Hydronic vs Mini Forced Air in Mobile Home

Steam95 · March 2019

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

Jamie Hall · March 2019

First thought... no prices. It's something we don't do...

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.

Steam95 · March 2019

Fair enough. My focus is not on specific dollar amounts, but more on the line of "Both options are rather expensive, but which is more cost effective and/or better?" Price unfortunately is a factor in my decision, hence why I'm doing it in the first place. Electric space heaters are too expensive to run.

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

lchmb · March 2019

Is the trailer on a foundation? If not you'll need to get all the piping up under the insulation or face freeze ups. So that will add a bit (lot) of labor. Although there is a difference in up front efficiency, you will not get 96% efficiency while running it for baseboard heat. With the wall unit you will get a fast response, with quiet heat. If it were me, I'd add two wall unit's properly sized for the space and be done with it.. easier, quicker, cleaner..

EBEBRATT-Ed · March 2019

I would be concerned with freezing as well as @Ichmb mentioned above. Could use glycol but that adds $$.
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)

Steam95 · March 2019

It is not on a foundation, however I have to get up under the insulation anyway. I have to repipe the gas and get a new meter, since the tankless heater requires more gas at max firing rate than the old meter and lines can supply. I will also be piping/repiping for a gas dryer, future gas fireplace, and a potentially bigger range. I also have to move the water lines around to work with the new location of the water heater, as well as the option to install a whole house water filter. Adding some hydronic lines wouldn't be difficult. Add to that that I'll have to have a heat exchanger and zone for the makeup air unit anyway. There's no way to directly gas heat the incoming air. All of the gas fired makeup air units I found while researching are industrial.

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

JUGHNE · March 2019

I have seen mobile homes that have had a second source of heat, wood stove etc.
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?

Steam95 · March 2019

I am aware of the potential for freezing water lines, and do have the thermostat programmed to run the blower for at least 30min/hr. I've never had an issue with freezing lines thus far, even when it's -20F and the electric space heaters are doing the heating in those parts of the house. If anything the hydronic lines will do a little bit towards keeping things from freezing.

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.

Larry Weingarten · March 2019

Hello, I’ve worked on lots of mobile homes and wonder about trying to balance the existing system. In addition to using registers to control air flow, you might be able to increase fan speed to get air to the far points. Checking for serious duct leakage would be good too. If you could get it balanced, that just might cost less 😜
Yours, Larry

Steam95 · March 2019

I definitely intend to do that once it warms up. Going to have an HVAC contractor come out and do a thorough inspection, and repair if needed. There is one register that hardly blows, so I definitely suspect that will need some work. Even so, the little bit coming through is enough for the small bedroom it's in.

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.

SuperTech · March 2019

Having worked in several mobile homes I can say that many have issues with the forced air heat. Trouble with the home heating unevenly and uncomfortably. The furnace constantly running, shutting on and off, high energy usage.
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!

JUGHNE · March 2019

Most of the mobile homes that show up around here come equipped with the LP kit. It is hanging in a small cloth bag attached to the gas valve. They may put the LP orifice in the furnace as it is very small and if connected to NG there is little risk of overfiring, if the situation is reversed then bad things could happen. Also the gas valve regulator spring gets changed with this conversion and must be adjusted for NG.

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.

Steam95 · March 2019

Thanks for the advice guys! Sorry for the late response, work has been a bit crazy the last couple of days. Work-eat-sleep repeat, haven't checked my phone much.

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

Jellis · March 2019

I managed to balance heat in a mobile home i was living in, i put new adjustable registers in for the whole house and adjusted each one. you basically close the registers close to the furnace and leave the ones far away wide open. I managed to keep every room within 3 or 4 degrees of 70 all the time, its a pain, but it works.

Steam95 · March 2019

I've tried. Even with the ones close to the furnace shut, it won't keep the two end rooms at set point. It's close, but not where I'd like it to be. Not to mention that the center rooms start to get a bit chilly with the vents shut. The whole point is to not have to choose between a warm bathroom/bedroom/living room and a warm kitchen.

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.

Jamie Hall · March 2019

Rinnai makes very good equipment, and the Q series is very good indeed. Having lived in a mobile home and struggled to get the heat even sort of even, I sympathize... might be a very good way to go. Consider panel radiators -- they are much less intrusive into the space being heated.

SuperTech · March 2019

Panel radiators are great. TRVs might help out with balancing the heat, but will add to the cost of the installation. I think a mobile home with a boiler and panel radiators would be the basis for a really nice mobile home.

Steam95 · March 2019

Would you recommend the Q series combi over the tankless water heater? I don't see much of a difference besides in one situation the potable water is heating the hydronic water, and in the other the opposite is true. Considering it will be doing much more domestic water heating than space heating, I'm inclined to go with the water heater indirectly heating the hydronic. Of course the tankless water heater runs at a lower temp, which may also make it more efficient. Or is is about even?

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.

kcopp · March 2019

Your biggest problem is finding a unit that is accepted for Mobile home use.... It does NOT exist.
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.

Steam95 · March 2019

The Rinnai RUR199in tankless water heater is approved for mobile home use. Though now that I've read the Q series manual, it turns out it isn't approved for mobile home use.

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.

Hydronic vs Mini Forced Air in Mobile Home

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