Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Boiler replacement for hydronic system with cast-iron radiators

sovavto Member Posts: 15

As we're planning to replace our old boiler (gravity fed, millivolt) with a new high efficiency boiler, I have a few questions.

While we do not think that a combi boiler with (tankless DHW) is our first choice, I have seen a "combi" boiler which combines hydronic heating with indirect DHW tank. <a href="http://www.htproducts.com/versa-hydro.html">http://www.htproducts.com/versa-hydro.html</a>

The boiler is to service 10 radiators on 2 floors with ~1900 sq.ft area. Most radiators are cast-iron column type radiators converted from steam long ago.

The DHW is to supply 2 bathrooms, 1 kitchen, and 1 washing machine hookup.

The location is New England. Cold winter nights may be in teens, rarely single digits. The house has been insulated over the past 2 years (walls/ceilings/new windows).

Few questions:

1.Which is more efficient in colder climates for DHW: indirect with a tank or direct/tankless hot water?

2. What is an acceptable temperature of hydronic hot water? The HTP Versa Hydro that seems like a nice option has a maximum temperature of 160 degrees F. Is that enough? My sense is that while 180 supply might be hotter, 160 supply will probably maintain a more even temperature.

3. Are there other combination boiler (aka system boilers) that have indirect hot water (I know there are lots of combi systems with direct hot water, but these aren't the ones I'm asking about)

4. Other than space saving considerations, what do you think is the trade off between the expense of an integrated combi system that requires less installation labor and the expense of labor for installing a separate boiler with an expansion tank and an indirect hot water tank?

5. Last but not least - if the system referenced above HTP versa hydro would not have been your top pick, which boilder/indirect hot water tank would you recommend?

Look forward to any advice you can provide. Thanks!



  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    it depends.

    "2. What is an acceptable temperature of hydronic hot water? The HTP Versa Hydro that seems like a nice option has a maximum temperature of 160 degrees F. Is that enough? My sense is that while 180 supply might be hotter, 160 supply will probably maintain a more even temperature."

    Since I am not a pro, I will not comment on your other questions. But for this question, the answer, as usual, is: it depends.

    If the radiation in the house was designed to be exactly the right size, and the insulation, windows, and infiltration have not been improved since the heating system was designed, you will need 180F water to heat the house..

    But to get the benefit of a modern high efficiency boiler, you will almost certainly wish to run with much lower temperatures. I have two heating zones in my house, one of which is heated by finned tube baseboard. Since my boiler is a mod-con and I wanted condensing to occur, I replace the old baseboard by some that is almost 5 times longer. This enables me to use 135F supply when it is 0F outside, 130F when it is at design temperature, and 110F when it is at least 52F outside. Needless to say, the return water temperature is less than the supply. This is so that some condensing happens almost all the time the system runs.

    That list of assumptions I gave is most likely to be partly wrong. Perhaps the system was designed with too much radiation. Perhaps your insulation has been improved and leaks sealed. Perhaps you now have better windows. In that case, you might get by with temperatures less than 180F. Perhaps you are willing to increase the size or number of radiators. All of these will permit lower temperatures.
  • sovavto
    sovavto Member Posts: 15

    Somewhat to my surprise, some licensed HVAC and some plumbing and

    heating contractors have previously provided me with unsound advice and

    proved to be less knowledgeable than one would reasonably expect.

    I do appreciate your suggestion. The main reason I posted on here is to get as many opinions as I can.
  • HeatKing
    HeatKing Member Posts: 2

    Unfortunately,  licensed does not mean that they are knowlegable in this certain aspect of the trade.  The best advertisement for a good HVAC professional is word of mouth.  I would ask around your neihborhood or family if they have any one to reccommend.  A good resource would also be your local supply house.  The supply houses usually can identify the hacks from the true professionals. 
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Define 2 Bathrooms

    What is in these bathrooms? Combi will produce 3.5gpm enough for 1 shower at a time....Not a human carwash. A single shower head....
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Not a human carwash.

    I once got a tour of a very rich person's house around here. The master bathroom was all tiled from floor to ceiling. It had two benches so you could sit while showering. It had two of those giant shower heads (perhaps 10 inches across) I do not remember ths size of the supply pipes. Each had its own temperature regulating valve so he and she could shower at the same time and use different temperatures if they liked.

    I suppose that was a human car wash.
This discussion has been closed.