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

dvdeusen

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!

Jamie Hall

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...

Steamhead

Jamie Hall said:

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.

Lance

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

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.

dvdeusen

An update here for posterity in case others are searching the forum for Air-to-Water heat pump + radiators use cases. Despite the sensible advice received here regarding how big of a project it would be to switch my system to hot water, I was excited enough about the prospect to go for it.

I was not able to find anyone willing to convert the existing steam radiators to hot water, so I purchased new radiators (using the opportunity to make them as large as possible to keep required supply water temperatures low - there is a combination of traditional cast iron radiators and wall-hung steel column radiators). As part of the home renovation we also did extensive insulation and air sealing, both to reduce the heat load overall and once again this helps keep the radiator water supply temps low.

I currently have an Enertech Advantage with hot water flowing to radiators (new homerun distribution), following an outdoor reset curve. The circulator is constantly running (there is a constant call for heat) and the outdoor reset curve is set aggressively in an attempt to provide just enough heat to maintain desired room temperatures. There are TRVs on each radiator set for individual room comfort. At some point I'll be receiving a system monitoring device from Enertech that will track a bunch of data, including the COP which I'm very interested in. Currently only 80% of the total system EDR is installed, with the remainder of the radiators to be installed when tile/floors are finished in the area of the house still under construction. In light of the partial installation, I'm expecting to be able to lower the outdoor reset curve a bit more in the future and hit higher COPs. However, in the past week of 30s/40s I'm seeing COPs in the high 3s to mid 4s when I check the screen on the internal unit.

The max output of the unit is 55k BTUs/hr, the house is ~3800 square feet. I'll report back when the unit experiences sub-10 degree weather with how the unit does. There is a built-in backup electric resistance heater in the internal unit that can kick on if the heat pump itself can't match the required capacity.

I'm very pleased with the system so far! As a PSA for other homeowners considering an Air-to-Water heat pump, a couple of important points:

1. The insulation and air sealing work is really important! If you have an old leaky house like I did, you really have to do quite a bit of work to make sure the heat load is lower enough that you can use the lower water temps to match the load. Otherwise, the COPs will be low and you might well end up paying more per unit of heat than you would with natural gas. The COPs also matter in terms of the actual emission reduction you'll achieve from switching to a heat pump while using grid power
   
2. If you want an Air-to-Water heat pump installed, don't wait until your current boiler breaks. It is quite difficult to find contractors to install this sort of system at the moment and the timeline from getting a quote to installation is long. I had the most luck going to heat pump manufacturer websites (Enertech Advantage, SpacePak) and using "find a contractor" tools there to get referrals.
   
3. "Converting" from steam at this point essentially means installing a new hot water system and radiators if you have the same luck that I did converting the radiators. For most people with steam systems, I'd prioritize making that system work well and doing insulation/air sealing work to reduce your emissions rather than installing a brand new hot water system. I was already tearing open a bunch of walls during renovations which made the removal of old steam piping and installation of new water piping not an impossible ordeal.
   
4. I found John Siegenthaler's writings/presentations on Air-to-Water heat pumps very helpful in this process. Browse Youtube for his presentations and review Idronics 27 and Idronics 25 as a starting point.
   

dvdeusen

Jamie Hall said:

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 wanted to follow up here and emphasize @Jamie Hall 's good point that reducing the natural gas burned in the basement is offset by the increased use of electricity from a grid that produces emissions. I want to run through the calculations of my specific project and leave a formula here with some resources that hopefully others could use when considering the emissions impact of proposed hvac/insulation changes.

The percent reduction in CO2 emissions resulting from switching a natural gas boiler to a heat pump should be as follows: 1 - [(2.58 X (Emissions per kWh) X (Gas Boiler Efficiency) X (1 - Percent Load Reduction from Insulation Work)) / (Heat Pump Seasonal COP)]. I'll run through each variable and then plug in my specific variables in the formula to give a concrete example.

• 2.58: This is just a constant to keep the formula looking less horrible. It's assuming a 5.3% line loss on the electric grid, 12 pounds of emissions per therm of natural gas, and uses 100000 BTUs per therm and 3412.1 BTUs per kWh conversions
  
• Pounds of CO2 Emissions per kWh: This is grid-specific - use EPA's Power Profiler to get the number for your grid. For my RFCE grid, I have 0.652 pounds for kWh.
  
• Boiler Efficiency: Steam maxes out at 85%ish, hot water boilers can reach the 90s. I'm plugging in 85% for my boiler.
  
• Insulation: We did tons of insulation work on a house that had almost none. This number can be verified in the future once my system starts recording kWh usage and we can compare to old energy usage, but ~40% is reasonable given we moved from zero to closed cell foam on the roof and basement rim joists, and 0 to densely packed cellulose in the walls.
  
• Seasonal COP: Coefficient of performance (measure of how efficient the heat pump is at moving heat). I'm hoping to achieve something close to 4 on average for the heating season when all the radiation is installed, but I'll pencil in 3.5 for now which is roughly what I'm achieving with 80% of radiation installed.
  

I'll now plug in the numbers here for my example: 1-((2.58 X 0.652 X 0.85 X (1-0.4))/3.5) = 0.77, which represents a 77% reduction in operational emissions as a result of both the insulation work and the switch to the heat pump. I want to emphasize again the importance of the insulation here. From the formula it looks like the insulation is only responsible for the heat load reduction, but it importantly also has enabled the higher COPs for the heat pump by enabling lower water temperatures for the radiators.

To show an example where good intentions of reducing emissions can go awry, let's assume no insulation work happened (so 0 instead of 0.4), the COP is reduced to a seasonal average of 2.5 (perhaps because there isn't sufficient radiation for lower water temperatures), and the grid has moved from RFCE to MROE (increasing pounds of CO2 emissions per kWh from 0.652 to 1.526. This formula shows us that this would actually lead to a ~34% increase in CO2 emissions, despite the removal of a gas boiler.

Of course, it should be noted that if recent trends continue, the emissions from the grid will very likely reduce over time since the proportion of energy produced by solar/wind is increasing, but it makes sense to be mindful of the grid sourcing when thinking through HVAC changes done for the purpose of reducing emissions. (Perhaps install solar and do insulation work first if you can manage it)

I hope this is helpful to homeowners thinking about reducing emissions through HVAC work, and please correct my math if you spot errors.

WMno57

What keeps the water in the outdoor A2WHP from freezing in the event of an extended power outage?

ethicalpaul

Where is the gas loss due to leaking distribution pipes? This is not a trivial amount.

dvdeusen

WMno57 said:

What keeps the water in the outdoor A2WHP from freezing in the event of an extended power outage?

The "water" in the system is actually 30% Propylene Glycol - good question! More context for others - the unit is "monobloc" so there is a water line from the outdoor unit to the basement rather than a refrigerant line.

The house also has a full-house generator (haven't completely removed dependency on natural gas yet - the roof of the home is a mix of slate and copper, both of which don't play nicely with solar panels) so a true system outage should be rare indeed.

dvdeusen

ethicalpaul said:

Where is the gas loss due to leaking distribution pipes? This is not a trivial amount.

This is a good point - it's not really a fair comparison since I'm including line loss for the grid yet no loss for gas. I'll try to find a reasonable estimate and update the formula.