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  1. #1

    Hydronic system buffer tank effect on modulating boiler


    Located in the Dallas TX area, in a 130+ year old home (completely renovated/insulated/etc) that has had a totally hydronic geothermal system (radiant floor and water coils) in place and functioning wonderfully for 10 years. Due to reasons to complicated to explain in a post we have lost our water source for the system. While exploring options for restoring the water source, I am considering adding a gas boiler to the system which would provide heat for this winter giving us more time to deal with the water source issue and if restored options (gas or electric geothemal) for future heating.

    Pertinent facts :

    approx 130K BTUs required
    past water temp for heating 120 degrees
    buffer tank 120 gal
    all pumps and tekmar dual point controls already in place
    if installed the boiler will also replace gas hot water (will add indirect)


    Because the buffer tank is already in place, won't it negate the efficiency difference between a modulating boiler and a "Bang - Bang" ? Won't the buffer tank and proper control set points actually perform the "modulating"? In this scenario is there any value in spending more in initial cost and on going maintenance without the efficiency payback?

  2. #2
    Join Date
    Jul 2004
    With radiant floor heating it is desirable to have some water mass in the system. Thus a buffer tank is, IMO, a plus. However, the 120 gallons needed for the geo system is excessive for the gas system. I'd recommend something along the lines of a 40-50 gallon buffer tank for the gas system.

    Now let's get to the efficiency thing. First, what appliance in your home other than your heating system can offer you any payback at all? Car or truck? Refrigerator/freezer? Counter tops? Wide screen T.V.? I don't know what you have in your home but why the emphasis on payback for the heating system when nothing else in the home provides a payback? How about just because it's more energy efficient than the competing unit?

    There are a number of efficiencies to be considered in a high efficiency gas boiler when compared to a basic boiler. First is the mass of the boiler. You could put in a Viessmann or Buderus condensing high mass (cast iron) gas boiler and use the mass of the boiler as the 'buffer tank'. Or you could go with a standard gas boiler and operate above condensing temperatures most of the time, again using the boiler mass as a substitute for the buffer tank. However, the mass of the boiler is part of the energy savings of a low mass condensing boiler. The fact that a condensing boiler can actually operate continuously at 120F and below is the very essence of a condensing boiler. Thus once one recognizes the advantage of operating at 120F or below, the use of a condensing boiler is obvious. Condensing the water of combustion on a continuous basis is a huge savings (960 Btu's/pound of water condensed) over that which would otherwise be lost up the chimney. So whether you choose a low mass boiler and by-pass the buffer tank or a low mass boiler and add a smaller buffer tank is, IMO, insignificant. The crux of the issue is that yes, a condensing boiler with its increased controls, complexity and maintenance would be to your advantage. Since payback is of concern to you apparently, then the variables that determine savings one over the other (high mass non-condensing versus high mass condensing or low mass w/buffer tank) is dependent on the But's needed over a winter, the combustion efficiency of the boiler selected, the cost of gas in your area and the temperature you keep your home.

    As far as the geo-thermal system goes, it sounds like you were using either a pump and dump or standing column well as your heating/cooling water source? There are several ways to do geo and using vertical dry wells or a trenched system also works. You might want to investigate that a little further before you jump into the gas system, since that will affect your payback or nullify any payback you might otherwise have determined with gas.

  3. #3
    Skippedover, thanks for the reply having spent some time reading this forum I value your opinions, let me expound a little and then get your feed back.

    Payback, I agree no other appliance is going to payback. I'm not removing a functioning inefficient system to replace it with a more efficient one and looking for the point at which I break even for making the investment. Rather I am looking at adding an expense to return heat to my home and I evaluate this expense in total (over time as well as initial cost). That being said I would more compare it to having to replace my refrigerator, the bigger I get the more it cost to operate, so I save money (both on the purchase and the operation) by buying what I actually need rather than the biggest one with the most "Bells and Whistles".

    If a condensing boiler is going to cost me an additional $3K more and not provide enough additional on going savings to "payback" that 3K over the life of the boiler (because I am in Dallas with marginal heat needs) then I'm just paying for "Bells and Whistles". The buffer tank (and existing controls) do the "modulating" from a comfort stand point and "IF" (the crux of my question) btus required ends up the same (ie. 130K btus for 5 min VS 75K btus for 10 mn) then my savings is back to the difference in AFUE value (or close to it).

    Add to that my concern that modulating systems are not simple to install and I am not sure I can find a sub that has very much experience with residential boilers in the Dallas area much less modulating boilers and I don't want to be somebody's training ground.

    I would not want to use a cast iron unit as my utility room is located on the 2nd floor and would prefer to stay away from the weight (no basements in Dallas). I was thinking a sealed burner with a stainless steel exchanger would be my best bet. Any manufactures you would suggest, so I can search for local installers?

    I am familiar with the various options for the Geo thermal. Our house is a large Victorian a 1/2 block off the downtown square. The only real option we have is vertical wells. The problem is we have spent the last 10 years developing the property with extensive gardens and water features which would be significantly damaged to come in and dig the wells, not to mention the cost of the wells alone would be around $30K (we have a 10 ton system that would require 10 - 300 ft wells). We might end up abandoning the geothermal. Our air distributions system is UNICO high velocity (ran the 4" ducts in the walls to avoid furdowns in the Victorian) and one option would be to add refrigerant coils to the existing modular blowers with HEFF condensing units, just hate to give up the Geothermal.

    Love to get some more feed back.

  4. #4
    Join Date
    Jul 2008
    Rochester NY
    The analysis on incremental investment in low temp situations is almost not worth the effort because you are always operating at condensing temperatures AND you avoid cycling losses, risk of condensation in a non condensing boiler, and cycling wear and tear. Avoiding dealing with 400 f exhaust is another plus.

    What seems odd is the assumption you need 130 k. I see you have 10 tons of geo? How big is this house? It seems odd that someone owning 10-20,000 sf home would think twice about getting a good vs crappy boiler. Where does this load come from?

    Another possible misunderstanding is your bells and whistles paragraph. When will a mod con EVER run at 130 in you circumstance?

    If you are wondering about strategy going forward a bigger picture needs to occur. Time for some engineering. If you just want simple answers, get the mod con.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to, or RESNET, and find an auditor near you.

  5. #5
    Join Date
    Jul 2004
    TRex, you've got numerous issues tied up in one decision.

    1. Stick with or abandon the geo.
    2. If sticking with it, how to solve the earth coupling issue.
    3. How much to invest in the geo solution.
    4. How much to invest in alternative solutions (heating & cooling)
    5. Cost of operation of geo versus gas and air-to-air cooling.

    Some of those answers are simple math. Others are less simple emotional decisions. I can help with the math, the emotional are all yours.

    The lowest temperature solution for the radiant heat is your best solution over the long term. If you've not had a blower door test done on the home as well as a load analysis, that's where I'd begin the project. Once you know the leak rate on the home, you might want to pay some attention there first, re-test and then have the load analysis done. The payback if your hold home is leaky could be substantial in a short period of time, given that you could purchase and use a smaller heating solution.

    I obviously cannot speak as to the expertise of heating professionals in your area when it comes to gas boilers. If necessary, I'd recommend communicating with either the manufacturer or better still, the local suppliers of the boiler of choice to find a high quality installer of the product. I'm sure they'd be willing to share a short list of companies with you.

    Once you find the company, then yes, a proper installation is required. As to maintenance, the chosen installation company should also be willing and able to provide service minimally once per year but preferably twice per year.

    My favorite gas boilers are the Triangle Tube Solo and #1 choice and the Lochinvar Knight as the #2 choice. I have no idea as to their availability or popularity in your area, of course. If you end up using he gas boiler for an extended period of time or permanently, I'd highly recommend a low mass, high AFUE, modulating gas boiler with a small buffer tank to smooth out the starts and stops where you're using such a low temperature. The boilers I've mentioned have outdoor temperature reset on them and thus will modulate in reference to both return water temperature and outdoor ambient. What other controls you have should be dedicated to the distribution system, while the boiler should be looking solely at the buffer tank temperature with the system designed as such. Having not seen your system piping arrangement, if it doesn't need the buffer, then obviously the boiler reference would be to return water from the radiant floors. It seems that using a smaller buffer tank would simplify the controls issues however, as guessed from afar.

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