Boiler Replacement Question: Combination Boiler and DHW or indirect tank?


  #1  
Old 01-17-19, 08:59 PM
B
Member
Thread Starter
Join Date: Nov 2010
Location: USA
Posts: 352
Received 0 Votes on 0 Posts
Boiler Replacement Question: Combination Boiler and DHW or indirect tank?

So we are likely going to be replacing our boiler and our water heater in the next few months. We're debating now between a combination boiler and on demand DHW system (I believe they are called a combi system or something) or doing a Hugh efficiency boiler with and indirect storage tank. There are pros and cons to both.

We have a 2.5 bathroom (1+.75+.75) house. Only three of us in the house (wife, daughter, and me). Our hydronic radiant heat is configured into abiut 6 different zones including both in-floor as well as a couple radiators.

Interested in what people think. The indirect storage tank is slightly more expensive than the combination system but most of thr dealers I've talked to seem to lean towarda the combi system and think it would work well. One of my big concerns with the combi system is the limit to how hot the water can get. We tend to keep our hot water a litter warmer than most.
 
  #2  
Old 01-18-19, 05:46 AM
D
Member
Join Date: Apr 2014
Location: us
Posts: 1,142
Received 61 Votes on 57 Posts
Why is boiler being replaced?

Many old boilers are over sized due to home improvements, i.e. insulation, new windows, etc.. Old boiler efficiency can be significantly increased by reducing nozzle size, adding out-door-set and electric stack damper. Most new boilers have those options.

Old cast iron boilers do not have established life span. My 60 years old Weil-McLain boiler has efficiency of 85% +
 
  #3  
Old 01-18-19, 06:21 AM
B
Member
Thread Starter
Join Date: Nov 2010
Location: USA
Posts: 352
Received 0 Votes on 0 Posts
So our old boiler isn't old enough to be a beast like yours. I don't believe ours is oversized. It's starting to be flakey. The zone control manifolds are also shot and replacements are no longer made.

In addition our hot water tank needs to be replaced and the cost for that is pretty significant unfortunately.
​​​​​​
Now could we replace all of those things and not replace the boiler? Sure! But we're looking at not a huge investment more to replace the boiler as well.
 
  #4  
Old 01-18-19, 11:48 AM
S
Member
Join Date: Oct 2013
Location: United States
Posts: 3,234
Received 85 Votes on 79 Posts
B,
I know you asked for thoughts but the sight below could be used to show you your options and provide you with some info on different boilers and what they offer unless you already have one in mind.

Other boilers and equipment are available by entering in search bar.

https://www.supplyhouse.com/Boilers-449000

Hope this helps a little.
 
  #5  
Old 01-21-19, 08:48 AM
D
Member
Join Date: Apr 2014
Location: us
Posts: 1,142
Received 61 Votes on 57 Posts
Boiler Sizing With Real Data, Not Calculated

One common static is 85% of boilers are twice the size needed. Because of many variables in construction, heat load calculations are more like estimates.

Collecting data on existing heating system provides actual heat load, not calculated.

For period of days log run time, degree days, oil/gas consumption.

A good source for run time is elapsed time clock wired to boiler solenoid circuit.
https://www.ebay.com/itm/Hour-Meter-...kKlH:rk:1:pf:0

For a few days or longer, log total boiler clock time, degree days and oil consumption.

For the period divide number of degree days by hours run to find degree days per hour.

If your design temp is 0 F then that is 65 degree days. Divide 65 by your dd/hour to find run time required on design temp day..

By logging gallons used during time period, degree days per gallon and actual firing rate GPH can be determined.

A simple way to find boiler output is 140,000 btu/gal X efficiency (if unknown use 80% to 85%) X actual GPH.

If boiler also supplies domestic hot water the dd/gallon will be slightly higher in colder weather.

Using run time data is more accurate than heat load calculations and boiler/nozzle rates. This provides actual nozzle gph flow rate. Charts with flow at different psi are not actual flow rate. A .65 gph nozzle at 160 PSI is actually firing at .79 gph.
 

Last edited by doughess; 01-21-19 at 09:05 AM.
  #6  
Old 01-21-19, 09:44 AM
S
Member
Join Date: Oct 2013
Location: United States
Posts: 3,234
Received 85 Votes on 79 Posts
Correct PSI ratings on sight below on page 8.

.65 @ 160 PSI = closer to .83 than .79GPH.

https://www.beckettcorp.com/wp-conte.../6104-BAFG.pdf

All this talk about oil is a moot point since I believe combi boilers, which is what was asked about are gas anyway, so oil info has no bearing on this conversation at all.
 
  #7  
Old 01-21-19, 02:47 PM
D
Member
Join Date: Apr 2014
Location: us
Posts: 1,142
Received 61 Votes on 57 Posts
Manufacturer nozzle rating are +/- 10%

Fuel pump pressure readings at 160 PSI are not always accurate. Delavan nozzle chart has note saying "to be used as general guide only".

Using those two numbers for gph on chart is not measured flow.

Measured rating of 0.79 gph is actual.

Clock hours X .79 gpm provides gallons used and matches dip stick oil tank measured quantities for same period.

Boiler btu is basic data, regardless of what type of system is selected. That is not "moot."
 

Last edited by doughess; 01-21-19 at 04:17 PM.
 

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