In hot water: A DHW logistical predicament

I recently looked at a DHW system at a local hotel. This was the third time we have visited this hotel to look at the mechanical rooms over the last in the last 20 years. The hotel is owned by a well-known national chain. We were the primary contractor on the last two visits, but this time, we would be working as a subcontractor.

The current DHW system consists of three hot water makers, each rated at 2 MBtu input (total Btu input is 6 million). The hot water makers are set to provide 160° F water to the three hot water storage tanks. The larger storage tank has an 800-gallon storage capacity, and the other two smaller tanks hold 400 gallons a piece (total storage is 1,600 gallons).

From what I am told, when everything is working, the system keeps up with the demand. This plant produces domestic hot water for hotel rooms, a commercial kitchen and laundry. Each of these “zones” has a 3-way electronic mixing valve for DHW output. Based on the amount of growth on the mixing valves and nearby piping, I suspect they no longer do much mixing. Although to be fair, two of the thermometers on the valve outlets read about 125° F, and one was 140° F.

All three hot water storage tanks appear to have been built in 1995. The hot water makers are dated 2007 — this makes sense as the last time I visited the site, the original heaters were in place. Based on the amount of growth on the copper pipe, fittings and valves, I am pretty certain the incoming water is not being treated. I am also pretty sure the manholes in the ASME-rated tanks have never been opened. I’m guessing there is a lot of sediment in the base of all three vertical tanks. Each hot water maker has a dedicated circulator pump that transfers the hot water from the heater to the tank, and back. All three circulators leak at the bearing assembly and are dripping on the galvanized vent piping below. It appears these pumps have been leaking for at least six months. The two 400-gallon tanks are leaking, but the 800-gallon tank appears intact (so far).

The contractor that asked me to look at the job does very little design work, and almost no rigging. They focus on service and maintenance contracts — occasionally, they will replace smaller equipment if required. We design most the systems we install and have done a lot of the rigging and demolition ourselves. At the minimum, we assist with demos and advise our subcontractors on what needs to be done.

For this hotel job, we would prefer to redesign the entire room from scratch. This would allow us to build a modern energy-efficient plant that would last for years, come with a warranty and reduce energy usage. However, the owners may not have the funds or want to allocate the money needed for such a large capital expense. As is often the case, we were asked to come up with a ‘simple’ proposal to replace the two leaking 400-gallon tanks and the nearby piping.

I forgot to mention the original tanks were likely lowered in place by crane — there used to be an opening in the roof above the mechanical room. The roof has been sealed since 1995. This mechanical room is located on the second floor and the roof is approximately 50 feet off the ground.

It appears that the original installers set the three steel ASME-rated storage tanks in place first. Then they set the three hot water makers in place in front of them. Afterward, they piped everything in and ran the heater venting from the heaters through the roof.

We are now faced with a challenge as the hot water makers block access to all three storage tanks, as does the welded gas piping, the domestic water piping and the breaching.

Our plan is to cut up the two existing steel tanks and remove them from the second floor in relatively small pieces. We specified four smaller 200-gallon vertical storage tanks equaling the same capacity that will fit through the doors and sneak behind the hot water makers without too much effort. We can lift most of the old and new equipment utilizing the huge overhead beams. With some planning and a little luck, it shouldn’t be too difficult or labor-intensive. Once the new tanks are in place, we will pipe them in and flush the system. We will bring the old cut-up equipment out on carts utilizing the service elevator. We will deliver the new equipment on pallets utilizing our pallet jack. Once the equipment is inside the mechanical room, we will use a series of chain falls hung off the beams to get the new tanks in place.

Our biggest challenge may be shutting off the domestic hot water to the building for a couple of hours. We were told that most, if not all, the isolation valves in the room are seized in the open position. Hopefully, we can close the main cold water line to the mechanical room. If not, I know someone at the local water department that owes me a favor. If I tell them in advance, they often show up as needed to shut the water main to the building.