Marey 16L tankless, Exhaust vent chimney feedback froze my coils

[hugybyr]

Purchased a Marey 16L tankless water heater last March. Finally got it hooked up this summer works OK but not great. Last week it got cold here in CT. and even though the unit is in my 50 degree basement the coils started to freeze. I found that the vent chimney was feeding cold air back thru the unit. This unit uses a 5 inch double wall pipe. How do I block off the chimney when I don't use the heater ? A new heat exchanger is $100 and now I need one but if it will just freeze up again why bother.

 

[Dana]

You can't safely block the flue. Most of the bigger-deal tankless water heaters come with electric resistant-heaters to limit the freeze-up potential, but even those aren't foolproof. (They're pretty foolproof in a CT climate, not so much in central Saskatchewan.)

The Marey 16L shouldn't be hooked up to a masonry chimney in a CT climate, especially if there isn't a boiler or furnace hooked up to the same flue. At the low intermittency of hot water use and the cooler exhaust of near-condensing 86-87% burner the masonry will run cold, and there will be excessive flue condensation which will be slowly destroying the chimney from the inside out.

It also shouldn't be hooked up to B-vent (double-wall galvanized exhaust piping), since that too will be destroyed by condensation within 5 years even in more temperate climates, (probably sooner in CT). See the "Venting Guide for Marey Heaters" section.

If you insulated your basement it would never drop to 50 degrees, probably never drop below 60 degrees, which may be enough to make the difference should you decide to resurrect the Marey and use $tainless venting (as it needed to be all along). Even if you decide to cut your losses on the Marey and move on to something else, a 50F basement is still a huge heat loss to the house when it's 15-20F outside, and no insulation between the basement and the great outdoors. Insulate it to current code min the basement indoor temp will rise, but the losses will be cut by more than 90%. There are some details to get right to avoid creating a mold farm, but it's not particularly expensive to do it right.

 

[Sponsor]

 

[Master Plumber Mark]

you already stated the obvious.... the unit works but its not great,,,,
and now you have it frozen up.... and its gonna cost you again...

have you considered just going back to a simple tank type heater for 450
and just chalking this up as a bad experience???

has your wife started to complain about this fiasco yet????
you are gonna have more than just your coils frozen if you dont
get something fixed and fixed right

maybe the universe is trying to tell you something....

 

[hugybyr]

You can't safely block the flue. Most of the bigger-deal tankless water heaters come with electric resistant-heaters to limit the freeze-up potential, but even those aren't foolproof. (They're pretty foolproof in a CT climate, not so much in central Saskatchewan.)

The Marey 16L shouldn't be hooked up to a masonry chimney in a CT climate, especially if there isn't a boiler or furnace hooked up to the same flue. At the low intermittency of hot water use and the cooler exhaust of near-condensing 86-87% burner the masonry will run cold, and there will be excessive flue condensation which will be slowly destroying the chimney from the inside out.

It also shouldn't be hooked up to B-vent (double-wall galvanized exhaust piping), since that too will be destroyed by condensation within 5 years even in more temperate climates, (probably sooner in CT). See the "Venting Guide for Marey Heaters" section.

If you insulated your basement it would never drop to 50 degrees, probably never drop below 60 degrees, which may be enough to make the difference should you decide to resurrect the Marey and use $tainless venting (as it needed to be all along). Even if you decide to cut your losses on the Marey and move on to something else, a 50F basement is still a huge heat loss to the house when it's 15-20F outside, and no insulation between the basement and the great outdoors. Insulate it to current code min the basement indoor temp will rise, but the losses will be cut by more than 90%. There are some details to get right to avoid creating a mold farm, but it's not particularly expensive to do it right.[/QUOTE Thanks for your reply. The basement temp sits at 50-60 and it is air tight from a lot of caulk and weather stripping. I found and ordered a 5" high wind back draft damper for the exhaust vent pipe but it is not a complete seal. THEN I found out thru some research that what is happening is that my oil furnace when it is running is probably pulling the fresh air it needs thru my 5" vent pipe. This seems to be a common problem even with the big bucks units with freeze prevention electric heat coils. If my unit was used a lot rather than a couple times a week MAYBE it would not freeze up but I doubt it. I'm not giving up just yet but it sure is getting expensive. My unit is a Natural draft unit so sealing it off is not an option. Maybe a small portable electric fan heater under it blowing up into it would keep from freezing but electric rates for my commercial rated meter at the shop
(last month $.77 a KWH !!!) will kill the whole tankles idea. I wonder if I could wrap water pipe heat tape around the coils? How hot do the coils get when these thing run?

 

[hugybyr]

UOTE Thanks for your reply. The basement temp sits at 50-60 and it is air tight from a lot of caulk and weather stripping. I found and ordered a 5" high wind back draft damper for the exhaust vent pipe but it is not a complete seal. THEN I found out thru some research that what is happening is that my oil furnace when it is running is probably pulling the fresh air it needs thru my 5" vent pipe. This seems to be a common problem even with the big bucks units with freeze prevention electric heat coils. If my unit was used a lot rather than a couple times a week MAYBE it would not freeze up but I doubt it. I'm not giving up just yet but it sure is getting expensive. My unit is a Natural draft unit so sealing it off is not an option. Maybe a small portable electric fan heater under it blowing up into it would keep from freezing but electric rates for my commercial rated meter at the shop
(last month $.77 a KWH !!!) will kill the whole tankles idea. I wonder if I could wrap water pipe heat tape around the coils? How hot do the coils get when these thing run?

 

[Stuff]

If your basement is air tight then you need to find make up air for the the furnace. How is the oil furnace vented compared to the water heater? You need to review the whole setup.

I hope you have some working carbon monoxide detectors.

A heater won't help much as a wind coming down the vent will overpower most small heaters.

 

[Master Plumber Mark]

Look on the bright side of the situation....
Spring is right around the corner and it wont be a problem once
the temps get up some... You will be able to get another 7 months out of
it before the problem starts up again... and you will need to change it out.

working carbon monoxide detectors would be a wise thing to buy.

 

[hugybyr]

If your basement is air tight then you need to find make up air for the the furnace. How is the oil furnace vented compared to the water heater? You need to review the whole setup.

I hope you have some working carbon monoxide detectors.

A heater won't help much as a wind coming down the vent will overpower most small heaters.

You are right- wait till spring. Tonight it going down to -6 with a wind. Think I'll go drain all the hot and cold pipes. The furnace has its own chimney and the Marey has a stainless double wall exhaust vent. I'm going to leave the inside basement door open to the upstairs and let my furnace burner pull down some warm air. What are your thoughts about wrapping the Marey's thin when bent outside coils with pipes heat tape? They were easy to solder when broken but I can only do that so many times. I also wonder if running a connection from my hot air ducks that heat the upstairs over to the Marey and let it vent behind the unit that sits away from the foundation wall on a 2x4 frames ? Gonna loose some heat but might be not all that much.

 

[Dana]

The volume of combustion air used by a furnace or boiler is tiny enough that leaving the basement door open won't be adding any heat to the basement. What's more likely to happen is that it will allow even higher stack effect infiltration, lowering the temperature of the basement.

Actively heating the Marey with hot air from the furnace would more than defeat any efficiency gains from going tankless.

If your basement isn't air-sealed and insulated it's likely to be 15-25% of your annual fuel use even if you're not actively heating the space. If you ARE actively heating the space an uninsulated foundation could easily be responsible for over a third of your fuel use. Insulating a basement is a higher priority than going with higher efficiency hot water heating.

 

[Jadnashua]

If, in fact, you have air sealed well, with a natural draft furnace, you MAY not have enough leakage to properly provide for combustion air. This is a problem as places get tightened up. You may be onto something in that when the furnace is on, it is pulling combustion air though the other unit's duct which is causing the freeze up.

 

[hugybyr]

you already stated the obvious.... the unit works but its not great,,,,
and now you have it frozen up.... and its gonna cost you again...

have you considered just going back to a simple tank type heater for 450
and just chalking this up as a bad experience???

has your wife started to complain about this fiasco yet????
you are gonna have more than just your coils frozen if you dont
get something fixed and fixed right

maybe the universe is trying to tell you something....

Well it looks like spring is here. Yup disconnected the thing and drained it out in Feb. so now it is time to reconnect and continue the battle. I'm thinking about building a closet around it with a hole in the front and then next winter use one of my hot air heater ducts to circulate around the heater before continuing the heats journey to the upstairs register. Or throw a plate over the exhaust pipe and pull it out when I,m running the unit ! I'm a typewriter repairman so I'm not to busy in the shop using hot water these days. Guess I should have gone with a boiler and a aditional coil for the hot water when I put the furnace in 12 years ago rather than a straight hot air unit.

 

[Standardairconditioner]

have you considered just going back to a simple tank type heater for 450
and just chalking this up as a bad experience???

has your wife started to complain about this fiasco yet????

Unless there are space saving constraints, or seldom use (like a ski cottage), I have no idea why people use tankless water heaters.

Water tank heaters of 2016 are so well insulated and efficient. You don't even have to wrap them anymore like Dad used to. Only fools believe they waste electricity. If hot water is needed once a day, then it pays. People really should shower daily, I am strong supporter for human hygiene.

Some people just like to do things the hard way, the stress makes them feel alive!

 

[Dana]

Standarairconditioner: Only fools in Connecticut heat with electricity if they're on the gas grid. The Marey 16L saves a LOT of electricity- it has no electrical connections at all (which is part of the problem, since it has no freeze protection heater strips like better gas fired tankless units do.)

Electricity is by far the most expensive way to heat hot water in high electricity cost locations such as Connecticut (about 3x the price Hydro Quebec customers pay), which is why he's trying to get by with this (inappropriate for the climate) low end gas fired tankless.

Would he be better off with a tank? Sure! Would he be better off with an ELECTRIC tank?

Not really. He could buy a new Marey 16L (maybe two) every year on the operating cost differences. That Marey tankless costs less up front than a standard gas-fired tank, but it's not worth the effort he's going through to (attempt to) make it work.

hugybyr: Do NOT just block the vent with plate when not in use! It's a code violation and a potentially serious health & fire hazard. If this is just a convenience for hand washing at a shop you may in fact be better off with a $200 bargain-basement electric HW heater, since your actual hot water use levels will be pretty low, compared to a residential application with multple people showering daily. If this place needs air conditioning or dehumidification, a heat pump water heater may make a lot of sense if subsidized.

A tankless coil in a boiler is an abyssmal way to heat hot water, especially in low hot water use applications. For the 10 gallons/day user it'll burn the vast majority of the summertime fuel in high idling costs, at extremely low efficiency.