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Hydronic vs Mini Forced Air in Mobile Home
Steam95
Member Posts: 14
Hello all,
I currently live in a mobile home, and it has some issues with the existing forced air system. Some of these can be fixed, but others are inherent problems with the design.
The two ends of the house are constantly colder than the middle. Specifically the kitchen and master bedroom. There is one heat register in each room, except the living room which has two. In other words, the same size register that heats a small bedroom close to the furnace is expected to also heat a large bedroom or kitchen at the farthest points from the furnace. Not only that, but the kitchen and master bedroom both have 3 exterior walls, and large windows... In addition, it turns out the furnace does not have the necessary capacity to maintain temp when it's -23F outside. (57kBTU in, 45kBTU out)
Currently, there is a 30 gallon electric water heater, and besides not holding enough hot water, it is costing me a fortune to run. I've opted to upgrade to a Rinnai RUR199in tankless gas heater, as it seems to be a good model, and is actually approved for mobile homes. According to the manual, it is rated for combination potable water/hydronic heating applications.
Originally I was considering small direct vent wall furnaces, but those are about $ a piece. I'm starting to think hydronic may actually be less expensive... Small panel radiators are quite inexpensive, at around $ per room, and pex is cheap too. Pumps and heat exchangers are where the cost comes in.
I plan to install a make up air unit (Electro Industries 1kw electric heated, with power damper, fan, and filter) as there has been an issue with cold air backing into the dryer if the bathroom fan or range hood are running. Fresh air is also nice. That said, I'd rather not pay to electrically heat the incoming air, so I'll heat it with a hydronic coil and disable the electric heating on the unit. Therefore I already need a heat exchanger, and at least two pumps. My question is, do I need a pump for each zone (kitchen, master bedroom, bathroom towel rack/radiator, and makeup air unit)? Or will a single pump (not including the pump on the potable side of the heat exchanger) with zone valves work?
Cost wise I'm not sure which makes more sense, but the installation of two combustion appliances and venting will probably add a good bit to the base price of $ for both gas wall heaters. Also, given the small btu load (combined 30kBTU) and gas being rather cheap it won't matter much, but the tankless water heater is supposedly 96% efficient, while the wall furnaces are 82%.
Any thoughts/opinions, safety concerns, maintenance concerns, etc. are all much appreciated!
Thank You,
Nick
I currently live in a mobile home, and it has some issues with the existing forced air system. Some of these can be fixed, but others are inherent problems with the design.
The two ends of the house are constantly colder than the middle. Specifically the kitchen and master bedroom. There is one heat register in each room, except the living room which has two. In other words, the same size register that heats a small bedroom close to the furnace is expected to also heat a large bedroom or kitchen at the farthest points from the furnace. Not only that, but the kitchen and master bedroom both have 3 exterior walls, and large windows... In addition, it turns out the furnace does not have the necessary capacity to maintain temp when it's -23F outside. (57kBTU in, 45kBTU out)
Currently, there is a 30 gallon electric water heater, and besides not holding enough hot water, it is costing me a fortune to run. I've opted to upgrade to a Rinnai RUR199in tankless gas heater, as it seems to be a good model, and is actually approved for mobile homes. According to the manual, it is rated for combination potable water/hydronic heating applications.
Originally I was considering small direct vent wall furnaces, but those are about $ a piece. I'm starting to think hydronic may actually be less expensive... Small panel radiators are quite inexpensive, at around $ per room, and pex is cheap too. Pumps and heat exchangers are where the cost comes in.
I plan to install a make up air unit (Electro Industries 1kw electric heated, with power damper, fan, and filter) as there has been an issue with cold air backing into the dryer if the bathroom fan or range hood are running. Fresh air is also nice. That said, I'd rather not pay to electrically heat the incoming air, so I'll heat it with a hydronic coil and disable the electric heating on the unit. Therefore I already need a heat exchanger, and at least two pumps. My question is, do I need a pump for each zone (kitchen, master bedroom, bathroom towel rack/radiator, and makeup air unit)? Or will a single pump (not including the pump on the potable side of the heat exchanger) with zone valves work?
Cost wise I'm not sure which makes more sense, but the installation of two combustion appliances and venting will probably add a good bit to the base price of $ for both gas wall heaters. Also, given the small btu load (combined 30kBTU) and gas being rather cheap it won't matter much, but the tankless water heater is supposedly 96% efficient, while the wall furnaces are 82%.
Any thoughts/opinions, safety concerns, maintenance concerns, etc. are all much appreciated!
Thank You,
Nick
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Comments
Having said that, I have lived in such a mobile home, and I know exactly what you mean about the forced air heat that's installed in them. I'm not normally particularly keen on using combis for heat as well as hot water -- they are usually much too big for the heating load. However, in this instance it might not be a bad fit -- particularly since space is quite limited. The fresh air unit could possibly have a coil in it. I would be very much inclined to duct it into the existing hot air ducts, but blank off at least one of the living room registers -- if not both of them.
I'd go for a single pump and zone valves.
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
Agreed that the tankless heater is far too large for the heating load, but supposedly it can ramp down to 15kBTU. Is that the only reason you're typically averse to combis for heat and hot water?
As for the fresh air, it says it cannot be ducted into pressurized systems (aka the hot air side of the furnace), but since the furnace just has a grille on the front for the cold air return, I can't really pipe it in. I was going to duct it to a vent right in front of the furnace, so it'll be sucked in and mixed with circulating air.
Why blank off the living room registers? The living room is rather comfortable. Note that the living room is quite open to the kitchen.
Any resources you'd recommend to learn pump sizing and such for hydronic systems? I've really only read up on steam.
Thanks!
-Nick
I agree with @Jamie Hall 1 pump with zone valves is all you probably need.
@DanHolohan has several books on hot water heating available on this site. "Pumping Away" I think would be the one (others may chime in)
I didn't figure I'd get full efficiency when only the hydronic heat is running, but I'd think it'd still be slightly more efficient than the wall heater. Not that it's a big concern with gas prices being low.
I'll definitely check out that book!
Thanks!
-Nick
They would have a problem with the water lines freezing up.
These would typically be under the floor along side the heating duct work. The supplemental heat kept the rooms warm but not the water lines. That is something to consider. They were advised to run the furnace fan constantly in sub zero weather as this may warm the water line spaces.
Also some had a self contained AC ducted into the supply.
There could be heat loss thru the outside AC air handler.
Have you clocked your gas meter to see if the furnace is actually burning the nameplate rating?
I haven't compared the furnace rating with its actual firing rate, but it seems like I should. I'm pretty sure it's running at full or close to full output though, as it doesn't lag that far behind when it's extremely cold. Another 10kBTU is enough to make up the difference. An HVAC tech told me he would install a 60kBTU furnace in a 1000sqft home in my area.
No central AC in the house, so I'm definitely not losing heat through that.
Yours, Larry
I still don't feel it will be able to be balanced completely, or be able to keep up to my satisfaction when it's incredibly cold. Had a few too many polar vortex days this year to take that chance. A backup/supplemental heat source isn't a bad idea either.
Still want the fresh air regardless, so I'll still have to run a hydronic zone, but maybe only one instead of 4.
However a hydronic system faces some of the same challenges created by the design of the mobile homes, leaky structure, insufficient insulation and a poorly designed distribution system in an unconditioned area.
I think a combi boiler like a Navien would be a nice fit. The use of glycol is obvious, as is the need to properly seal all penetrations and insulate the piping. Panel radiators are better choice then fin tube convectors since I usually see a lot of furniture and stuff up against the walls in mobile homes.
It's not a bad idea, good luck with it!
It is very doubtful you still have an LP orifice installed, but the pressure may not be adjusted high enough.
Also some come pre-piped for LP and would be possibly to small a pipe.
So clocking the gas meter would tell you if the furnace is putting out the max possible. In severe weather the pressure could drop, that would be the time to check manifold gas pressure.
It should be noted that it is a newer mobile home (2005), so the insulation is not that poor, nor is it terribly leaky. A poorly designed distribution system in an unconditoned space I'll give you though.
I'm hesitant to use a Navien, as unlike other manufacturers, they say nothing about mobile home use. Rheem explicitly says not to, while Rinnai simply states that if used in a mobile home it must be installed according to 24CFR subpart H. Having been around my grandfather (an insurance agent) taught me that if it doesn't say you can, you can't. I would also be hesitant to use a boiler primarily aimed at central heating, as it is going to primarily serve for domestic hot water, with a very small supplemental space heating load. Is that a concern or does it not matter? If a combi boiler makes more sense, would a rinnai Q-series be a good choice? The E-series seems a bit underpowered for my domestic hot water needs.
I wasn't sure if glycol is necessary so long as the lines are run with existing water lines (next to heat ducts), since none of my other pipes have ever frozen. I'll account for it though, since it certainly can't hurt.
As for the orifice question, the LP orifice is still in its bag hanging in the furnace, so I know it has the NG orifice installed. NG is far more prevalent around here than LP, so pretty much everything comes set up for NG. Also, I'd hope there aren't many piped for LP with lines inadequate for NG. Code pretty clearly prohibits that.
https://www.law.cornell.edu/cfr/text/24/3280.705
Mine has piping adequate for NG, just not for a tankless water heater, especially since it's not piped for a gas water heater in the first place. Only gas appliances are the furnace and range. Everything else is electric.
Thanks
-Nick
Add to that the issue of not maintaining temp anywhere when it's extremely cold out, I still need an auxiliary heat source. Not a lot, but enough that the system doesn't struggle so hard.
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
Definitely going to use panel radiators. I'd love to have radiators in every room, and just use the forced air to filter/circulate and provide AC in the summer. That would, however, likely freeze my water lines, since they rely on the duct heat.
Rather than TRVs, I'm planning to go with zone valves and a thermostat for each zone. Maybe tie it into my ecobee4. I'm not really sure if one can set up multiple zones on it.
Axman-Anderson makes a conforming boiler but it not wall hung.
I just went through this a few years back for a customer who had a Quietside combi illegally installed in her Mobile... It is/ was a piece of junk.
I looked far and wide for a conforming unit. The only unit for a wall hung was a Noritz NR98 tankless water heater that I was able to get approval from Noritz to run @ 180F....
I was not happy to go this route but I had little choice. It works... But I wont do it again.
Looks like I'll just stick with the tankless water heater with a heat exchanger for the hydronic zones. I don't need the high temps, and the 140F max hot water setting should be plenty for the hydronic zones. They are simply supplemental heat, not the main source, so I don't need a whole lot of performance out of them.