Sunday, November 7, 2010

HRV, On-Demand Water Heater, and More!

I arrived back home after 2 weeks off-line on vacation last Sunday, and briefly checked out progress on the job. Then I immediately flew to Washington, DC, early the next day for a conference, coming home on Thurs.  A lot has been done in the 2 weeks I was gone, more than in the previous 2 months. Today, I finally had time to poke around in more detail. Here's what's been happening.

Fiberglass batting is not air proof, air can leak through. When our roof was replaced in 2003, the builder did not install any ventilation under the plywood decking as recommends. Ventilation allows the warm, moist air from inside to vent to the outside, but also of course results in energy loss through the thermal envelope. So in many places, warm indoor air came up against the cold roof decking in winter and condensation accumulated on the plywood. When the water remains more than 24 hours, mold will grow. We had many places on the decking where mold  was growing, especially along the ridge in the hallway, since that is the highest spot in the house and is naturally where the hot air accumulates. There were also a couple places on the south side of the house with mold on studs and siding. Since I have a mold allergy (one of the many reasons why we are doing this remodel), I insisted that the mold be removed. We had a mold remediation  company come in and treat the mold. The closed cell foam insulation that we are having installed doesn't allow air to penetrate, so it can be sprayed directly on the roof decking and siding to seal against air leakage.

Before we started the remodel, we had in our living room an old pellet stove without any thermostat that needed to be started by hand. I really wanted a newer model with thermostat and automatic starting, but Santa Clara County has a ban on wood burning appliances due to pollution during winter inversions. So we instead opted  for a 90% AFUE gas Empire Comfort Mantis fireplace. This is about the most efficient gas fireplace that you can get. It has a condensing firebox that retrieves the heat of condensation from the steam formed by combustion, and takes combustion air from outside rather than within the home. The only downside is that it won't work when the grid is down because the outside venting requires a fan. But neither would a pellet stove, since it requires electricity for ignition and the screw that retrieves the pellet from the hopper.

On Fri., they installed the fireplace. Here's what it looks like without the copper surround:
and here is what the plumbing looks like coming out the back:
One pipe is for the combustion air intake, the other for the combustion gases exhaust. Unlike a lower efficiency furnace/fireplace, the combustion gases are not hot enough to simply float up a chimney and water would also condense out on a metal pipe, so a normal chimney/flue can't be used.

Structural Reinforcement
Another reason why we started this work was because the drywall along the ridge line in the hallway cathedral ceiling was cracking. Not just in small areas, a big gap opened up right along the ridge line from the front bedroom to the middle skylight and it would expand  and contract depending on the temperature, raining down little bits of plaster when the temperature rose. Fixing this has turned out to be an enormous problem, and has been responsible for holding up the job for at least a month and a half. We have gone through two structural engineers trying to get some kind of solution  that would immobilize the roof enough to prevent future cracking without having to take the roof off and put in new trusses. The basic problem is that the roof beams installed originally were not sturdy enough to support the weight of the skylights and the plywood decking without twisting some. Originally, the roof was wood shake shingles which are lighter, but we replaced it with composite shingles because they last longer, which required the plywood decking. This increased the weight of the roof. But even without the decking, the original roof supports are really not sturdy enough to support the full weight of the skylights - some of our neighbors  with the same model  house have a similar problem with cracking. It does not mean  that the roof will collapse in a major earthquake, just that it is not stable against thermal expansion and minor movement.

The Project Manager Who Shall Not Be Named had  the framers install  metal  plates at the joints but that was insufficient for some of the forces. So the new structural engineer and Paul came up with a series of what look like 10x4's that run up to the ridge beam and transfer some of the load onto the rafters further down:

Here you can see how the beam  runs from the skylight, on the left, down along the rafters, on the right,  distributing the weight of the skylight more evenly along the roof. The new beams are bolted to the rafters with heavy bolts:
Paul says that the beams will not likely prevent cracking during an earthquake but they  will  prevent the twisting with temperature and random minor movement which seemed to have caused the original problem.

The HRV system has been partially installed. We have two HRV units, one in the attic above the garage and one in a chase next to the east side upstairs bedroom. Here's a picture of the one in the chase:
The HRV unit is the Fantek SH704. The hard metal pipe running across above the unit is the existing bathroom ventilation from the downstairs master suite bath. The foil clad soft ducts on the left and the fiberglass clad ducts on the right are the new HRV ducting. The metal bar across the top is the mounting frame for the HRV.

When I first saw the metal bar, I was a bit upset. We had agreed to hang the HRV from wires in order to avoid transferring vibration to the house frame. After all, this unit is right above the master bedroom and next to another bedroom (which we use as The Lovely Wife's sewing studio) . It looked as if they had simply bolted the metal frame to the ceiling rafters and the HRV to the metal frame. However a closer inspection showed that the HRV apparently has  a kind of shock absorber on it:

I think a rubber grommet is missing and must be installed yet in the shock absorber. So I'm willing to try it out to see whether or not there is any vibration. 

Some details still must be worked out. The attic HRV, on the other side of the house,  needs its humidity drain installed somewhere, and I'd like it to go into the new downspout that will be on that side of the house. And the intake and outlet vents on the chase HRV are uncomfortably close, and also close to the master bath venting:
I've asked Paul if we could locate the intake vent further up toward the roof peak where it it is less likely to draw in stale air from one of the exhaust ports. The new chase we installed running from the HRV chase to the hall exhaust vent seems to have enough room that one could run a second duct, depending on how large the duct is. The exterior venting has yet to be determined on the front side for the attic HRV. I've requested that they vent out where the existing attic vent is located, and vent in from the side of the garage. Not sure if they can do this, though, maybe higher on the attic wall would be better. 

The interior ducting for the HRV looks like this:

Here the ducting runs in the chase next to the upstairs bedroom. An insulation guy is going to have to climb back in there to insulate the stud bays above the master bedroom and the back of the bathroom wall. It's a tight fit, I climbed back in there myself today. But because the ducts are flexible, you can shove them around, unlike standard forced air ducting. So I think it should be possible.

This next picture shows the chase in the front bedroom closet where the exhaust duct runs from the attic HRV to the exhaust vent in the hallway:
The framer put this chase in so  we could run the HRV duct, and a similar one in the east side upstairs bedroom. The attic HRV draws in warm, stale air from the top of the cathedral hallway through the hall vent into this duct and exhaust it, after transferring the heat to fresh, incoming air. The vent cover for the HRV looks like this:

I've requested from Paul  if we could get vent covers that look a little less intrusive. White on a colored wall is just too noticeable for my tastes.

On-Demand Electric Hot Water
The framer put in a closet above the upstairs bathroom door for the on-demand hot water heater and also for storage. The Tempra 20 Steibel-Eltron on-demand hot water heater was also installed, though it can't be actually hooked up to the power line until PG&E comes and installs 200 amp service:

Notice the lack of any venting? That is one nice thing about an electric hot water heater as opposed to gas. Because there is no combustion, it doesn't need venting, it's just another electric appliance like the washer or refrigerator. So we can use this closet like any other closet, for storage. It just happens to have a hot water heater in the back.

I checked out how the plumber fitted it into the solar hot water plumbing, and I am not quite sure if I understand. I've asked Paul to check. It should be in series  with the solar plumbing, with the hot side of the solar tank running directly up to the on-demand heater, and the hot side of the on-demand heater running to the mixing valve downstairs and then into the domestic supply. This allows the hot water from the solar tank to rise up to the on-demand heater between use, eliminating a gout of cold water that enters the on-demand heater when a hot water tap is turned on, which could cause the on-demand heater to fire for a couple seconds till the flow from the solar tank hits it.

Some people, like Allison here at the, don't seem to like on-demand electric hot water heaters. I suppose, if they are not just being used as a backup to a solar thermal hot water system, they could be a problem, since they do use large amounts of electricity, especially if they need to heat the water from 60 degrees F or so to 120 degrees F. But when they are being used as backup, the incoming water is rarely below 100 degrees F and in summer, they won't be used at all. As my previous blog post showed, on-demand electric backup for solar becomes more carbon efficient than gas at some point *if* (and this is a big if) the grid is migrating to non-carbon energy sources. Of course, if you are offsetting the electricity use with home solar PV, as we are planning to do, then on-demand electric backup becomes essentially carbon free. It is also possible to offset a gas on-demand backup with solar PV too, but a gas on-demand heater then requires complex venting.

Next up: PG&E installation of the new 200 amp service, a decision on the solar PV installation and selection of an insulation contractor!


  1. "I've requested from Paul if we could get vent covers that look a little less intrusive. White on a colored wall is just too noticeable for my tastes."

    If your paint supplier is full service, then you ought to be able to purchase the same color paint as the walls, but loaded into aerosol paint cans. Then you just prime and spray paint the vent to the same color as the walls.

  2. Great, thanx for the tip, Brent. We'll ask about it.

  3. James, yes, you're right about the reason I think electric tankless water heaters are usually not a good idea. In small applications where they won't be used a lot and can eliminate the hot water losses in long plumbing runs, they're fine. As back up to solar water heating, they're fine. When used to supply hot water for a whole house, however, they draw a huge amount of current, and even the electric utilities, which love to sell more kilowatt-hours, discourage their use for that application.