Bosch greenstar combi vs heat only

oztpath

Hello everyone,

Hoping to hear from members who have experience with these boilers. First time poster on the forum.

My local HVAC company will install a greenstar combi for a good deal less than a greenstar heat only with indirect tank. We are converting over from a 40 yr old CI boiler in our house with only 1 zone serving all the convectors.

My question is, If I go with the combi and down the road if I decide I want to add an indirect tank to the system is it really possible with the combi boiler's built in controls? Waiting to hear what my installer says, just wondering if anyone has done this with the greenstar units.

Thanks in advance

Dave

kcopp

I have installed both.

Yes an indirect is more money.

You will be happier w/ the hot water output and the overall operation if you do the separate tank.


I have never had anyone complain about having too much hot water... just saying.

Eastman

What kind of convectors do you have?

oztpath

we have the house's original 1938 Trane convectors in 30" high convector housings. They are mostly triple row with some quadruple row 3/4" copper fin tube with brass end tanks.

My question is not whether an indirect is better, but only whether I could add one down the road to the Bosch Greenstar combi version. The Bosch combi is the same price as a standard efficiency Replacement Buderus iron boiler with chimney liner.

kcopp

Yes you can add it later.... But at a cost.
The combi boiler will need to be upsized to get it large enough to produce the hot water needed for a combi. If you add it later you will then have a boiler that is now oversized for the space heating part.

Eastman

That's the real problem, the boiler ends up being the wrong size if it's used for on demand DHW. Do you know what the heat loss is for your house?

Gordy

Maybe @GW will chime in he does a lot of these.

I'm not sure if the controls would work, or if control boards could be swapped to control an indirect.

oztpath

Our fuel use heat loss showed for the coldest day of the year Jan 1 2018 hi temp of 10F low of -1F we averaged 130k btu/h for the 24hrs subtracting for the DHW heater. Current boiler is a Burnham 240k btu, monozone. House is 4000sq ft, 4 walls of uninsulated stone, single pane windows.

HVAC company will install a greenstar 151k btu combi boiler which they say can modulate down to 30k something btu on low fire.

So it is possible for the greenstar combi to have a indirect tank added down the road if needed? I'd rather go the route of adding an indirect later if needed.

kcopp

Again... yes you can add it later....

oztpath

If the onboard heat tonic controls would need to be swapped to do it then I would hesitate

oztpath

Kcopp, thanks for your reply. I dont mean to sound like I'm questioning you, but do you have experience with actually doing that on a greenstar combi? Did you have to swap control boards or anything to make it work? Thanks.

Gordy

When you run an indirect off a regular boiler the controls allow for DHW priority, and a separate indirect pump control.

A combi works by a flow switch. It needs a certain flow rate from the DHW side to light the boiler, and start heating the water.

Since a combi also uses a separate heat exchanger to heat the DHW. If you were to somehow be able to use the combi side to heat an indirect you would lose some efficiency.

I personally don't know if you would just not use the combi side, and use the boiler to heat the indirect. If so there would need to be a different control board.

I'm not saying you couldn't, and I'm not really saying you can as is, and simply hook up an indirect.

I guess I've never seen someone buy a combi, and want the possibility down the road of using an indirect instead.

kcopp

I have installed both. HO & Combi...
Stopped using them a few years back for another Brand.
Same control board....
They have a wiring harness for an indirect.

oztpath

Gordy,
Thanks for your detailed answer. It makes sense. I used the search function before I posted and saw a few posts where members reported adding a tank after their combi boilers. I suppose I just want sure whether this greenstar is different or there were some members that were experienced with them.

Honestly the combi is a big enough nut to crack at once without the indirect costs added on. I would like one in the future but it isnt in the budget at this time. I'll try calling bosch tomorrow to see what they say. I'll report back what I learn.

GW

I'm a tank guy, i don't care for combi. Sometimes there's not a choice. I'm sure there's a way to trick the controls to make a tank work on a combi. I've worked on Viessmanns with an internal pump/Diverter valve and tricked that system, but i haven't tried (had the need) this on a Bosch. You would need to get a controls guy, many heating dudes aren't into controls as much as other heating guys. If you ask the question and get a long pause----you aren't talking to a controls guy!

oztpath

Just an update - the operator at Bosch wouldn't even let me talk to their technical support team since I'm a homeowner. My contractor sales guy says that its possible to convert later but he didn't elaborate.

GW

relays, you can trick the system to do whatever you want

oztpath

I'm sure you're right that it can be done by a knowledgeable craftsman.

My installer's supplier told him the control boards on the combi and heat only boiler are different so it's not as simple as changing software settings.

Thanks to everyone who replied, you've been really helpful in my planning.

poolin4you_

Have the combi pre heat to an electric water heater set it to 120. Cheaper than a indirect and it won't run much.