Combined radiant and forced air Geothermal Retrofit - need advice and opinions
I initially started this as a reply to another thread here:
but thought it best to start my own so as not to threadjack too badly.
I was just at a customer's home this morning. The house is 7 years old with a 93% 2-stage bryant propane furnace and a 3.5 ton single stage AC currently serving their HVAC needs. When the house was built, the HO put radiant tube in the entire basement as well as the rooms on the upper floors with hard surface flooring. It's a total of 4 zones (5 if the garage is included - served by a hot water heater with radiant floor currently) of hydronic and the forced air is also zoned for the main and upper floors.
I'm thinking a water to water unit makes sense, but I'm trying to balance things out. The entire house isn't going to be able to be heated by radiant so I'll need to have a lot of space heating by the forced air unit and the higher temperatures required by a water coil on top of the furnace compared to the lower temps of the radiant floor adds a bit of a wrinkle to everything.
I have a perfectly serviceable furnace to use for backup, otherwise I very much like the use of a boiler as both auxiliary and DHW production. I'll offer that as an option, but I don't known that it's a necessary expense at this point for the HO. I'm looking into the Waterfurnace Synergy3D as a potential option - it seems like it might be a good fit, but that would be doing away with the furnace and then we're probably looking at electric as auxiliary backup, which really isn't a big deal I suppose as the current furnace doesn't have a VS blower either.
I'm pretty sure I can do it a number of ways - I'm trying to minimize the amount of equipment that will need to be purchased while maximizing the efficiency of the system.
I think the biggest decision I need to make, or at least one of the options, is whether or not I want to keep the furnace in the equation. If I keep if, auxilliary heat is taken care of, but it saddles me with a non-variable speed motor (could be swapped for a retrofit, but that's not the same) and a split geo unit (which I think excludes the Synergy). That makes the forced air side of things pretty simple, but leave the hydronic problem unsolved.
For the Hydronic I'm considering (1) Synergy3D or equivalent dual capable unit (2) Water to Water Geothermal (3) condensing wall hung boiler (4) High efficiency water heater.
I suspect something like the Triangle Tube smart multi energy tank might place a key role in allowing a tie-in between domestic hot water production and space heating.
The controls side becomes a whole 'nother problem to solve. There will be 2 forced air zones which have overlap on the 4 hydronic heat only zones. I certainly see where two pieces of equipment greatly simplify that equation, but again, trying to be cost effective as well. I would probably prioritize forced air heating as that covers the entire home and radiant floor would be for comfort rather than actual heating.
Still very much in the planning stages but really appreciate the assistance and any guidance or advice.
I dont' know if this would work but if you use a Synergy rather than sizing with a 4 ton unit maybe you could do a 5 or 6 because 1st stage will be 60-70% of units total capacity putting you about 3.5 tons(assuming the AC is currently sized properly) yet will give you more capacity for heat so that your radiant system is not running 100% of the time on a cold day or so that your forced air will have a chance to shut down on a cold day and warm the radiant tank back up.
For a situation like this though I would typically do the two system option with a dedicated water to water unit (sized for that load only) and then a hybrid system with a refrigerant not water coil for the ducted heating and cooling(also sized to the load). I am just not a fan of chilling a ton of water and not having the option of the radiant during ducted cooling. In my climate we need heating in the morning and cooling in the afternoon and it is a huge energy waste to switch from one to the other with a water system and 50-80 gallon storage tank.
The synergy system is interesting as is your suggestion of dedicated water-to-water. In theory, if the synergy system is sized to the heat load of the house, the combined radiant and forced air need shouldn't exceed the available capacity of the unit, even if it does switch between the two. I do worry that using a single piece of equipment to provide both forced air space heating and radiant floor may allow for a situation where floors get cold or rooms not covered by radiant do, neither of which is an attractive proposition in what will not be an inexpensive system no matter which direction we take.
At this moment I'm trying to figure out in my head how to combine domestic hot water production and radiant floor heat using a Geo w2w unit AND the built-in desuperheater of a forced air Geo. If I use something like the TT Smart Tank, I'll end up with one big tank of 120 degree water - which is great for radiant and DHW, but doesn't allow for much buffer for the desuperheater to take advantage of and doesn't give me an auxilliary option to make sure domestic production is at 120 degrees when it comes out of the taps. I think I might need 3 water tanks which seems excessive to me...
I think on the domestic water side if they have propane its time to use the KISS philosophy, Keep It Simple Stupid
Personally I would not worry about trying to take advantage of all hot water production because you will have a marginal benefit from it. If your propane, hookup a Rinnai, those are already about half the cost of a conventional water heater as far as yearly usage, put in a 50 gallon desuperheater storage tank for the Rinnai to pull from and be done with it. Saves space and gets the job done for about the same yearly cost as the complicated controls you were thinking of but probaly will cost less upfront and be easier to work on too.
Otherwise there is one last option for water, run a dedicated NSW018 unit, they are INCREDIBLY inexpensive IMHO compared to some of the water to water units and after tax credit can sometimes be done for the price of upgrading to a tankless after tax credits, this is certainly not going to work in all situations but could be simpler than trying to make the radiant floor W2W unit work to do water heating as well and would save you some money on not having to purchase the desuperheater addition for the other two units.
Is this going to be vertical or horizontal loop?
What is the Manual J calc for this house? Radiant portion and forced air.
Well, unfortunately for the HO, they're on Propane. They've currently got a 73gallon natural draft propane water heater, so while geo hw production might be marginal, it is still far better than their current situation. They have a $5k a year propane bill so I'm pretty sure as long as I can show the savings and they can afford the system they'll probably go for it.
I'm a huge believer in KISS and you're convincing me that keeping hydronic and forced air separate will contribute to that in a large way. Now the question become whether or not the hydronic side is a W2W geo unit or something like a Rinnai or condensing boiler.
If you look at the video in the link below it is a similar story, customer was spending up to $5K per year in propane, he had a 5 year payback on the system in the video including new top of the line marathon water heaters, and two zone system costs lol.
Sadly the water heating was an afterthought, I brought it up and they didn't want to change the water heater too. Middle of the install we put in the new marathon and he is amazed how good it looks and that maybe he should just convert to all electric and have 0 propane usage, well at this point equipment was ordered and loop was installed so we couldn't go dedicated geo water but he is now between $900-$1,200 a year in total water heating and space heating/cooling costs. The system was installed in November and has so far only cost about$600 in additional electric costs while having no propane costs, we are a very light cooling load here so I doubt he will spend much more in this years heating/cooling cycle and his loop temps have been consistant at a 38-40EWT.
I actually watched that video earlier today. it looks like a nice installation, although it doesn't have a hydronic radiant application that I can see. Are you using one of the marathon tanks as a buffer or both full temp?
There is no radiant on it, just similar cause the customer converted from popane to geo and added water heating lol and its a waterfurnace.
Originally Posted by frumper15
One marathon is a desuperheater storage tank(no electric hooked up) and the other is the actual water heater with a 4500 watt element in it.
"because if Radiant has priority over forced air it will just run all day long trying to satisfy the radiant load".
The Hydron Module combo unit can be configured with shared priorty.
"Shared priority: In shared priority mode, the
unit operates in the hot water priority mode
and in forced air priority mode in an alternating
sequence. There is a field selectable timer,
which will allow settings of 20, 30, or 40 minutes
for switch over time. The unit will always start
in hot water priority mode at the first call for
hot water (aqua-stat input). Then, based upon
the timer setting, the unit will switch to forced
air priority for the selected amount of time. For
example, if the timer is set for 20 minutes, and
the unit has a simultaneous call for hot water
and thermostat, after 20 minutes of hot water
operation, the control will switch over to forced
air priority for 20 minutes, and then switch back
to hot water priority for 20 minutes, and so
on. If the aqua-stat call is satisfied, forced air
operation is allowed at any time; likewise if the
thermostat is satisfied, hot water operation is
allowed at any time. There is a five-minute antishort
cycle timer between modes."
I have a Hydro-Temp combo system, it can heat air and water at the same time.
I do alot of retros just like this w to w the best way to go. get a hydronic coil to put on top of the gas furnace. size storage tank for 6 to 10 galon per ton get a B&D tank, and do everything with one geo unit and keep it simple. the worst thing with combo units is that your heat load is way more than your cooling load and your way oversized then you have problems.
How do you handle domestic hot water in that situation? Do you use the Desuperheater with a buffer tank and then another powered (propane or electric) unit for production? I see the benefit for having a buffer tank on the unit itself and for the radiant and air coil, it makes a lot of sense to pull from that tank. However, in the summer months, that tank is going to be cold water, so you can't really integrate the domestic and radiant water because that doesn't work for half of the year. Also, how are you handling the differing water temperature requirements of radiant floor and an air coil on top of a furnace? Are you storing the water at 150 degrees and using a mixing control for the floors?
Couple of things, your not going to get 150 water out of a geo unit, you will run 115 to 120 and that is good for floors and coil, if you want less for floors just put in a mixing valve. If you go water to water you have to have a buffer tank. Means domestic hot water seems to be such a big issue just use desuperheater. the only way you can integrate domestic and radiant together is with a double wall heat exchanger. I don't know what electric rates are but electric water heater and desuper. if you need alot of hot water you could also get a small w to w and make it just for domestic, a little more expensive but works great, i have a school where we have to 5 ton water to water just for domestic hw
Originally Posted by frumper15
A couple of things - thanks for the heads up on water temps - I have done a w2w geo yet so I wasn't sure what to expect. I do plan on using the desuperheater with a buffer tank for the domestic - has always been a nice "freebie" on the other forced air systems we've done.
You're not the first person to suggest using a dedicated w2w unit for the domestic and radiant. I guess the question I have is around the configuration - you say a little more expensive, I'm thinking dedicated loop field, pumps, etc. Should I not be thinking that? Is it as simple as plumbing it inline with the water to air unit and sizing the loop field to be large enough for both? Is that how people are doing multiple units in a single home?
I do think I've figured out how to do everything well with a single water to water unit - I'm working on a design and would like to share it, but I don't know if this thread should maybe be moved to the pro side - what's the criteria for what can be shared over here?