In Hot Water, Part II

A couple of days ago, I received a comment on the post called In Hot Water from a reader asking for more information about our Margin Gem and hot water system.  Her questions were quite good, and I felt they deserved their own blog post.

As we are entering the heating season, our Margin Gem has been fired pretty regularly.  In fact, we've only had a handful of days in the last month when we've had our electric hot water heater going.  There have been days when we've intentionally fueled the fire in such a way as to keep the stove heating while we are gone during the day, and it has almost always had a fire in it overnight.  However, we still haven't been firing the cookstove at full tilt like we will when our weather is truly cold. 

While the stove is in use, we are quite happy with the amount of hot water that we have--and unless there are unforeseen circumstances (on one night, a log didn't burn the way I had expected that it would), the water that we do have is HOT! 

Below is a picture of the firebox of the Margin Gem taken while standing in front of the stove.  I let the fire die down enough this afternoon so that we could remove the piece of the stove top that covers the firebox in order to snap the picture.  The pieces of metal in the corners and in the middle of the right side are the air jets for Margin's Air Jet re-burn design.  At times when you open a top lid, you can see flames curling from those holes.  At the top, the right side, and the lower half of the bottom, the light gray panels are firebrick.  The entire left side of the firebox is the waterfront. 

Top view of the Margin Gem's firebox with water-
front on the left.

Just like hot air, hot water rises.  Thus, cooler water from the bottom of the range boiler flows into the bottom of the water jacket.  There, it is heated by the fire and then flows up out of the top of the water jacket to the top of the range boiler. 



The pipes carrying water to and from the waterjacket.
(Ughh!  I hate how a camera flash makes dust that, in
normal light, is imperceptible to the naked eye
 suddenly appear unduly tragic!)

The Vaughn range boiler behind the Margin Gem. 

In the picture above, the pipe which enters the tank on the left side is the one which carries the hot water into the boiler.  The two pipes coming out of the top of the tank are both exits.  The one on the left has a temperature pressure relief (TPR) valve on it for the tank and is connected to the other two TPR valves in the lines going to and from the waterfront and is plumbed into the basement to exhaust near a floor drain.  The pipe coming out of the top right side of the tank is the hot water which serves the house.  When we open a hot water tap in the house, the hot water flows out of the top of the tank and down to a mixing valve which is located where the cold water enters the right middle side of the boiler.

The mixing valve which allows us to temper the hot water before
circulating throughout the house.

The end of the pipe that is attached to the TPR valves.

The above picture shows the end of the pipe which is attached to the TPR valves.  It is located in the basement and is aimed toward a floor drain which is a few feet away.  The dark stain on the concrete below the opening is from a few drips of water which leaked out during the Margin Gem's first firing.  No noticeable moisture has occurred since then.  Originally, I thought that this pipe should have been plumbed directly to the sump pit in the basement, but the plumber felt that this was unnecessary.  I was concerned about this because I was afraid that if a valve should release, there was a chance that someone could be standing in the way of the hot water coming out of the end of this pipe.  However, the only time that anyone would be standing there would be if he or she was using the shower or the wringer washer.  As it turns out then, the plumber was right because both of these activities would use enough hot water and release enough pressure to preclude the valves from releasing.

To date, the TPR valves have not released.  I'm surprised at this because some of the things that I have read seemed to indicate that this could happen on a fairly regular basis, and if it does, each TPR valve is supposed to be replaced after it has released three times.  These valves were purchased from Lehman Hardware and are supposed to release at 210 degrees or a pressure rating of 125 PSI.  We do bear in mind, though, that as I said earlier, we have not yet been operating the cookstove at its peak output for long periods of time, so we'll see what happens when it is truly cold outside.  The one thing that we also know is that colder outdoor weather will also mean that the water entering the system will be much colder, too.  This may be sufficient to offset the additional heating capacity of the hotter fire.

One thing that is different about heating water with the cookstove as opposed to electricity or gas is that as the water heats, if no hot water has been used for awhile, the first time that a hot water tap is turned on there is noticeably more water pressure for a second or two while the pent up pressure in the system releases through the faucet.  This pressure is neither dangerous nor excessive, and might not even be noticed by people who routinely have more water pressure than our gravity fed system provides us.

I recorded in my original post about wood heated hot water that we had had some initial difficulty in adjusting the tempering valve due to its having been damaged slightly during installation.  Because the indicator plate on the black part of the valve fell off, we have no idea where the temperature of the valve is actually set.  I measured the hot water temperature at the tap this morning at 169 degrees.  That's HOT!  If we had children here, we would definitely adjust the valve to a more reasonable setting, and we may have to adjust it down as the stove is regularly kept burning at a hotter rate.

Our blog reader asked if we noticed a drop in the space heating capacity of the range because it is also heating our water.  My answer to that is that we don't know how the Margin Gem would have acted without the waterfront because it came equipped with it and you cannot fire a range with a waterfront unless the waterfront is connected to a water system.  However, here are two observations that may be helpful:

1) One thing that I have noticed about the Margin Gem that is different from the other two cookstoves that I have used is that the left side panel of the stove is always cool enough that you can touch it.  I don't know whether this is because the water jacket is on the left side of the firebox or just due to the design of the stove (note the louvers in the side panel in the picture above).  I like this feature because now my legs don't have to roast while I'm standing over those pesky dishes that have "stir constantly" in their directions. 

2) The presence of the range boiler in the kitchen adds a great deal of thermal mass to the whole set up.  When the water in the boiler is hot, you cannot comfortably lay your hand on the outside of the boiler.  If the fire is allowed to go out and little hot water is used, the boiler will radiate noticeable heat for quite some time.  When you figure in the radiant surface area of the boiler, it seems to me like any change in room heating capacity is negligible if it exists at all.

Some things that you read indicate that having a woodstove heat your domestic hot water cools the combustion in the stove sufficiently to increase creosote buildup.  While it is true that the waterjacket has creosote covering the side of it, this is normal and actually beneficial.  It is also true that coals and firewood resting against the waterfront don't seem to burn quite as hot as in the right side of the firebox sometimes.  However, as far as increased creosote in the rest of the stove or chimney, all I can say is that I haven't noticed it.  Perhaps this is due to the Margin Gem's air jet re-burn design.  At any rate, I would chalk it up to the stove being airtight and the fact that we aim for a long slow burn overnight and while we are away during the day rather than the presence of the waterfront.

Overall, we are very happy with the Margin Gem and Vaughn range boiler so far.  I wish that we'd been heating our water with wood much earlier now that we know how easy and efficient it is.