Bosch Boiler Control

rocksolidsr

I was wondering if you could please help in determining the best method to control my radiant floor heat using the Bosch Greenstar Combi 151P.

Below is a picture of my current setup.



My main questions are:

1. With the setup that I have is the boiler modulating its flame or is it 100% when it is on
   
2. With this setup I have my water temperature set to 140 and when just running zone 1 the boiler is on for about 1 minute and off for about 4 minutes and continues to cycle like that. I believe this is short cycling, if I were to switch controls would this be eliminated?
   
3. I do not have the external system supply temperature sensor for system supply pipe, is this required, if so where can I purchuse one/
   
4. Would it be better to use the FW200 or a CRC200 with CZM100
   
5. Since my setup uses both pumps and zone valves would I even be able to use the CRC200 with CZM100
   

Thank you very much for your time,
Shane

Ironman

That's not a good setup the way it now is.

If the larger manifold is controlled by one stat, then the actuators on it are not necessary.

Regarding the 2 zone manifold: you got two micro zones which will cause the boiler to short cycle and die early. You may have a problem getting sufficient heat from them, too.

The circulator going to the small manifold is mounted improperly and will fail prematurely like that. It has to mounted with the motor horizontally like the other one.

The best solution would probably be combine all of the loops on one zone, install the FW200 and get its reset curve setup properly. If you get the reset curve set right, the t'stat will be almost unnecessary and just act like a high limit.

rocksolidsr

Thanks for the reply!

The larger manifold does not have actuator those are just the flow adjustment knobs

If I'm going to just combine the other zones could a CRC200 be used with indoor reset, is one option better than the other (FW200 vs CRC200)

Thanks for the note about the motor I will try and get that corrected.

Just curious how others do radiant heat that have smaller zones with bigger ones, or is that just not done?

Ironman

You don't want micro zones on a low mass mod/con. If you absolutely have to have them, a buffer tank should be added.

However, if you use ODR, and get it adjusted correctly, a thermostat is unnecessary. I've got multiple jobs setup like this including one next door and it stays at 68* throughout the heating season.

I've used the IDR feature on jobs with cast iron radiators, but not on a radiant floor. I can't really say how well it would do there.

Ironman

Is the floor, or any part of it, a slab?

rocksolidsr

No it is all on the first floor, staple up, no slab.

Do you know if the boiler will modulate the output without having the outdoor reset, or is that necessary to achieve the modulation

I guess my main question is will having the FW200 help with the short cycling

I tried setting the Service function 3.C: Switching differential, I went all the way from 0 to 3o and it didn't seem to make a difference. Do you have any idea why that wouldn't change anything (not sure how much you know about a Bosch Boiler)

fenkel

Even.with the fw200 installed, you'll still  have shortcycling...
Whats your total heatloss calculation?
Whats your smallest zone your heating in btus?

Your unit modulations is 4.4 to 1...so your zones need to be bigger than 34k btus... even then you'll have some short cycling..
You might be able to lower your output btus%, but in order to do that, you need the total design btus.

Ironman

The FW200 will adjust the water temp to match the load. If you make the system one zone, as I suggested, then there would be no micro zones.

The boiler will modulate as it approaches its target temp, with or without ODR. But I think you’re missing the point. As @fenkel pointed out, the MINIMUM firing rate is 34k btus. If you have good extruded aluminum heat transfer plates installed, you can expect 4-5k btus output per loop with 250-300 ft. loops @ 8-9” OC. At best, that may barely match the minimum firing rate if all loops run at the same time.

Your boiler is severely over-sized for space heating because it’s sized for domestic. It takes 151k btus to provide about 3 gpm domestic. This is the down side of combi boilers. If you add a buffer tank to prevent short cycling in space heating, you’ve probably spent as much as a PROPERLY sized boiler and indirect tank would have cost - without the associated issues.

rocksolidsr

Instead of adding those two microzones to the main manifold, would it be the same if I just set the t-stats to always call for heat, therefor all zones would be running, that would essentially be the same right?

Attached is a PDF report from LoopCAD which depicts my setup.

In my boiler settings there is an option to set the maximum space heating output, would changing that help, or would I not be able to reduce it further than the 34k btus?

What would be acceptable cycle times for a boiler (i.e. X minutes on, X minutes off?

GW

FW200 Will make cycling worse, not better. Combination boiler, not sure why people install it so often.(unless your house is kinda big)

Ironman

The absolute minimum run time should be 10 minutes, but the whole point of a mod/con is to get it to run as long as possible on as low of a firing rate as needed. This produces the most comfort and efficiency. In theory, if a properly sized mod/con is selected, it should almost never shut off until it reaches WWSD. It should just modulate down.

By choosing a 151k btu boiler where a 25k was needed, you've essentially defeated that purpose. A buffer tank is your only reasonable alternative.

fenkel

 I just looked at your heat loss calculation..based on the supplied information....Your boiler is WAY to large for this system..
I agree with  ironman, you need a buffer tank  to make it more  efficient.. but its gonna take a big buffer tank of about 30 to 40 gallons to reduce the short cycling...

Ironman

Changing the boiler output only addresses its maximum firing rate. It won't do anything to lower the minimum firing rate. That's fixed by the manufacture.

rocksolidsr

Thanks all for the feedback, I really appreciate it!

So I turned on the other two zones last night and it ran all night. Initially it did help with the short cycling (on - 10-15min, off - 10min), but once the temperatures were brought up it was back to about 1 minute on and 5 minutes off. So if I got a large buffer tank wouldn't that just do the same thing, I think I am missing something pretty fundamental?

Also in the two smaller zones that I turned on over night, the rooms stayed at 61°F all night and didn't get any warmer. This is probably due to the water temperature and the outdoor reset should help this when I get that installed. But I would have thought it would have increased the temperature at least a little. The supply temperature is about 140 and the return temp is about 110, and that leads me to another question.

I have set the temperature differential to 10 degrees but when I watch the boiler it kicks on at 110 and turns off at 140 so...I'm not sure why it isn't following the setting.

If I were to get a different boiler would I want something with a MAX BTU of around 25k.

Would this boiler still be too big
https://www.pexuniverse.com/bosch-greenstar-100-zwb-28-3-combi-boiler

Ironman

I feel that you're not grasping that the problem is that you have a COMBI boiler. Again, the main issue in using one is that in order to make enough domestic hot water instantly, the boiler has to be sized to THAT load and ends up being grossly over-sized for your space heating load. The boiler that you posted a link too is just a smaller version that will only do 2 gpm domestic but still has a minimum firing rate of 28k btus which is higher than your space heating load (25k) on the coldest night of the year.

The proper setup would have been a boiler with a minimum firing rate below 10k and an indirect tank to heat your domestic, not a combi.

Your easiest solution is to now install a buffer tank with enough mass (at least 30 gal.) and have one zone.

rocksolidsr

ok so if I install a buffer tank, will this cause my boiler to fire all the time at 34k BTU, b/c that doesn't seem efficient to me?

Or will it run longer to heat up more water and stay off longer b/c there is more hot water to service the system

Right now it is firing over 8 hours/day and consuming quite a bit of propane

Again thanks for answering my many questions

Ironman

Adding a buffer tank will increase run time, but how long that will be depends upon the size of the buffer, the load imposed vs. the minimum firing rate of the boiler.

Again, I don't think you're seeing the total picture: whatever heat that goes into the buffer is not lost, it eventually goes into the house.

Second, the most efficient operation of a mod/con is for it to run as long as it can on the lowest firing rate necessary. Your conception seems to be fixed on "bang bang" equipment where it's either 100% on or 100% off. That doesn't apply with modulating equipment.

rocksolidsr

But the particular boiler that I have has a minimum firing rate of 34k BTU, so are you saying that for most efficiency I would want my boiler running at 34k BTU all the time, is that accurate?

So then adding the buffer tank solves the short cycling which I get needs fixed, but wouldn't that make the system consume more fuel as it would be running more, meaning the only real way to consume less would be to get a properly sized boiler, or am I still not getting it, (I don't think I am )?

I looked at the boiler when it is firing and it is at its minimum ~39%, so it isn't 100% on

GW

Here’s another way to look at it. Let’s say one house is 25 years old, has low mass fine tube baseboard, total water volume of the heating system is 8 gallons.The house across the street is 100 years old, it has cast-iron radiators. Total water volume of that heating system is 80 gallons. 

Let’s pretend for a minute the heat loss between the two houses are the same (Let’s pretend the newer home is twice as big as the
older home)

Lo and behold, both boilers take a nap on the same cold December day. Both homeowners install the same combination boiler. Let’s pretend the heat loss is about 60,000 or 80,000 BTUs, or whatever. But the combination boiler is 151,000 BTUs input, much larger that the home needs 

So, which house will have a boiler cycling on and off issue? The house with more water in the heating system will be more like a choo-choo train (large mass), the house with less water would be like a Datsun pick up, lighter mass. So, water equals mass. Mass equals less cycling. A buffer tank increases mass.

Once the larger mass system gets warmed up, you are not wasting energy as the boiler keeps on dribbling in more or less BTUs as the weather changes 

Ironman

rocksolidsr said:

But the particular boiler that I have has a minimum firing rate of 34k BTU, so are you saying that for most efficiency I would want my boiler running at 34k BTU all the time, is that accurate?

So then adding the buffer tank solves the short cycling which I get needs fixed, but wouldn't that make the system consume more fuel as it would be running more, meaning the only real way to consume less would be to get a properly sized boiler, or am I still not getting it, (I don't think I am )?

I looked at the boiler when it is firing and it is at its minimum ~39%, so it isn't 100% on

You still don't seem to see that no matter how long the boiler runs, the heat is going into your house, and therefore, it's not being wasted. The buffer tank is only storing it like a battery stores energy.

If you eat well, work out, take a shower an then rest, does your body stop your heart so it won't "waste energy"? No, it adjusts your heart rate, blood pressure, breathing, etc to match the load. Same principle with modulating equipment.

neilc

rocksolidsr said:

But the particular boiler that I have has a minimum firing rate of 34k BTU, so are you saying that for most efficiency I would want my boiler running at 34k BTU all the time, is that accurate?

So then adding the buffer tank solves the short cycling which I get needs fixed, but wouldn't that make the system consume more fuel as it would be running more, meaning the only real way to consume less would be to get a properly sized boiler, or am I still not getting it, (I don't think I am )?

I looked at the boiler when it is firing and it is at its minimum ~39%, so it isn't 100% on

Imma gonna try,
your minimum 34K is still more than you need for the house,
your max, and I didn't go back to find it, is what? 200 / 250k?
which you'll never use for heating,
BUT need(?) for the domestic hot water.
You could have bought, should have been sold, a max 35K (or less to match house), and the 35K would have backed down to ?, 10K, and run long at less temp and satisified home comfort on less than design days.
This would have been piped to indirect storage for the domestic hot water. This would have been the primo job, and you would not be seeing these short cycles heating the house.

rocksolidsr

thank you all for the help and explanations! It is greatly appreciated. I will look into adding a buffer tank.

rocksolidsr

So after looking at prices for a buffer tanks, was wondering if it would be possible to use an electric hot water heater as a buffer tank, and what the connections would be?

Ironman

It will work fine, I've done it before.

The connections are 3/4" unless you use the tapping where the element is.

Don't use the cold water inlet at the top as the internal dip tube is only 1/2". Instead, remove the drain valve and use a brass nipple and Tee to bring water into the tank. Then reinstall the drain in the Tee.

There are a few different ways to pipe it (2 pipe or 4 pipe) and even some different control strategies. Look up Caleffi IDronics and find the issue that addresses buffer tanks.

A simple 2 pipe (one in, one out) may be the easiest.

fenkel

Check this site for an article written by Hot Rod...
Make sure you use the correct size tank...
Youll need at least a 35 gallon tank...
Use the formula to determine correct size..

GW

I bought a 20 gallon HTP buffer for my house, You’ve spent this much money, for an extra several hundred you might as well jump in and do it correctly.

rocksolidsr

where did you get the 20 gallon buffer tank

fenkel

Try a search on google.... or type in Vaughn buffer tank or heat flo buffer tank.
Use the sizing calculator on vaughn  web site to size your buffer tank correctly...

GW

Local vender had to special order it. Apparently buffer tanks are not a thing yet.

rocksolidsr

Would this setup work?

Hot supply would be piped into the hot pipe on the water heater and the return would be connected to the drain plug.

I'm not sure I understand what you are saying about the drain...

rocksolidsr

does that image look correct for hooking up the hot water heat as a buffer tank?

Is that appropriate to use tee's where it is coming off of the supply and return?

Ironman

No, the system flow has to go THROUGH the tank and I'd put it IN at the bottom and OUT at the top.

Simply Teeing in like that will produce no flow through the tank.

rocksolidsr

oh man not sure what I was thinking with that first image.

is this what you were thinking.


Did you say to replace the drain valve with a brass nipple b/c the drain valve is plastic?

What is the Tee for that you mentioned?

Ironman

The nipple and Tee are to go where the drain valve is so that you can use that as an inlet. Then, put a good brass drain in the Tee.

Ironman

Like this:

rocksolidsr

thank you! that makes sense

rocksolidsr

Does anyone know where I can get the supply temperature sensor part number 8737 700 289 0, I called a local place and they wanted $200, that seems a little high for a temperature sensor

Ironman

Try Supplyhouse.com.
I think you're looking for the "system sensor", not the "supply sensor" which is an internal part of the boiler.

rocksolidsr

oh ok, yes I'm looking for the external supply temperature sensor. I cannot seem to find the part number for that, you wouldn't happen to know would you?

Once that is connected is that temperature the one the will be displayed on the boiler? Or will it still be the internal one?

Ironman

No, IDK the part number and I'm not sure which temp the boiler will display. All of that should be in the manual.

rocksolidsr

So I was able to get my hot water tank installed as a buffer tank and installed an FW200 and also put in the external supply temperature sensor. I am sorry to say that I feel as though the boiler is still short cycling.

Here is a graph of the boiler before the above changes


Here is a picture of the setup with the changes from above


and here is a graph of the new output


It definately helped with the keeping the water at a constant temperature but the boiler only seems to be running for about 3 minutes and off or about 3 minutes