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Steam/Forced Air System - Convert to Steam/Hot Water or All Hot Water?

dvdeusen
dvdeusen Member Posts: 1
I've lived in my 1927 northern New Jersey home for about 3 years. The original part of the house (Zone A) has 1-pipe steam radiators and a dedicated AC with ductwork. There is also an addition (Zone B ) that uses a forced air furnace/AC with ductwork. (All heating using natural gas).

Zone A's heating works well from a comfort perspective (after @JohnNY and Gateway/Toro fixed it up 2 years ago); I have VentRite #1s on all radiators adjusted so that they heat up evenly. Zone B, however, is a comfort nightmare. Some larger rooms with bigger loads have almost no ducts, other rooms have far too many, and all of the ducts are very leaky.

The third floor of the house (Zone A, steam & dedicated AC) is being extensively renovated (either this fall or next spring) and would require all the steam radiators to be moved substantially or replaced with an alternative.

I've had several energy home diagnostic companies evaluate the house to give info on existing insulation and air tightness. They also have provided options for insulation/air sealing improvements. I've also pulled temperature data for my area and attempted room-by-room heat load calculations (both for "current state" and "future state" post insulation-improvement).

My Goals:
1) Reduce comfort issues
2) No more forced-air heating
3) Use less natural gas

My plan is to first move forward with the insulation/air sealing improvements, then use a hot water heating system to replace the forced air for Zone B and the steam on the third floor. After doing the post-insulation heat load calculations, I'm interested in getting an air-to-water heat pump system (with outdoor reset control) that could efficiently handle most (although not quite all) of the heating load of these radiators as well as handle the cooling via two hydronic air handlers. I currently only have one cooling plan but I see two potential hydronic paths forward for which I would love to get input.

Cooling Plan
Run insulated cold-water lines to two hydronic air handlers. (Ductwork for Zone A is fine, would have new ductwork/ducts run for Zone B ). An external heat pump would replace the two existing AC condensers.

Heating Plan A
The steam system is largely unchanged, besides the loss of the third floor radiators. Hot water radiators will be added to the third floor and all of Zone B.

An air-to-water heat pump will be the primary source of heat, heating a buffer tank to a temperature specified by an outdoor reset control. All radiators have thermostatic valves and a variable-speed pressure-regulated circulator operates continuously when heating is "on".

When outdoor temps hit a low temperature (My current thinking is 25 degrees; temperatures below 25 degrees only occur less than 5% of the heating season and this corresponds to where the intended radiators need water temps approaching 130, the limit of the heat pump system) and the heat pump cannot efficiently produce the water temps needed for the radiators, the heat pump will turn off and the steam boiler will heat the buffer tank using a heat exchanger.

Pros: Less invasive, uses existing steam boiler
Cons: More natural gas usage, particularly in shoulder seasons. Steam boiler becomes oversized with the loss of some radiators. (Current boiler sized for 472 EDR. Current radiators constitute 407 EDR, after third floor radiator loss this would drop to 315)

Heating Plan B
Similar to A, but all radiators would operate as hot water radiators. The steam radiators in Zone A are generously sized - importantly, they are sized sufficiently large that they can meet the loads of the rooms with the same water temperatures as the new radiators in Zone B.

I believe the radiators, though currently configured for 1-pipe steam, could be modified for a hot water system. After reading this post:https://heatinghelp.com/systems-help-center/converting-from-steam-to-hot-water-heat/, I'm pretty sure that my radiators have the "top push nipples." I believe this valve https://danfoss.com/en-us/products/dhs/radiator-and-room-thermostats/radiator-thermostats/radiator-valves/ra2000-conversion-valve/#tab-overview would enable the conversion.

In this scenario, buffer tank (heated by the air-to-water heat pump) sources the hot water for all radiators. Instead of the steam boiler acting as the colder weather heat source, a mod con boiler would take its place.

Pros: Less natural gas usage (heat pump can handle all heating in shoulder seasons where its efficiency is high)
Cons: Much more invasive (I assume all the cast iron steam piping would be removed and replaced)

Open Questions
  • Which heating scenario seems most sensible? (perhaps neither? is there a plan C that I'm not considering?)
  • How big of a job is it to switch the radiators over to hot water, and do these conversions work well?
  • Is it a problem that post-insulation the steam radiators would be very oversized for the rooms? (eg would this lead to any short cycling or big temperature fluctuations in the rooms?)
  • Can the steam boiler actually serve as the back up heat for the buffer tank in "Heating Plan A"?
All feedback is appreciated!

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 16,324
    Well let's see. You ask... First question. I would definitely go for a modification of plan A. The modification being that I would leave the third floor radiators in place, as is, heated by steam. You yourself say that Zone A is working well for you.

    My friend, if it ain't broke, don't fix it.

    The change in structure heating load in Zone A from the insulation and air sealing will not, of itself, affect the operation of the system in that zone. What will happen is that the boiler will run for less time overall, and you may find it expedient to change from one cycle per hour -- if that's where you are now -- to two, if the system heats quickly enough when the thermostat calls. Your con for plan B -- much more invasive -- is a masterpiece of understatement.

    You can, as you suggest, also heat a buffer tank for zone B from the steam boiler and use some of its excess capacity for that. Same principle there as an indirect water heater, but the hot water resulting being used for heating.

    Your second question, how much of a hassle is it to convert radiators from steam to hot water? If you are fortunate and the radiators themselves are of the type with top passages, not really all that much. If you are lucky. Keep in mind, however, that the radiators will need to withstand ten times the pressure to which they have been subject up to now. The odds of at least one them leaking and having to be replaced completely are very high. There's also the minor detail of running all new piping for at least the returns from them -- and as you suggest it might not be a bad idea, while you are at it, to run all new feed piping as well. Your second half of the question, do these conversions work well. In general, not really. Or to put it another way, except for the possibility of zoning, they are never better than the steam system they were converted from, and usually worse.

    Your third question, would the radiators post insulation lead to overheating? No. That's what thermostats are for. As I noted under the first question, you might be able to switch to 2 cycles per hour if you are now running 1, which give you even more even heat.

    Your fourth question, can the steam boiler be used for backup heat? Certainly, and the modifications needed are very simple (in addition to the indirect used as a buffer tank and its thermostat/aquastat, you will need some pipe and fittings a couple of valves, and a very small pump).

    Now I sense an underlying motivation: reduced natural gas usage. Well, maybe. Leaving aside the political question of when we will all be forced to eliminate all fossil fuel usage, how much reduction overall you can make is very very much a function of where your electricity comes from and the COP of the heat pump. Most of your power comes from natural gas fueled power plants. They have an internal efficiency, when all is going well, of converting about 40% of the heat in the natural gas to electricity. Then some is lost in transmission, so the end result is that about a third -- 33% -- of the heating value of the natural gas used in the power plant actually makes it to your house. A good steam system -- and if @JohnNY has anything to do with it, yours it good -- has an efficiency of around 85%. Bottom line there is that if the heat pump COP at a given outdoor temperature is less than about 2.5, your boiler will use less gas to heat the house than your heat pump would. Even if your heat pump can manage a COP of 5 -- very high -- you would still burn only half as much gas with a heat pump as you would with a boiler.

    I hope this helps...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    mattmia2
  • Steamhead
    Steamhead Member Posts: 14,700

    Well let's see. You ask... First question. I would definitely go for a modification of plan A. The modification being that I would leave the third floor radiators in place, as is, heated by steam. You yourself say that Zone A is working well for you.

    My friend, if it ain't broke, don't fix it.

    The change in structure heating load in Zone A from the insulation and air sealing will not, of itself, affect the operation of the system in that zone. What will happen is that the boiler will run for less time overall, and you may find it expedient to change from one cycle per hour -- if that's where you are now -- to two, if the system heats quickly enough when the thermostat calls. Your con for plan B -- much more invasive -- is a masterpiece of understatement.

    You can, as you suggest, also heat a buffer tank for zone B from the steam boiler and use some of its excess capacity for that. Same principle there as an indirect water heater, but the hot water resulting being used for heating.

    Your second question, how much of a hassle is it to convert radiators from steam to hot water? If you are fortunate and the radiators themselves are of the type with top passages, not really all that much. If you are lucky. Keep in mind, however, that the radiators will need to withstand ten times the pressure to which they have been subject up to now. The odds of at least one them leaking and having to be replaced completely are very high. There's also the minor detail of running all new piping for at least the returns from them -- and as you suggest it might not be a bad idea, while you are at it, to run all new feed piping as well. Your second half of the question, do these conversions work well. In general, not really. Or to put it another way, except for the possibility of zoning, they are never better than the steam system they were converted from, and usually worse.

    Your third question, would the radiators post insulation lead to overheating? No. That's what thermostats are for. As I noted under the first question, you might be able to switch to 2 cycles per hour if you are now running 1, which give you even more even heat.

    Your fourth question, can the steam boiler be used for backup heat? Certainly, and the modifications needed are very simple (in addition to the indirect used as a buffer tank and its thermostat/aquastat, you will need some pipe and fittings a couple of valves, and a very small pump).

    Now I sense an underlying motivation: reduced natural gas usage. Well, maybe. Leaving aside the political question of when we will all be forced to eliminate all fossil fuel usage, how much reduction overall you can make is very very much a function of where your electricity comes from and the COP of the heat pump. Most of your power comes from natural gas fueled power plants. They have an internal efficiency, when all is going well, of converting about 40% of the heat in the natural gas to electricity. Then some is lost in transmission, so the end result is that about a third -- 33% -- of the heating value of the natural gas used in the power plant actually makes it to your house. A good steam system -- and if @JohnNY has anything to do with it, yours it good -- has an efficiency of around 85%. Bottom line there is that if the heat pump COP at a given outdoor temperature is less than about 2.5, your boiler will use less gas to heat the house than your heat pump would. Even if your heat pump can manage a COP of 5 -- very high -- you would still burn only half as much gas with a heat pump as you would with a boiler.

    I hope this helps...

    This.
    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • Lance
    Lance Member Posts: 168
    Your Goals: Increase Comfort, Eliminate Forced hot air, reduce natural gas use.
    After reading your extensive outlook, my first response is to help you the right way will require some thought. The largest component of your comfort system is not the equipment, it is the building, and secondly the distribution system for heat and cool. Equipment sizing and application comes last after the building is ready and your ability to adopt any changes to or replace the distribution system. Built in 1927, means the structure is loose and poorly insulated. I would recommend the building be appraised for these issues. what has been changed and what could be changed to make improvements here first. If you reduce the heat and cool loads in this area, it will affect all choices made in engineering the distribution and equipment. A blower door analysis that pressurize the building can determine air leakage and pin point solutions. Add insulation is secondary to sealing the open holes the building leaks and breathes with. Next an ACCA Manual J heat loss and gain measurement must be done based on the building refinements for insulation and infiltration. Heat and cool are opposite duties and while we can blend them into one forced air system, having a hydronic system currently is a great asset that should be retained if possible. The principles I follow in building a home and comfort system are this: If its complicated, make it simpler. Address how each part of the building is being used. Bathrooms get 115% heat added. Cooling 100%. Since hot air rises and physics cannot be ignored, there will always be 20-50% of the lower level cooling load migrating to the upper. If 50% cool load is calculated for both levels, build 30% cool capacity for lower and 70-75% capacity for upper. With heating not so much disparity is required as your system is built from the bottom up and heat rises. In cooling we must move air either by fan and duct or by gravity circulation with valance fin coils. Cost prohibits fin coils compared to forced air. But an option for Ductless heat-pumps could be a good solution for those pesky spaces that just don't seem to fit a central temp control. They can provide quiet airflow and backup heat if the heat system is off line. I would go into more but its your baby and good luck.
    Lance Bent Melroy P&H Inc.
  • mattmia2
    mattmia2 Member Posts: 3,431
    dvdeusen said:

    I
    Can the steam boiler actually serve as the back up heat for the buffer tank in "Heating Plan A"?

    I was thinking this before I even got to the end.

    I have a couple other thoughts. For the renovated floor, it will probably be easier and less expensive to execute moves of some radiators instead of replacement of that system on that floor. You will have to do more careful planning in the design phase but it should make the build phase simpler. There are lots of options to replace some of the radiators with a different design for some of the more challenging areas(but you do need to be careful to maintain balance with the rest of the system)

    You could add a TRV between the vent and the radiator in some strategic locations to make the steam more predictable if you have to in places with variable loads or just very different loads, IE a kitchen or someplace with a lot of solar gain or a section that has new insulation while the other sections have not had the insulation upgraded yet.

    I can see some of this you want to do to increase comfort and other parts you just want to do which is ok, but don't expect to save enough in gas to make up for the cost of some of these things.