Residential retrofit maintains historic appearance

In the middle of the Front Range, looking down over the Denver skyline and plains, sits a historic property built in 1933 that is undergoing a complete transformation. Our task was to replace the existing steam boiler heating system with a hot–water, forced-circulation heating system to provide better temperature control. The other important factor was by eliminating the original steam piping mains in the basement, the new owners would gain valuable headroom space. 

The specs

The home was mostly stripped down to the studs, but it retained much of the fine woodwork.  The house was built in the Southwest style of architecture with white stucco and dark stone exteriors, and a red tile roof. Unfortunately, the exterior walls were either two-by-fours with no insulation or uninsulated concrete block. The ceilings throughout, and in the two vaulted great rooms, were also uninsulated.

For a total of 6,700 square feet of living space, the existing system consisted of two not very old 199,000 Btu/h steam boilers in a two-pipe steam arrangement supplying a variety of heat emitters. The great rooms had floor-mounted and enclosed cast iron radiators. The basement and garage had ceiling and wall-mounted cast iron radiators. The unique parts of the house were the 19 recessed wall unit heat emitters that were built by an unknown manufacturer.  The question became, do we replace them or try to salvage them? 

We disconnected and disassembled one of the units so it could be air tested to see if it was a viable candidate for conversion from steam to a hot-water system. It held the 30-psi air pressure test, which was a good sign, so we decided to try to salvage them all. Each unit was then disassembled and air tested. Two failed the pressure test, but the rest of the 17 were good. This was great news since we could then reinstall the original heaters to keep the original historic look of the home.  

The recessed wall units had a cast iron front mounted radiant panel plate and a row of fin tube elements in the cabinet, so it was both a radiator and a convector. A label was found on an inside panel, which turned out to be made by Modine Mfg. Co.

I called them to inquire about these units, and if they had any specification data for them. They told me Modine had a fire that wiped out much of their archival material many years ago.  We had no Btu output data for these units, so output estimates were based on other known, but somewhat different emitters.

It was obvious that upgrading the building envelope was going to be of major importance for this approach to work because when converting to a system that runs on 180° F maximum hot water supply temperature instead of approximately 200° steam at the altitude of the home, the Btu output of the emitters is significantly reduced. We performed ACCA Manual J heat load calculations based on the new envelope design and insulation values that were specified. Additionally, one of the great rooms featured a massive fireplace that needed to be properly sealed when not in use. Old wood fireplaces with drafty dampers are a common and preventable infiltration point and a high heat loss area. 

The conversion

To convert to a hot-water system, the existing steam traps were removed from the radiators and recessed wall units and replaced with new hot-water valves or thermostatic radiator valves (TRVs).  In some areas, multiple cast iron radiators are connected and controlled with a single zone valve and thermostat. In other areas, electric actuators are mounted on TRVs installed in the recessed wall units. These are also controlled by wall-mounted thermostats.  Several radiators are installed with non-electric TRVs and get heat whenever the system is on. Four of the larger zones are Wi-Fi-connected for remote monitoring and control. The homeowners are often away, and this allows them the ability to turn up the system so the house is warm when they arrive.   

The heating design for the smaller rooms in the lower level basement area calls for modern steel panel radiators and towel warmers, each with a non-electric TRV. Each panel radiator controls its own output based on the manually adjusted TRV dial setting and is essentially a zone unto itself. We installed a few new cast iron thin tube style radiators to provide heat where the failed recessed wall units had been.  

We chose to install a new, modern 299,000 Btu/h, high efficiency, modulating condensing floor-mounted boiler with outdoor reset and DHW priority. A 119-gallon indirect tank provides the DHW. The single system circulator is a smart pump that varies its speed based on downstream variations in demand as zones open and close. This single system circulator supplies all the variety of heat emitters in the home through a new, two-pipe parallel distribution piping system routed through ceilings, floors and walls.

The project is near completion with the lower level radiators, final trim, thermostats and final commissioning remaining. The main level heat has been operational this winter during interior construction.