they are self powered (such as batteries) there shouldn't be a problem.

Wired properly, the relay won't get twice the signal -- it will just see a closed switch through one or the other or both, as the case may be.

Redbaran

Eric, your are correct as it relates to a sensor for the low loss header - by 100E I am referring to the 100 family of boilers. E indicates the current control platform - there are (6) boilers in 100E family (2 Kombis (KE) and 4 Heat Boilers (HE). The 100 is Viessmann's "base" model for wall hung modulating condensing boilers. I hate that term because the 100E family does a lot; that being said, the 200E does more (and you pay more for it...)
Your thought process is good, but the value of zoning is just that. If your goal is to prevent the boiler from tripping the high limit (and that's a good goal - this should not be a regular event) I would go to weather compensated operation: The boiler will have to be recommissioned (not a difficult process) and you'll need the outdoor sensor install (it comes with the boiler) Make sure it is installed on the North or North West wall. Be sure it's away from things like dryer vents, etc (anything that can skew it's accurate temperature sensing). Set a curve with a high limit under 180 (the curves are adjustable so you can fine tune as you go). Check out Viessmann North America YouTube and search for heating curves - you'll find some well done short videos.
I believe you posted a picture on another thread - it's a very nice job. The pipe work is clean and well done. That being said, I think the boiler is oversized. From the picture, there are 3 3/4" loops on the heating side - 199,000 btus is a lot to unload into that system (The only way to know for sure is an accurate heatloss). One of the features of the 100E is the ability to derate the boiler on the heating side; I have mixed feelings on this - the boiler should be sized by way of the heatloss; but look at your Installation and Service Manual page 63 - System Configuration/Boiler Address 596.0. The proper derate would of course be based on an accurate heatloss, which would have made this suggestion unnecessary. It's a way to get a little closer to a load match.

EBEBRATT-Ed

For now, just disconnect the Sunroom thermostat and use the first-floor thermostat to open both zone valves or both circulators whichever you have. Put jumpers from the first floor zone valve or circulator relay to the sunroom zone valve or circulator relay.

The boiler will not turn down enough for a small zone. The answer is a dump zone or a buffer tank

erickyung

@Jamie Hall I have checked all the thermostats' connections where accessible, all are white to white and red to red - no guaranty inside the wall where wires can cross. I did see a jumper from R to RC at the base of the thermostat. There is no C wire which I wish I have so some day I can upgrade to smart thermostats.

@Redbaran You must be a pro and know a lot about Viessmann products. I will read about Address 596.0. A bit of a background why my contractor picked that boiler size. He did ask me a bunch of questions about my house, family (of 6) and hot water usage. My house is just a bit over 2,700 sq.ft. of floor space, not including the unfinished basement. Although the house has good insolation all around, there are many spots where I can feel air leaks in, particularly the two giant intakes in the basement for the old boiler that just got replaced. I have yet to remove and seal them. I do agree with you that B1HE-199 is oversized but if I can solve the sunroom issue, I will crank up the heat and give it a workout. It's been only a month and I've learned so much about boilers - at times I wish I got a traditional Burnham boiler and let it cook.

@EBEBRATT-Ed Agree, that seems to be the safest route for a novice like me. From my observations, the sunroom zone calls for heat 1 or 2 times more than the two main zones throughout the day (not sure about overnight), even though its T-stat has 3F lower all around. I still would love to have both T-stats operable that way each room gets its own heat, except when the sunroom calls for heat, the boiler can dump the excess to the first floor.

I know enough to be dangerous but still cautious because the whole system is new and costs a fortune.

erickyung

I will update which route I take:

I will go with my instinct and put 2 t-stats in parallel for the two said zones. I believe all I need to do is leave all the t-stats wires where they are and add a jumper wire from W to W of the two said zone. I think R terminals are tied together on the Taco control board based on their schematic diagram. I did check all t-stats and all connections everywhere I have access to, W to W and R to R. Those wires could cross in the wall, in that case it could be bad for the Taco control board.

Or I will disable 1 t-stat and let the other control 2 zones.

I know enough to be dangerous but I am still cautious because the system is new and it costs a fortune.

109A_5

Hello @erickyung,

I think I would add a relay (or two). The coil of the relay would go to the zone valve motor or circulator power for the First floor connections at the Taco SR503. The N.O. (Normally Open) Relay contacts would be connected in series with the Sunroom's thermostat. That way the Sunroom's thermostat can only actually call for heat if the First floor is calling for heat. The Sunroom may have wider temperature swings, since it can only get heat (if needed) when the first floor is calling for heat.

120 VAC relay coil for circulator, 24 VAC relay coil for zone valve.
With Taco SR503 just circulators I would guess ?

You could expand on that idea with a second relay. The second relay coil is connected to the Second floor zone valve motor or circulator connections and the N.O. (Normally Open) Relay contacts would be connected in parallel with and the First floors N.O. (Normally Open) Relay contacts and those two contact sets are in series with the Sunroom's thermostat.

So if either the First floor or Second floor is calling for heat the Sunroom can get heat, but never independently on its own.

Easy fix if you can wire up relays safely and correctly.

erickyung

I've added a jumper from first floor's W to sunroom's W and it seems to work the way I wanted. I will monitor for new behaviors and update later.

109A_5

Hello @erickyung,
With that configuration I would think if the Sunroom has greater heat loss you would end up overheating the first floor.

Since you are heating both areas whenever either thermostat calls for heat.

Do you have the thermostat in the Sunroom set lower than the First floor's temperature setting ? So in effect the Sunroom's thermostat is really mostly doing nothing now. And the Sunroom just gets its heat when the first floor is heating with no actual individual control of that zone.

If that gives you the comfort you need than OK.

109A_5

Hello @erickyung,
Looking at what you have. If the basic desire for the Sunroom is to keep it above freezing and not really normal comfort temperatures you could connect the Red wire of the Sunroom's thermostat to the White wire of either the First floor thermostat OR the Second floor thermostat (the Second floor may have less actual duty cycle). That way the Sunroom can only get heat when another zone is calling for heat.

erickyung

@109A_5 It seems to work well so far for the entire day but today does not represent the entire winter season. Here is my t-stat setup:

First floor and Sunroom (-3F)
Wake(6:30am 68F), Leave(11:00am 65), Return(6:00pm 68), Sleep(10:00pm 62)

I've noticed at Wake time the first floor's t-stat runs to almost 11:00am when outside temperature is around 20s to 30s. The sunroom gets to 65F quicker but continues to get extra heat (+2F) due to first floor's t-stat. Around 2:XXpm and 4:XXpm, the sunroom's t-stat calls for heat and the first floor gets some heat as well. I have not seen either t-stat calls for heat overnight, maybe when outside temperature gets down to lower 20s or below.

I should log inside temperatures at various time better but I believe the first floor drops 1F every hour and the sunroom probably loses close to 2F every hour. The guestimate on the heat loss is probably on the high side.

They don't run out of band that much. My reasoning is like this:

- Both t-stats call for heat at Wake and Return, it's a non-issue.
- At Wake time, first floor's t-stat calls for heat a bit longer, the sunroom gets a a bit more heat but not too much.
- In the afternoon, the sunroom's t-stat calls for heat once or twice, the excess heat goes to the first floor. It's quick to raise the temperature in the sunroom. At Return, the first floor will get to set temperature quicker.

I will see in a week or two if I need to make adjustments or different setup.

Your suggestion of connecting the red wire of the sunroom's t-stat to the white terminal of either first or second floor's is noted. That's an awesome trick that I wouldn't even think of - it took me a bit to realize that's how to connect the sunroom's t-stat in series with first or second. Your previous post about N.O. relays is way over my head, I couldn't picture it still.

Thank you all for helping! I will update (if anything) so it will benefit other people.

109A_5

Hello @erickyung,
OK, as long it works for you, and serves your needs. The relay(s) N.O. contacts just effectively locks out the Sunroom's thermostat unless one of the other zones is calling for heat.

Moving the Red wire for the Sunroom's thermostat to the White wire of another zone does the same basic thing, but can only utilize one of the other zones for Sunroom heat.

Just presenting options, so if in the dead of winter the situation changes.

EdTheHeaterMan

If you have two thermostats set to 65°F, and you wire them Parallel, Do you get 130° water in the radiator? (65 + 65 = 130). What if you wire them in series? Do you get 32.5° if only one is calling. You need both calling for 65°. I think that is how it is supposed to work.

Asking for a friend

EDIT: Nevermind... I was thinking of capacitors

erickyung

It's been working well with the t-stats of the first floor and the sunroom tied together. The boiler runs longer with less on/off in between. I am thinking of tying all zones (first floor, second floor and sunroom) together. From what I am reading, this boiler is good for radiant heating setup with low water temp running most of the day. I don't mind having it run most of the day at 160F throughout the winter season. If the outside temperature drops to the teens or lower, I can increase to 170F. I hope to get the outside temperature sensor installed soon so I can just setup the heat curve which can adjust to the outside temperature by itself. There are always some people in the house so I may consider getting rid of the thermostat temperature setback.