All posts by adminsma

Synthetic Log Fireplaces and Carbon Monoxide

Propane gas fireplaces are popular here in Mexico and especially San Miguel de Allende. Because there is often no heating “system” per se, these temporary use heaters are ubiquitous. Most synthetic log types are designed to burn with a yellow/orange flame to look like real logs on fire. Yellow mean incomplete combustion, blue/white means more complete, clean and hotter combustion. Yellow means more monoxides of carbon, more air pollutants, more health risk. So…..

  1. Dont use them for longer than an hour or so…
  2. Don’t leave them unattended…
  3. Don’t use them if you start to get a headache or feel nauseous…
  4. Do not use them over night while sleeping….
  5. Do not use them close to curtains, furniture and other combustibles….
  1. Do have a carbon monoxide detector in the room…
  2. Do crack a window for fresh air…
  3. Do make sure they are cleaned each season…
  4. Do make sure they have a functioning safety valve with pilot…
  5. Do make sure there are no gas leaks when on or off…..
  6. Do install a separate shutoff valve outside the fireplace nicho/opening if possible…
  7. Do install a closeable vent up and to the exterior in the top of the nicho and make sure it is open…(yes, you will lose some heat but when you turn off the heater you close the vent and the warm air in the room will not escape).

Note: more modern models (more expensive too) have oxygen sensors so that when the room runs low on oxygen, the safety valve shuts down the fireplace and gas. I have heard complaints that these models with sensors are “tempermental”.

What size water heater? How long will a shower last?


Do a google search and if you don’t pull your hair out trying to get a simple answer then you are from another planet. This is terribly complex (for a non-engineer like me)  but I am going to make it simple by assuming many things for you.

The easy and conservative answer is this:

A 50 gallon gas DHW will allow two 10 +- minute shower at 110 deg F with a 2.5 gpm shower head (with my huge list of assumptions mentioned below). Depending on the recovery rate another person may have to wait at least 40 minutes  or so to take another 20 minute shower.  So what does that mean. It means take a 7 minute shower so you can do dishes, run a load of laundry and still take another shower an hour later. It means buy a 75 gallon if you have a couple of kids. It means buy a 100 gallon quick recovery (high BTU output) commercial DHW heater if you don’t want to plan and sequence usage or if you have a “thing” about long hot showers or never want to run out (also investigate on-demand/tankless heaters).

 Here is the partial  list of factors:

Inlet cold water temperature (avg)50 deg F (colder in Alaska, warmer at Equator)
Hot water heater (nat. or propane) temperature140 deg F (over that US code requires mix valve)
Hot water heater size (adjust with the formula)50 gallon (189 liters)
Distance of furthest showerLess than 50 ft
Draw Down (30 deg temp drop)70 % or 35+- gallons on a 50 gal tank
Shower head rating2.5 gpm (use 3.5 if you remove flow constrictor)
Temperature of a hot shower (min)110 deg F at the head (cools quickly as it leaves)
Ratio of Hot to Cold water (see formula)Approx. 70% hot 30% cold
First hour rating (FHR)Approx. 50 gallons
We are ignoring mixing valve calcsToo complicated for me, too many variables.


  1. A couple (2) can use a 40 gallon heater by adjusting use pattern but a family of 4 will have trouble so if you have a 40 I will just say it is undersized (don’t argue).
  2. If you take more than a 10 minute shower I hope you are doing it for therapeutic reasons. If you keep your 2.5 water saving shower heads (I hate em…!) then you get a 14 min shower. I take a small drill bit to my 2.5 heads to increase the flow. Do I feel bad…? NO!
  3. Quick recovery DHW heaters and boilers for floor heat connected to domestic hot water change everything. The BTU input can shorten recovery time considerably.
  4. On-demand heaters change everything. This information is for gas heater with a storage tank. Do not get a 11 liter per minute on demand heater and expect to take long showers while any other appliances are being used (dishwasher, clothes washer etc). On demand heaters are great energy savers but get a good plumber so you don’t have sudden “cold plugs” and insure you have the flow rate and pressure needed. Low flow shower heads are notorious for causing cold surges as the heater shuts down (happens more when pipe size and pressure add to the problem). Adding a pressure pump and expansion tank are probably required equipment.
  5. If you live in a warmer climate with mild winters, you will have quicker recovery and a better FHR.

Skip the Algebra if it isn’t your thing……Formula: X is gpm hot, Y is gpm cold, 140 deg F hot, 50 deg F cold. 110 deg F is desired “hot shower” temp. If you like it scalding then you will have a few minutes less of hot shower. Just increase the 110 deg F desired temp. This is the “Ratio Formula” for hot to cold.

140 X + 50 Y = 110 (X+Y)  Hot and Cold in, 110 hot shower out.


30X-60Y=0 or 30X=60Y so……X=2Y  

You need twice the hot as cold to maintain the 110 deg F. (I know, it sounds wrong, other sources often say a shower uses 70% hot to 30% cold). So we are close to the 2 to 1 ratio. Without an assumed ratio you cannot calculate but you can get a bucket and measure how many gallons at a given temperature (use a gauge) you get before the “draw down” drops you to below your desired shower temperature.

So using slightly conservative numbers,  a 50 gallon hot water heater will give you 34+ gallons of hot water. Add another 16+ gallons of cold (ratio above) to the approx.. 34 gallons of hot water to get about 50 gallons of shower water at 110 deg F.  50 divided by 2.5 gpm gives you a 20 minute shower (or 2 @ 10 minutes). I think this number is the best you can expect from a 50. With mineral build up, higher gpm because you drilled the flow constrictor, slower FHR and seasonal changes you will be closer to 14-15 minute shower max.

If you like it really hot hot, reduce to 14 (2 @ 7) minutes. If you can reduce to 10 minutes then you can get 4 people showered by waiting 40-60 minutes after the first 2 showers. All bets are off if you wash dishes or clothes during shower time.

Good luck and don’t hassle me over my assumption and bad math. Just send me the better math formulas or make your case for longer shower numbers. Better yet, just say the size you like for a couple, family of 4, etc living in say Santa Fe, NM or San Miguel de Allende, Mexico (cold winters, warm summers).

Ron Smith (cranky ex-general contractor).

Exterior Tile and Concrete Sealers

You want a penetrating sealer made with siloxane/silanes. I like Sika products, use Sikaguard 70 if you can find it here at Home Depot mexico. At between $2,000 mxn and $3000 mxn for 19 liters it’s not cheap but you can spend more for the product I use, Imperquimia Aquasil Life Style which costs $6800 mxn for 19 liters. But the coverage is much better than SikaGuard and you only need one coat.

They are not acrylics and do not form a seal at the top of the tile. They breathe and penetrate concrete, tile and other porous masonry surfaces without changing the look of the material. Any top coat sealer will degrade in the sun. Read the review at the link here and then begin your search with your tradesman to find the product locally. If you have a failing finish on your tile it has to be stripped to get the porosity back so the penetrating sealer can get in as deep as possible. Just sprinkle water on the surface and if it does not absorb and turn dark you have to strip or wait to seal with a penetrating sealer.

Watch out for waterbased formulas with silicone. I am not a fan yet of these hybrids that have acrylic mixed with some silicone but I think Okon S40 is a good product if you can find it here. I generally prefer oil or solvent based penetrating sealers although there are some waterbase siloxane/silanes that pentrate and perform well. Home depot carries Thompson’s Waterseal Multi-surface but their website and info at Home Depot is lacking information about what it is in it and the reviews by users are horrible. They lowered the VOC’s (volatile organic compounds) probably to meet California standards and made it thicker and stickier and it creates a mess on your concrete/masonry surfaces. Always do a small test patch first.

Go to Services and then Subcontrator Referrals on this website to find several tile refinishers that you can contact. Stay away if they do not recommend a penetration sealer for the exterior.

On Demand Hot Water Heaters, Save Energy or Save Water

Here in San Miguel, sometimes the flow rate on your shower heads and faucets isn’t enough to keep your on-demand heater on and thus allowing “slugs” of cold water to enter the system. If you are handy you can choose to use more water by removing the constrictors in the shower heads and drilling a small hole (when necessary) in the orifice to increase the flow rate so your on demand heater doesn’t keep turning itself off thus allowing the cold water to enter the line.  If your pressure is a bit low (you are up high in the house and you have a gravity feed tinaco without a pressure boost pump) then you may be signing up for frustration with an on-demand heater.

Solar preheaters save energy and can work with a wider range of pressures. It is a great combination and will reduce your propane gas consumption.

I have heard from enough owners and plumbers on this subject to say that in San Miguel de Allende you need to check your system carefully before installing an on demand heater:

  1. Do you have a 3/4″ gas supply line from your propane tank to the heater (needed for a 185,000 btu heater)? Long distances from your water tank (tinaco) to the heater may cause insufficient flow and tempermental heating if your line is the typical 1/2 inch copper.  A recommended minimum heater size of 185,000 btu’s needs a good flow rate from the propane tank.
  2. Do you rely on street pressure to fill your water tank (cistern or rooftop tinaco)?  Some houses have no cistern and no pressure pump. So they are stuck with gravity pressure from the rooftop tinaco. This means low pressure on the top floor to showers which improves as you go down to lower floors in the house. On demand heaters need a minimum flow rate of around 7/10 gallon per minute.  You can test this by just running your hose or shower into a bucket for one minute.
  3. Do you have a boost pump to increase water pressure to your water lines, filtration system etc? Do you have good pressure everywhere in the house?
  4. Will you calculate your high or peak demand useage to correctly choose your heater size and does this demand match the delivery rate capacity of your water line (1/2 inch lines with rooftop pressure may not be enough unless your tinaco is perhaps 2 to 3 floors above the heater)
  5. Does your filtration system (whole house type) maintain enough pressure to avoid cold water “slugs”?

I cannot improve upon David Grubbs information on tankless hotwater heaters here.

Water and Moisture

Keeping water outside the house, preventing it from coming in through the roof, walls, floors, doors and windows can be a challenge when you have the kind of hard rains we have here in San Miguel. Many homes do not use thresholds and weather-stripping at the doors so windblown rain can really come in fast.


More frustrating is the infiltration of moisture through walls, ceilings and floors. The problem is finding the source of moisture which may not be on your property. The water can be traveling through the soil or through other house walls for very long distances. The only protection you have is to put waterproofing coatings on the outside of your foundations, floors and walls. If you have buildings up against your walls then generally you are stuck with the problem unless you can excavate on both sides of the wall and foundation and this is most often not possible. There are some coatings that can go on the inside of the walls and floors, there are French drains and vent holes that can be added but all of these options are almost never 100 percent effective and the moisture usually returns at the same spot or someplace nearby as the moisture finds the easiest path to escape.
Despite these well known infiltration problems, builders and architects continue to build foundations and stem walls without proper damp proofing (or none at all). If you already own a home and there is an empty lot or a garden next to your exterior walls, before the access is permanently blocked, put on a waterproofing membrane (better than a coating) and install French drains if you can slope the drain to daylight (if not, all you are doing is creating an underground pond for water to collect and eventually enter your house). Many architects and builders need better specifications and installation practices. Asphalt/bitumen based “roof coating” rolled or brushed on is helpful but the current standard (of course I am talking about the U.S.) is a membrane that combines these materials with polymers in a tuff sheet sold in rolls that can be torched on or self adhered. Bitumen or asphalt based brush on coatings (impermeabilizantes) are better than nothing but if you can access the lower walls and foundations then I recommend spending the money to install a membrane. As coatings get more sophisticated (polyurethanes etc) they become more effective. Wikipedia has a decent discussion of “waterproofing” and “dampproofing” that is helpful.


I came home one night to my recently rented house during a hard rain and found a cascade coming down the stairs. The wind direction was just right to maximize the amount of rain getting in under the unprotected terrace doors and the terrace drain was partially blocked by leaves and debris. So we adapted the door sweeps which stopped the main flow but we still need a towel on the floor during certain storms. Good thing we have Saltillo tile floors.

First, if it isn’t too late, put in thresholds on the unprotected exterior doors, the ones that are not under a portal (yes, I know this changes the “feel” of how the indoor and outdoor space flows).  Next, add door sweeps with flexible weather stripping on the bottom to minimize water infiltration. Some rain will still get in if you have French doors. In a perfect world, French doors belong under protective overhangs and portals.

Second, are the sides and back of your house plastered or raw brick ? It is common here to wait for your neighbor to finish their house and cover up the unfinished sides of your home. This is one of my “pet peeves” about builders and developers here in San Miguel. If you cannot afford it then okay, you have to wait. But if you can you should at least plaster one coat and throw on a coat of paint that matches your other elevations. If you don’t, hard rains may enter and damage the plaster and paint on the inside of those walls.  Here is an unfinished wall with no moisture problems inside perhaps because it faces east. The other west wall had to be plastered to stop the moisture damage. Note the white paint could have been tinted to be more like the house. It was a pretty quick fix. Note the white efflorescing below the parapets. More on that in another post.


Moisture in walls grows mold. I have done many mold inspections in San Miguel and in the U.S. I have yet to find dangerous black mold (stachyboris) and asperguilias here in San Miguel. But as I learned from clients here, any mold can be harmful to a person with sensitivities, allergies or with compromised immune systems. See more in the “harmful mold” post .