Run New Combi and Old Burnham in Parallel?


  #1  
Old 10-15-09, 08:51 AM
D
Member
Thread Starter
Join Date: Oct 2009
Location: NY
Posts: 1
Received 0 Votes on 0 Posts
Run New Combi and Old Burnham in Parallel?

I have an unusual idea, and would like some real world input.

I'm running a 1976 Burnham natural gas boiler with 3 zone baseboard in my 1961 1500sqft split with single pane windows and poor insulation.

My electric water heater needs replacement, and I'm planning to change to gas. I found a supply house with a new old stock Bradford Combi unit M2-C-TW75T10CN, which has a 60,000 btu space heat exchanger for $1000, incl. powervent. I need a powervent anyway, even on a straight tank heater, and was quoted $800 by a different supply house just for the tank heater.

I'm assuming the efficiency of the old Burnham is not as good as even the mediocre 82% from the combi. At the same time, I estimate my peak heat loss to be about 75,000 btu, but only for about 15 days per year in downstate NY.

I need someone to tell me what would be bad about plumbing the combi heat exchanger in parallel with the burnham. It seems I could split the baseboard return to the combi, and isolate the burnham with a power switch and zone valve, then bring the combi output into the loop between the Burnham and the circulator.

For those 15 cold days, I don't have a problem manually flipping a switch to power the burnham and open the zone valve, so that the flow passes through both boilers equally. For 15 days I wouldn't think any fancy balancing would be needed between the boilers, half the load through each would be good enough.

I don't want to spend much more than $1000. Are there any reasons this approach wouldn't work, or would be bad?

Thanks,
 
  #2  
Old 10-15-09, 02:41 PM
Who's Avatar
Who
Who is offline
Member
Join Date: Jan 2006
Location: San Jose
Posts: 2,175
Received 1 Vote on 1 Post
For those 15 days I'd wear a sweater. If you want a safety blanket pipe it in. I'd be more tempted to just have it on a bypass where AFTER the combi the water can either go on to the zones or can be bypassed through the boiler so that it can add some BTUs to the already warmed water. 3 ball valves. One on the regular run and another before and after the old boiler.
 
  #3  
Old 10-15-09, 03:10 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 4 Votes on 3 Posts
Controls for something like this are crucial... from a safety standpoint. You would want any controls to 'fail safe' and not let a boiler fire up without the proper valves, etc...

I don't think a zone valve is what you would want to use though. A separate pump for each boiler, feeding the primary loop, with a check valve properly placed would isolate the unused boiler.

Take a look at some of Tekmars diagrams, and at their controls which will 'stage' two boilers automatically.

tekmar Literature

I would think that the safest setup would be to convert the system to a primary/secondary affair, with each boiler on a separate secondary loop. Then you could run either or both, depending on your needs.
 
  #4  
Old 10-15-09, 04:03 PM
M
Member
Join Date: Sep 2009
Location: U.S. Midwest
Posts: 1,340
Received 0 Votes on 0 Posts
There comes a time in your life when there are three watchwords: simplify, simplify, simplify. To hell with the ultimate in efficiency and automatic everything.

Trying to implement automatic controls for two boilers doesn't meet my criteria.

Here is another idea: set things up so that you can cut in/out either/both boilers manually, and forget all the automation. When you leave for two months in the winter to your condo in Rio, line up your system to be most reliable, and turn down the thermostat.
 
  #5  
Old 10-15-09, 04:37 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 4 Votes on 3 Posts
I agree 100%, as long as it can be done SAFELY!

Example: let's say they were plumbed in parallel, and ball valves were installed to manually cut in/out the desired boiler. Then, let's say that the expansion tank is somewhere placed so that the isolated boiler is not connected to it when it's valved off. Then, let's say someone accidentally turns ON the valved OFF boiler... boiler fires, pressure skyrockets, relief valve opens, dumps steamy hot water, nobody around to notice, water feed also on the other side of the valved off boiler, boiler dry fires and melts down, causing a house fire...
 
  #6  
Old 10-16-09, 01:33 PM
M
Member
Join Date: Sep 2009
Location: U.S. Midwest
Posts: 1,340
Received 0 Votes on 0 Posts
The more I think about it, I fear this whole idea of paralleling the two boilers is likely to end with disappointment or worse. Trooper has pointed out some potential pitfalls that would have to be avoided, and there are likely others.

This is part of what you posted, on the same topic, two days ago at the bobvila.com website:

"...It seems that they could just share the heating load 50/50 with no fancy balancing, just a Y junction in the heating loop.

"I can commit to $1000 plus 3-4 hours of plumbing but not much more."


I assume the $1,000 is for the surplus combi?

I think you may be underestimating the cost, effort, and complexity for this project - installing a second boiler, piping them up in a safe manner, all the necessary controls, and duplicate safety features, etc. My advice is to forget the whole idea. If I understand your situation correctly, your existing gas-fired boiler can carry the load, right?

Also, October might not be the best time for a DIYer to tear into the heating system.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: