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Retrofitted Gravity Hot Water designing further upgrades

I have a hot water radiator system, single loop (yep) gravity eureka tee based system with about a 4 in diameter main supply. The supply piping to the radiators actually get smaller the further along the loop run you get. All the iron pipe is in place and seems in pretty good shape. There is some small delay in heat at the last radiator but not all that much really. Originally it was a 1912 coal fired system (the original boiler is still in place). There was an expansion tank in the attic now removed and the system has been retrofit with a circulator, air eliminator, expansion tank, single zone with a bypass to the return side of the high mass cast iron Boiler (Dunkirk WPSB circa 2001 almost looks straight out of "Pumping Away"). The circulator is on the supply side. The Dunkirk has an aquastat set to 150 though originally the cast iron supply had an aquastat supplying at 180 (it is still there). The rads have an EDR around 90,000 BTU (at 150 F water) but the boiler is sized at 299kBTU (seems large to me there is also seems to be some short cycling). I still need need to do a heatloss calc for the house. I will also be doing some substantial upgrades to my enclosure.

It seems the folks who installed the systems and upgrades knew what they were doing. The last rad on the circuit is on the southeast and is in the kitchen. The thermostat is located on an interior wall on the north side of the house. The system actually works pretty well no real complaints, except I feel like my bill is a little high, there is a lot to be said for that.

So why post? I recognize that a 17 year old boiler may last a long time still but also may be on its last legs. I am also interested in possibly doing some upgrades for efficiency and even unconventional things. I think my system could run more efficiently and that my boiler could likely be downsized.

Upgrades first--

I think a reset on my system makes good sense.

I would consider installing TRVs as well. The house is historic (1912) so I do have some concerns about installing ugly TRVs and with threading on nice looking ones from Europe. Since it is a single loop eureka tee based system there is already bypass piping on the single loop. I also know that since it is a gravity retrofit some hot water will naturally head up the risers even when the TRV is shut. The circulator should minimize that some but not completely.

There are possibly two places that have had Rads removed that may benefit from being reinstalled. They were likely removed in the 30s.

Adding a loop to go directly to my domestic hot water (seems like at one time it did that but not currently set up that way).

Should I do a constant circulator system? What are the benefits/negatives for this on a gravity based retrofit with the type of tee I have?

I would consider installing a condensing/mod boiler. This seems like a no brainer, except I may want to go all electric for personal efficiency reasons.

Unconventional ideas-

I am particularly interested in possibly installing something like an air source heat pump or a residential reverse cycle chiller to deliver chilled water to my rads or pull a loop off (valved for the summer) something similar to commercial chilled beams.

So what do folks think? Anything I should just forget about outright? Anything I should do without question?



  • SuperTech
    SuperTech Member Posts: 2,206
    Without question do an accurate heat loss calculation for your house, and look into the various different Tekmar controls for outdoor reset and system control. I would avoid the TRVs on a single pipe system. Proper flow is critical for all radiators to work properly.
  • jbeck
    jbeck Member Posts: 15
    Constant circulation - check the cost of electricity for running the pump.

    Look into dirtmag that removes ferrous material in the iron pipes....I am thinking about installing one as my water is very black.

    I also don’t have an air scoop and want one of those.

    I removed my TRVs because they were causing trouble. The plumber installed them backwards...I didn’t want to risk it so I took them out and installed regular valves.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    Would you consider a mod/con? High mass iron is just about as good as a radiant slab as far as a modcon is concerned.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Emisivity
    Emisivity Member Posts: 6
    So I want to be clear that my single pipe isn't a true single pipe like a baseboard. There is a "bypass" available because eureka tees do not introduce the pressure difference, the water does that itself. Maybe you know that already. My thought is that the TRV would allow water up to the point of the TRV but the majority of the water would move following the circulator. Maybe not. What I am hearing is that a TRV for a single pipe gravity system isn't worth the potential headache. It may work but too iffy to be worthwhile. Great for me saves me money. I am a bit of an efficiency fiend though.

    I would absolutely consider installing a ModCon. I want to understand more as to why they are so good with High Mass Iron vs. something like a heat pump. I especially like the possibility of ramping down even further on the temperature. As I tighten the envelope and eventually install dedicated ventilation with and ERV or HRV. I will need a lower and lower delivered temperature. What is it about how mod cons work that make them so great for a High Mass gravity system? Is it just the ability to ramp down the output and rely on the mass of the system?

    At the same time I have an interest in eliminating all local combustion. Where I am at has horrible winter air quality primarily attributable to combustion. It may not make sense to eliminate the combustion.

    My water is not all that black honestly. It seems pretty darn clean. I am concerned about fouling by going to a low mass boiler though, or water quality causing similar issues.

    I'll be doing a proper room by room heat loss calc and will likely even include something like a blower door test to get better results than "average" "tight" etc. Air changes are usually the greatest determine factor in heat loss calcs and tightening up the envelope can go a long way.
  • Gordy
    Gordy Member Posts: 9,546
    With your room by room heat loss survey, do a room by room edr survey. To see how balanced the edr is to the load in each room. Also gives an idea of how low you can run those water temps. A mod/con is a good marriage to a system like yours. Those large diameter pipes are like a buffer tank.

    Trvs are an excellent option for zone control.
  • Emisivity
    Emisivity Member Posts: 6
    Great suggestion on the room by room edr. Essentially that is what I did when I calculated the total edr load. Correlating to the room by room heat loss is smart.

    So some folks like TRV for this system others don’t. Those of you who like them why don’t you agree with those that don’t.

    I am heating on the mod/con that due to the overall mass of water in the system, plus the mass of the roads, the mod on can ramp down and use the thermal inertia as a flywheel. So the Modcon ramps way, way down. Is that why?

  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    And the inherently low return water temps with a high mass system make mod/cons run in their highest levels of efficiency.

    With a non/con boiler you need to protect yourself from low return temps, with mod/cons that low return temp leverages condensation and over 90% efficiency.

    The modulation allows for much lower supply temps, while still allowing the boiler to run for very long periods. Essentially sizing itself to the load based on how cold it is outside.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!