Critique this low temp small apt combi heater/DHW system design?

Jells

I have a 450 net ft apartment that needs a heating system. I can't use a mod-con unit, it's a rowhouse with no way to bring in air and no place to drain condensate. I only have a brick flue. It's a rental, so low maintenance is a higher priority than efficiency anyway.

I want to use a 40 gal 35k BTU DWH, a 60k BTU rated exchanger block, and 2 K120 Twin-Flo kickspace heaters in wall cases, each rated ~8500 BTU at low speed and 140 deg. It will be piped with a 3/4 pex loop and Monoflo tees.

An identical apartment has sufficient heat from a DWH and 30' of baseboard, so at best there's 10,200 BTU of output there with 140 deg water. It seems to me I'm safely within my requirements. Am I right?

Sizing the exchanger block has baffled me since the calculators are hard to apply to this situation for me given the low temps and other variables. Looking at the selector tables it looks like it should have plenty of headroom with the 60k derated for the temp and flow, but for the extra $40 perhaps I should just get the 125k and take the issue off the table.

I understand the rest of what the exchanger system needs in terms of 2 pumps, expansion tank, feed pressure reducer, etc. From the HX on it needs to look like any other hydronic loop on a boiler.

I'm needing a 'combi' type system on the WH, but it seems to me to be smarter to use a standard atmospheric tank with an exchanger block & 2 circulators than the absurdly expensive ($3600) BW Combicor that seems unavailable anyway. At least when the WH eventually fails I'm not paying to replace the whole exchanger system too.

Similarly, can anyone explain why I should spend $1700 on a Taco X-pump block when 2 006 pumps and a block will cost <$600?

Also, why do the DWH with heating capability have side taps? What difference does that make from using the regular taps? I've seen mfr diagrams for using the regular taps for room heating also, and had contractors install using the top taps even when the DWH had side taps.

Thanks for any constructive feedback.

PS: The cost of actually laying radiant subfloor is not sensible in this old, funky rental.

hot_rod

You will need to run the water heater up around 150- 160 to easily get 140F out. Look at the heaters and see what the BTU output is at the various SWTs. If you can cover the load with 130F then the tank could run at a lower temperature. 120 or lower and the air is not very warm coming out of a fan convector.
If you do run above 140F, install a thermostatic mix valve to protect the DHW temperature. The CombiCors always had a valve with them as they could run 160F

Close approach design is where you can run the A side of the HX at 3- 5° hotter than you want from the B side, which of course takes a larger HX.
The on line software for sizing HX makes it pretty easy to come up with the size options. No harm in oversizing the HX except for cost. I'd guess a 5X12 8 or 10 plate would move that load easily.

Keep in mind the efficiency derate for the water heater 35K X 78% gives you about 27,000 BTU/hr output.
less if you are at high altitudes. So when you are making DHW and heat, that is what you have to work with.

If you are handy, it is fairly simple to build up a HX module. I'd use Webstone purge ball valves to flush the A side of the HX, as it will scale up with hard water.
Utica boiler makes a pre-plumbed module if you want an off the shelf option.

Jells

@hot_rod thanks so much for a fantastically informative answer, without telling me I'm an idiot for doing this! 

Of course I would put a mixing valve on, so basic I didn't think to say it. All my water heaters have them, I pay attention to what I read here about legionella, and for like a four family building with a 75 gallon tank it makes for more hot water.

Are there basic DWH that can go up to 150 or 160?  And if it's so easy to build a HX module why would anyone spend $1,600 on the Taco X?

Tom_133

The Taco X-block has a heat exchanger built in. So you would technically get both things you want, and the controls to make it do outdoor reset when the temps drop or vice versa. They do the work all in a clean easy to use package. If you do it yourself in a clean package then you dont need the x-block

https://www.supplyhouse.com/Taco-XPB-1-Taco-X-Pump-Block-1-25-HP

GroundUp

People buy the X-block for 3 reasons;
1. They don't know how to build one
2. They don't have time to build one
3. They're lazy

Jells

GroundUp said:

People buy the X-block for 3 reasons; 1. They don't know how to build one 2. They don't have time to build one 3. They're lazy

Really, they spend $1,000 rather than do four extra plumbing connections and wiring up a thermostat relay?

Jells

hot_rod said:

Close approach design is where you can run the A side of the HX at 3- 5° hotter than you want from the B side, which of course takes a larger HX.
The on line software for sizing HX makes it pretty easy to come up with the size options. No harm in oversizing the HX except for cost. I'd guess a 5X12 8 or 10 plate would move that load easily..

I didn't see one on Supplyhouse that matches your description.The B&G 125k BTU I was looking at is only 3x8, but 20 plate. Will it do that <5 deg drop? If it's as simple as calculating the comparative total sq inches of plate then it should be the same as a 5X12 8 plate.

GroundUp

Jells said:

GroundUp said:

People buy the X-block for 3 reasons;
1. They don't know how to build one
2. They don't have time to build one
3. They're lazy

Really, they spend $1,000 rather than do four extra plumbing connections and wiring up a thermostat relay?

It's quite a bit more than 4 connections and a relay, but yes.

Jells

GroundUp said:

GroundUp said:

It's quite a bit more than 4 connections and a relay, but yes.

Thanks, but now you've got me worried that I need to be educated on this, since there's obviously something I'm missing. The HX has 4 taps just like the X-Block. But on each side you're adding in a pump to the loop. 4 connections. The WH side bronze pump runs 24/7 so we don't get Legionella, the heating loop needs cast iron pump, a 24v supply & relay for the thermostat 24v circuit. What am I missing? (obviously besides all the rest of the devices on the heating loop as I mentioned above like the PRV, expansion tank, etc)

mattmia2

The specific design of the hx and how it causes turbulence in the water at a given flow affects the transfer in a plate hx so it isn't exactly just surface area to compare different manufacturers.

wmgeorge

For a small apartment why are you making it so complicated? A water heater for the DHW and a small one or a small HW boiler for the heat? For the price of a plate heat exchanger you can have a heating only boiler. You can turn off the heating only boiler when not needed.

Alan (California Radiant) Forbes

@GroundUp

People buy the X-block for 3 reasons;
1. They don't know how to build one
2. They don't have time to build one
3. They're lazy

My number one reason for using X-Pump Blocks is the small footprint. Oftentimes, we just don't have the room.

GroundUp

wmgeorge said:

For a small apartment why are you making it so complicated? A water heater for the DHW and a small one or a small HW boiler for the heat? For the price of a plate heat exchanger you can have a heating only boiler. You can turn off the heating only boiler when not needed.

Where are you buying boilers for $200? Plates are cheap

GroundUp

Alan (California Radiant) Forbes said:

@GroundUp

People buy the X-block for 3 reasons;
1. They don't know how to build one
2. They don't have time to build one
3. They're lazy

My number one reason for using X-Pump Blocks is the small footprint. Oftentimes, we just don't have the room.

You are absolutely correct- sorry I missed that one!

wmgeorge

GroundUp said:

wmgeorge said:

For a small apartment why are you making it so complicated? A water heater for the DHW and a small one or a small HW boiler for the heat? For the price of a plate heat exchanger you can have a heating only boiler. You can turn off the heating only boiler when not needed.

Where are you buying boilers for $200? Plates are cheap

Along with all the rest he has listed? Small gas water heaters are about $500.

Jells

Alan (California Radiant) Forbes said:

@GroundUp

People buy the X-block for 3 reasons;
1. They don't know how to build one
2. They don't have time to build one
3. They're lazy

My number one reason for using X-Pump Blocks is the small footprint. Oftentimes, we just don't have the room.

Nice work! What's the device to the top right of the block? Not familiar with it.

Jells

wmgeorge said:

GroundUp said:

wmgeorge said:

For a small apartment why are you making it so complicated? A water heater for the DHW and a small one or a small HW boiler for the heat? For the price of a plate heat exchanger you can have a heating only boiler. You can turn off the heating only boiler when not needed.

Where are you buying boilers for $200? Plates are cheap

Along with all the rest he has listed? Small gas water heaters are about $500.

Huge cost difference aside, the footprint, noise and dependability differences make that a non-starter. Been there, done that. 1st contractor installed a Minitherm boiler and an indirect tank. Huge footprint in a tiny room, it was noisy with the relay and damper, and because it was on the top floor a venturi effect in the flue caused the venting system to shut down the boiler in high winds. Ripped it out and put in a DWH and circulator. Only problem I've in the decade since was a stuck check valve.

Jon_blaney

Does your code require that the DHW and heating be separated? You can get fancoils that can be used with an open system.

EternalNoob

GroundUp said:

Jells said:

GroundUp said:

People buy the X-block for 3 reasons;
1. They don't know how to build one
2. They don't have time to build one
3. They're lazy

Really, they spend $1,000 rather than do four extra plumbing connections and wiring up a thermostat relay?

It's quite a bit more than 4 connections and a relay, but yes.

taco x-block is $1650 not $1000 or am i missing something. cost of 2 circ pumps plus FPHX plus MV plus switch relay, still under $800, half the cost. but time... and space...

Jells

Jon_blaney said:

Does your code require that the DHW and heating be separated? You can get fancoils that can be used with an open system.

Regardless of what code allows, it's been said to me many times here that running potable water through the heating system isn't safe as Legionella can breed in the loop when it's not running. The idea is to use the HX block to separate them. If by fancoils you mean a kickspace type heater, that's the plan.

Jells

EternalNoob said:

taco x-block is $1650 not $1000 or am i missing something. cost of 2 circ pumps plus FPHX plus MV plus switch relay, still under $800, half the cost. but time... and space...

$1000 was referring to the difference in price between the xblock and your shopping list. I was figuring <$600 for that.

Jon_blaney

Most systems that are discussed here are radiant floor system designed to run at a temperatures below 120 degrees, prime legionella growing temperature. Your system is low volume and at 140 degrees, a temperature that kills legionella. Your system would take treated public water at 50 degrees, heat it to 140 degrees, circulate it and reheat. When will the legionella grow. It is no different than the flow in the normal DHW system. Warm water sitting in pipes overnight will give you legionella with your morning shower? Put a timer on the circulator, and if it is code, pipe it direct. Oh, don't let the kids play under the hose next summer because that is were they my get legionella.

Jells

Jon_blaney said:

Most systems that are discussed here are radiant floor system designed to run at a temperatures below 120 degrees, prime legionella growing temperature. Your system is low volume and at 140 degrees, a temperature that kills legionella. Your system would take treated public water at 50 degrees, heat it to 140 degrees, circulate it and reheat. When will the legionella grow. It is no different than the flow in the normal DHW system. Warm water sitting in pipes overnight will give you legionella with your morning shower? Put a timer on the circulator, and if it is code, pipe it direct. Oh, don't let the kids play under the hose next summer because that is were they my get legionella.

I was told the biggest risk was from the water sitting in the pipes in the off season. Seriously, every time this type of system is discussed I get the lecture on how dangerous it is! I have several without HX installed by contractors and no one has gotten sick in decades of use, but I'm trying to do better.

Jon_blaney

You can get a time to run the circulator on a schedule. Like I said, all system are not the same. That is the problem with rules of thumb.

Jells

Jon_blaney said:

You can get a time to run the circulator on a schedule. Like I said, all system are not the same. That is the problem with rules of thumb.

That's a fascinating idea, just run it like once a day for 5 minutes to clear the loop, not unlike a freezer defrost cycle. I'd love to hear what others here think, it would save me a lot of money and time!

wmgeorge

Jells said:

Jon_blaney said:

Most systems that are discussed here are radiant floor system designed to run at a temperatures below 120 degrees, prime legionella growing temperature. Your system is low volume and at 140 degrees, a temperature that kills legionella. Your system would take treated public water at 50 degrees, heat it to 140 degrees, circulate it and reheat. When will the legionella grow. It is no different than the flow in the normal DHW system. Warm water sitting in pipes overnight will give you legionella with your morning shower? Put a timer on the circulator, and if it is code, pipe it direct. Oh, don't let the kids play under the hose next summer because that is were they my get legionella.

I was told the biggest risk was from the water sitting in the pipes in the off season. Seriously, every time this type of system is discussed I get the lecture on how dangerous it is! I have several without HX installed by contractors and no one has gotten sick in decades of use, but I'm trying to do better.

Ditto, never heard of this... according to the Mayo Clinic & OHSA less than 20,000 deaths per year from Legionella and most are from cooling tower drift and hot tubs.

Alan (California Radiant) Forbes

@Jells It's a Caleffi Discal vertical air separator.

Jells

Alan (California Radiant) Forbes said:

@Jells It's a Caleffi Discal vertical air separator.

Thanks, never saw that, looks a lot pricier than the usual 1/8" Taco or MoM device. Is there an advantage besides vertical?

Alan (California Radiant) Forbes

Thanks, never saw that, looks a lot pricier than the usual 1/8" Taco or MoM device. Is there an advantage besides vertical?

It's directly in the pathway of heated water and offers a large enclosure for the water to slow down and give up any air bubbles. The enclosure also contains a mesh that air bubbles stick to and accumulate to a point where they become buoyant, float to the top and are released.

Jells

Alan (California Radiant) Forbes said:

Thanks, never saw that, looks a lot pricier than the usual 1/8" Taco or MoM device. Is there an advantage besides vertical?

It's directly in the pathway of heated water and offers a large enclosure for the water to slow down and give up any air bubbles. The enclosure also contains a mesh that air bubbles stick to and accumulate to a point where they become buoyant, float to the top and are released.

Thanks. Clearly you have a client there willing to pay for top quality components and design. Me, I still sweat copper.

Alan (California Radiant) Forbes

Thanks. Clearly you have a client there willing to pay for top quality components and design. Me, I still sweat copper.

I was going to click the "Like" button, but I don't really like the fact that you aren't able to install something that would serve the client well without being scrutinized for spending too much money.
I live in fear of offending people for what I say, but I think you mean well and would like to be able to install good equipment, but they may quibble about the price.

Jells

Alan (California Radiant) Forbes said:

Thanks. Clearly you have a client there willing to pay for top quality components and design. Me, I still sweat copper.

I was going to click the "Like" button, but I don't really like the fact that you aren't able to install something that would serve the client well without being scrutinized for spending too much money. I live in fear of offending people for what I say, but I think you mean well and would like to be able to install good equipment, but they may quibble about the price.

Fair enough. I work on rental properties and that's not an industry where you hear people say "money is no object!". You wouldn't believe what I hear about what goes on in the brand new high-rise rentals in my city where people are spending a fortune to heat their poorly insulated apartments using PTACs.

rsilvers

Is that air separator as good as the "micro bubble" kind?

hot_rod

rsilvers said:

Is that air separator as good as the "micro bubble" kind?

That is a micro bubble separator, can be used vertically or horizontally and is very easy to service, disassemble and clean.

EternalNoob

Alan I think the question maybe was more like what's the advantage of the discal air separator in the vertical orientation vs horizontal type (more typical).    I'm curious too.   I've been under the assumption that the horizontal is better so the bubbles have a long horizontal run to settle in the top of the pipe. 

EternalNoob

Regarding the to x-block or not to x-block question, I am very curious to know if anyone on here has used the Sanden CO2 A2WHP with 2 pumps and FPHE rather than the x-block that Sanden seems to strongly recommend for combi systems.   I think they might even say they won't warrantee installs without the x-block and I find this kind of annoying to be roped into buying an expensive product.

Alan (California Radiant) Forbes

EternalNoob said:

Alan I think the question maybe was more like what's the advantage of the discal air separator in the vertical orientation vs horizontal type (more typical).    I'm curious too.   I've been under the assumption that the horizontal is better so the bubbles have a long horizontal run to settle in the top of the pipe. 

That's my understanding as well.

hot_rod

The bottom portion of that Discal is actually the same principal as the horizontal only type Discal. The passageway forms a horizontal chamber 
Notice also how micro bubbles form on the hot surface wall of the boiler, thus includes low temperature mod con boilers

GroundUp

wmgeorge said:

GroundUp said:

wmgeorge said:

For the price of a plate heat exchanger you can have a heating only boiler.

Where are you buying boilers for $200? Plates are cheap

Along with all the rest he has listed? Small gas water heaters are about $500.

You literally specified that a boiler is the same price as a plate heat exchanger. A plate heat exchanger is under $200. The end.