Furnace bypass to allow lower setpoint without heat exchanger damage?

[Innovate1]

Put a condensing furnace (and AC) in my shop building a couple years ago. Perhaps I should have done a minisplit but I got a really good deal on the furnace and I am not changing it now... I keep it at 60 which is very comfortable to me for work there. But it would be nice to lower the temp for long periods when I am not there. I do have some plumbing so need to keep things from freezing (including the traps and such in the furnace). Duct bypassing is typically used for other things like pressure relief in zoned systems but it seems like some bypass air would raise the furnace intake air temp and lower the allowable set point without condensing. I did significant searching but found nothing like this. Am I missing something? I don't have a return air duct - the return is just a filter box and an inlet louver. One place I saw a recommendation that the bypass tap into the return at least 6 feet away to allow some mixing of the air streams and I don't have that. I do have a supply duct running the length of the ceiling so pressure to cause flow in the bypass duct should be fine. It's also two stage but I might just disable the second stage when doing bypass to help. Just starting to think about this and would appreciate any constructive feedback.

 

[4x4Pete]

What you're considering isn't done as it is for a zoned system and not to help protect the furnace from condensation. You will be better off using heat tape(or whatever it takes) to protect the furnace trap and other plumbing for that matter if you're away for extended periods. The furnace will be fine with cold start ups as long as it gets heated enough to vaporize the condensate on the heat exchanger. I suppose it could be done but would be complicated. Which usually means less reliable.

 

[Innovate1]

Hard to heat tape in the covered walls. And I only want to set back not turn off completely.

 

[PCustoms]

What am I missing, can't you just set thermostat at 50?

 

[u3b3rg33k]

A picture of your setup is worth 1000 words...

also a condensing furnace has different needs than an 80%. low intake temps will make an 80% condense internally and rust it out.

the primary HX on a condensing furnace sees pre-heated air from the secondary HX. freezing is the concern.

 

[Innovate1]

What am I missing, can't you just set thermostat at 50?

Condensation in the primary heat exchanger will ruin it. It is meant to run hot enough to only have condensation in secondary heat exchanger but with cool enough return air it will have condensation in primary HE. It's a well known issue. Min set point to avoid it is typically 55 to 60.

 

[fitter30]

Just let it ride. Firing a furnace cold is going to condensate from start up from the first hx changer any carry over will come from the secondary condensing hx . Have seen
heat 90+ % furnaces with cracked hx changers cracked with a flue analyser but no one was sick.

 

[danski0224]

Have seen heat 90+ % furnaces with cracked hx changers cracked with a flue analyser but no one was sick.

That's because it is almost impossible for an induced draft furnace to actually leak CO into the circulation airstream without having a safety, like a rollout switch, trip FIRST.

The flue is under negative pressure.

I just looked at one yesterday. CO out the *** in the exhaust flue (PVC) and zero CO in the supply duct, as checked with a combustion analyzer. No super obvious signs of issues by "looking at the flames", but I knew something was wrong when the customer said that "I'm resetting the flame rollout switch".

Those that look at furnaces without using a combustion analyzer are not doing the job right.

 

[u3b3rg33k]

That's because it is almost impossible for an induced draft furnace to actually leak CO into the circulation airstream without having a safety, like a rollout switch, trip FIRST.

The flue is under negative pressure.

I just looked at one yesterday. CO out the *** in the exhaust flue (PVC) and zero CO in the supply duct, as checked with a combustion analyzer. No super obvious signs of issues by "looking at the flames", but I knew something was wrong when the customer said that "I'm resetting the flame rollout switch".

Those that look at furnaces without using a combustion analyzer are not doing the job right.

psssh, back in my day we put our face in the exhaust stream and breathed in REAL DEEP.

if nothing happened, it was good.
if it knocked you on your *** 1/4 of the way done, you got problems, SON!

 

[Innovate1]

psssh, back in my day we put our face in the exhaust stream and breathed in REAL DEEP.

if nothing happened, it was good.
if it knocked you on your *** 1/4 of the way done, you got problems, SON!

Luxury! You were lucky. We had to live in the middle of the street in a cardboard box. (proper attribution to Monty Python...)

This has really gone off track. At least no one has suggested I rip it all down and build a bigger building.

Seriously though, it seems like a simple thing to put in a bypass and raise the temp. If I need 60F minimum air entering the furnace and have 110F air out (just guessing for an example) if I have 45F room air and mix 25% of the 110F air I get 61F air into the furnace. Would want some way to disable bypass when heating to higher temps and for AC as it will seriously decrease efficiency.

 

[PoorUB]

Luxury! You were lucky. We had to live in the middle of the street in a cardboard box.

You had a street? And a box?

 

[PoorUB]

Another thing you can do is check the temp rise across the furnace and run it near the upper limit. Most manufacturers will require about a 50F degree temp rise, so with 70 degree return they want 120F supply. Run the temp rise at 130-140 and it will clear the moisture faster.

 

[Innovate1]

Another thing you can do is check the temp rise across the furnace and run it near the upper limit. Most manufacturers will require about a 50F degree temp rise, so with 70 degree return they want 120F supply. Run the temp rise at 130-140 and it will clear the moisture faster.

Wondering how you are thinking to accomplish that... Restrict the output airflow? It's a variable speed blower so I would expect it to increase power to try to maintain airflow - most variable speed main blowers do that. Of course there is a limit but not great to run the blower at the power limit. I did verify it's two stage - 40 and 60k BTU. Wondering which provides the most temp rise. The blower speed is lower for stage 1 so I would expect them to be similar.

I also looked at the details of the return inlet. The filter box is against the side of the furnace and that is against a wall. They just cut the plywood on both sides of the wall and put a large grill on the other side. I suppose I could route the bypass duct on the other side of the furnace but it would stick out quite a bit to get around the gas and combustion air pipes. I'm thinking the supply side ducts are not allowed to be wood (?) so if I route the supply to the return how far does it need to be metal (or non-flammable)? I could run a metal duct down inside the wall to the grill and figure I at least need to do that I could line the return through the wall with sheet metal without much trouble but not sure that would be required by code.

 

[PoorUB]

Wondering how you are thinking to accomplish that... Restrict the output airflow? It's a variable speed blower so I would expect it to increase power to try to maintain airflow - most variable speed main blowers do that. Of course there is a limit but not great to run the blower at the power limit. I did verify it's two stage - 40 and 60k BTU. Wondering which provides the most temp rise. The blower speed is lower for stage 1 so I would expect them to be similar.

There is often jumpers or some way to set the temp rise, via blower speed. I don't know what furnace you have so no idea. Some are set automatically by the control board.

 

[u3b3rg33k]

Luxury! You were lucky. We had to live in the middle of the street in a cardboard box. (proper attribution to Monty Python...)

This has really gone off track. At least no one has suggested I rip it all down and build a bigger building.

Seriously though, it seems like a simple thing to put in a bypass and raise the temp. If I need 60F minimum air entering the furnace and have 110F air out (just guessing for an example) if I have 45F room air and mix 25% of the 110F air I get 61F air into the furnace. Would want some way to disable bypass when heating to higher temps and for AC as it will seriously decrease efficiency.

ok but this is GJ so please, DO tear it down and build again.

it's REALLY easy to do what you want with a bypass.

get yourself a 24VAC zone damper, and a heat only thermostat set for heat 60 or whatever. let it sense the return/room air (BEFORE the bypass). when the room is up to it's temp, it'll satisfy and stop bypassing. the hard part is sizing the bypass accurately, because it's only the right size at one room temp (hopefully your minimum setpoint).

or you could get fancy with a modulating damper, economizer style, and let the controller work it's freeze protection magic on the mixed air sensor. a Jade W7220 + actuator can be had for reasonable money on *bay.

 

[Innovate1]

Thanks for that and the specific parts suggestions! I was thinking the simpler system, maybe even a manual damper but an automatic system would be nice.

Wondering why I don't find anything on this. Maybe most people don't figure it saves enough to be worth doing? I need to do some estimates on the savings before embarking on it but fuel will continue to go up (maybe some short term relief but longer term it's bound to go up) and cost for doing it myself is pretty modest so thinking it would be worth it.

 

[beltfeed]

What about changing the thermostats anticipator setting to give you the longest heat-on run cycle. Sure this will give you bigger temp swings, but the furnace will run longer putting more heat into the heat exchangers.

 

[u3b3rg33k]

Thanks for that and the specific parts suggestions! I was thinking the simpler system, maybe even a manual damper but an automatic system would be nice.

Wondering why I don't find anything on this. Maybe most people don't figure it saves enough to be worth doing? I need to do some estimates on the savings before embarking on it but fuel will continue to go up (maybe some short term relief but longer term it's bound to go up) and cost for doing it myself is pretty modest so thinking it would be worth it.

don't waste your time with a manual system. you're NOT going to be there to actuate it every time it's needed, so either put one in without a damper, or make it automatic. or don't worry about it and see what happens.