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  1. #1
    Join Date
    Nov 2019
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    re-install cast iron radiator hydronic system

    This past summer bought a 1933 house in metro Boston with a forced hot water hydronic system complete with cast iron radiators and a 40+ year old peerless gas boiler. Against my better judgement, we proceeded to remove all the radiators, replace the boiler with a Bosch Greenstar 100k (95% efficient) with indirect DHW, and an Unico high velocity system for heating and cooling on both floors, zoned separately. So basically hydro-air heating. The condenser is a Bosch two-stage unit with a heat pump. I'll note that I just had a blower door test which came back at 16 ACH50 for the 2000 sf balloon framed house, and we're trying to bring that down with air sealing.

    Fast forward to the heating season and the system is pretty unbearable. The heat pump is set as primary heat down to 40 degrees F, and then the hydroair system takes over. The blower speed is turned down as low as it can go, and it still sounds like a windstorm outside everytime it turns on. The high velocity system just makes it feel drafty with the cool air temp from the heat pump, though the boiler is not much better. At just below 40 degrees F, the boiler heats supply water to cose to 160 degrees F, which I assume means it's never able to condense because I suspect the return water temperature isn't more than 20 degrees lower. The system does modulate temperature, but I've never observed it below 130-something, and I wonder if it's just cooling down then. The system also turns on multiple times per hour for 5-10 min at a time max, then stops, to try to maintain temperature. I don't know if this means the boiler is close to short cycling. When it was 25 degrees outside last week, the hydroair just never turned off for hours continuously, but at least the exiting air temperature was warm enough to not be uncomfortable.

    In a nutshell, all of my fears about the new system compared to the old have come true. The one thing the system does well is maintain even temperature everywhere, and I suspect that it will cool the house well and the aspiration/air movement will help with summer comfort.

    Am I crazy to want to put back the CI radiators and use the new boiler for what it should be good at doing? We gutted the low height cast iron piping in the basement for future head height, but all the CI piping to the second floor is still in place. In an ideal world, I'd hope the system could be repiped with oxygen barrier pex for home runs and zoning by floor. Maybe even switch out some of the CI rads for Runtal panel radiators and a bathroom towel rack. I'm not sure if these two different hydronic systems would be compatible, or if we'd just have to abandon the Unico heating coil and repipe the boiler for radiators. If none of this is possible, any advice to get the most out of the current system? I've set the indoor temp to 72 degrees with minimal setbacks to try to maintain overall comfort (audio and temperature).

    This is definitely a costly mistake, but I'm taking the long term view for comfort. Thanks for your thoughts, very much appreciated.

  2. #2
    Join Date
    Jan 2012
    Location
    Broomall, PA
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    It’s definitely doable running homerun pex and hooking back up to the existing piping.
    Do not make a second costly mistake. Do hire a hydronic expert to (re)design the system after doing a complete heat loss and properly sizing actually, over sized the radiation is good, as it allows lower SWT and the ability for condensing.
    Hydronic systems are always more comfortable than scorched air systems, and radiant is even more comfortable.

    I suspect the loud noise your experiencing is from incorrect duct design/sizing. Is it the same for AC?

    Do you still have the old rads?
    If I do a job in 30 minutes it's because I spent 30 years learning how to do that in 30 minutes. You owe me for the years, not the minutes.

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    Sounds like the Unico installation was far from the instructions. There's a minimum number of runs per ton. There's sound attenuating 2" duct to help quiet it down. Your contractor must not be versed in high velocity. Unico has good tech support, if you keep the system, your tech can contact them.

    I grew up in a house with a converted gravity system and big old radiators. For most of the winter, unless it was below zero, we kept the water at 135 in the 1950s boiler. So perfect place for a mod con.

  4. #4
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    Oct 2009
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    God's country - Shenandoah Valley, VA
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    Don't try to mix modern rad's with the old cast iron ones and don't create micro zones that will short cycle the boiler. Worst case scenario, you shouldn't have more than two space heating zones, maybe only one if the system is properly balanced.

    As Steve and BL said, the Unico system wasn't done correctly if you're have those issues.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  5. #5
    Join Date
    Jun 2012
    Location
    Northern NV
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    Might look at the heatinghelp dot com site. They are almost exclusively "wetheads" and was where I went for guidance when I was in the wet end of the trade..

    That said, modulating, condensing "boilers" with out door reset and CI radiators is a lovely thing to behold... Have done several.
    “A dying culture invariably exhibits personal rudeness. Bad manners. Lack of consideration for others in minor matters. A loss of politeness, of gentle manners, is more significant than is a riot.”
    ― Robert A. Heinlein

  6. #6
    Join Date
    Jan 2012
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    Broomall, PA
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    Quote Originally Posted by Juan Madera View Post
    Might look at the heatinghelp dot com site...
    I think all 3 posters above you are from HH.com. I know Bob and I are there.
    If I do a job in 30 minutes it's because I spent 30 years learning how to do that in 30 minutes. You owe me for the years, not the minutes.

  7. #7
    Join Date
    Nov 2019
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    9
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    Thread Starter
    Thanks, Steve, Bob, BL and Juan for your thoughts so far. I thought I did my homework for the system installer -- we got 6+ quotes. They're one of the few in the area who do high velocity, and had recommendations from family friends. We went with the high velocity system so we could keep ductwork to a minimum and not take up too much of the limited closet space or open up plaster walls. When we did the install walkthrough was told we made the correct number of ducts for the system -- 14 on the first floor (~1100 sf) and 11 on the second floor. (~900 sq) They used spacepac 2" ducts for the better insulation, and the rest Unico. I guess I'll go back to them and see what else they can do to fix it. I haven't tried the AC yet since the install finished last month.

    Unfortunately, the demo company (we had to gut the basement as well to install the 1st floor ductwork in the basement) also took away the radiators. The (hopefullly) silver lining is there are some local companies who supply old CI rads and will also strip and refinish. We would have had to do that anyway so I suspect it could cost a similar amount to find and paint replacements.

    The more I study the system, the more I'm convinced the heating system isn't set up properly. We have a leaky addition off the back of the house with baseboard convectors (~20 feet) and this morning when I had that zone call for heat on its own, the boiler cranked up to 170 degrees (outside temp ~38 degres), then it cycles every 5-10 min. Its only saving grace is that that FHW zone is supposed to be supplemental heat and the hydroair also supplies the room as well which keeps the entire 1st floor burns a little longer -- though they're still 15-20 min @ 160 degree SHW temp. I can't see how the system ever reaches condensing conditions.

    Re: zoning, is there any benefit to two zone upstairs/downstairs vs a single zone with TRVs on each radiator? Also, how would one zone with CI rads and one zone with steel panel radiators perform? Just thinking through all possibilities to get educated from everyone's expertise here. Thanks once again.

  8. #8
    Join Date
    Oct 2009
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    God's country - Shenandoah Valley, VA
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    The short cycling with the fin tube BBs is exactly what I cautioned about. That's only a 10k btu load and the lowest modulation on that boiler is 19k btus. That means the boiler's only gonna run for about 5 minutes with only that zone calling. You want to keep the number of zones to an absolute minimum as the more zones there are, the more the boiler will short cycle. Micro zones like that one should also be avoided.

    Panel rad's with TRVs migh be your best option, but again, you don't wanna try and mix them with cast iron rad's. One is low mass, high temp; the other is high mass, medium temp. Their operating characteristics and require SWTs are very different. What you're considering is one of the most common mistakes by people (even contractors) that don't thoroughly understand hydronics.

    Could it be done with one floor cast iron and another floor panel rad's? Yes, but it would require separate loops, additional mixing controls and pumps and separate water temp zones. That might be reasonable if you were building on an additional floor and already had cast iron rad's in the existing part, but building the system from scratch, you'd be better off using the same type of emitters throughout the house - unless you like spending $$ unnecessarily.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  9. #9
    Join Date
    Nov 2019
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    Thread Starter
    I've gone back and looked at the heat loss calculation for the house (done room by room using slant/fin app) and have some questions. These are my calculations for the 2012 sf house:

    Outdoor temp = 5 degrees (design day) the heat loss is 76k btu/h.
    outdoor temp = 30 degrees the heat loss is 47k btu/h
    outdoor temp = 55 degrees the head loss is 18k btu/h

    I also calculated the btu output of the radiators in the house and it comes to 118k btu/hr @ 180 degree water temperature. So that looks like 150 degree water temperature on a design day. 130 degree water temp when it's 30 degrees outside, and 100-110 degree water temp when it's 55 degrees outside. Is this math correct?

    Now, some questions:

    1. What's the lowest temperature that's reasonable for the SWH temperature? It seems like the radiators were sufficiently oversized to now take advantage of lower temperatures and a mod/con boiler.

    2. Should the entire house be on one zone, or is two okay? My calculations suggest each zone would be too small on its own to prevent short cycling above ~30 degrees outside.

    3. Is it reasonable to assume (for a single zoned system) that once it's over 55 degrees outside then the boiler will short cycle since its maximum turndown is <20k btu/h?

    4. Finally, how to deal with the sunroom which is on hot water boaseboard and looks to be undersized? It's the one on its own zone, and was with the old cast iron boiler as well. My design day heat loss calculation suggests the room needs 20k btu/h (it's a lot of windows over unheated crawlspace, vaulted ceiling, and not well sealed/insulated). The room only has ~22 feet of baseboard which is ~12k btu/hr @ 180 degree water temp. What's the best way to make sure this room matches up with the rest of the house for when the boiler is doing its thing. Convert to higher output baseboard and/or add more? Most of the room is baseboard as it is. I take the caution given in earlier posts about steel radiators on the same zone as CI rads, but wonder if increasing the radiator output of the room to ~20k btu/h with steel baseobard and add a panel radiator while also adding TRVs would work. The room is always 3-4 degrees colder than the rest of the house, so could actually use the quicker warm up time to its advantage.

    Thanks once again, this is a real education for me!
    Best,
    -Jon

  10. #10
    Join Date
    Oct 2009
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    God's country - Shenandoah Valley, VA
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    Go back and check your construction data on the load calc. You're at almost 38 btus per ft2. I've never seen even old leaky farm houses need more than 32 btus per ft2.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  11. #11
    Join Date
    Nov 2019
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    Thread Starter
    I will go back and run the numbers again. Temperature range was +5 degrees F to +70 degrees F. Since the interior walls are balloon framed and not insulated, they're open to the uninsulated fieldstone foundation basement and I included them as "cold partitions". Don't know if that was correct. The entire front of the house is brick and uninulated, just true 2x4 framing 16" on center. I also called an all glass sliding door as a large double glazed window. FWIW the ACH50 came out to 16, whch was double what the home performance specialist thought it should be.

    Any thoughts as to what else I could have done wrong? I measured every room and window dimension. Does the slant/fin app overestimate?

    edit: I removed someconservative assumptions and I'm only down to 70k btu/h @ +5 degrees F
    Last edited by jonny7117; 11-24-2019 at 02:41 PM. Reason: add information

  12. #12
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    Oct 2009
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    God's country - Shenandoah Valley, VA
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    Most mod/con manufactures start their offerings at 80k btus, so even with what looks like a somewhat inflated look calc, your below that.

    Cast iron rad's usually are sized to where 160* SWT is sufficient for design temps.

    As far as the sun room goes, I'd look at adding panel rad's or Smiths Heating Edge low temp base boards. If you use something like the HTP UFT080 boiler with a 10 to 1 turn down, short cycling won't be an issue if the smallest zone is 15k btus or higher. If not, a small 20 gal. buffer tank will solve the problem.

    Find out what the lowest necessary AVERAGE Water Temp is required for the CI rad's at design and size the sun room emitters from that. This way, you won't have to create two separate WATER TEMP zones.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  13. #13
    Join Date
    Sep 2002
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    27,236
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    Let's start by having a look at what you have now -

    How are the air outlets arranged? Are they in the corners of the rooms or somewhere else?

    When you say: " . . . on both floors, zoned separately" - do you mean two seperate Unico systems - or do you mean zoning by dampers in the main air ducts?

    Is there one outdoor cooling unit? Or is there two outdoor units?

    How many tons of cooling do you have?

    How many air outlets are there in total?

    How large (in BTU's) is the hydronic coil in the return duct(s) of the Unico stayed(s)? What make and model is it?


    PHM
    --------




    Quote Originally Posted by jonny7117 View Post
    This past summer bought a 1933 house in metro Boston with a forced hot water hydronic system complete with cast iron radiators and a 40+ year old peerless gas boiler. Against my better judgement, we proceeded to remove all the radiators, replace the boiler with a Bosch Greenstar 100k (95% efficient) with indirect DHW, and an Unico high velocity system for heating and cooling on both floors, zoned separately. So basically hydro-air heating. The condenser is a Bosch two-stage unit with a heat pump. I'll note that I just had a blower door test which came back at 16 ACH50 for the 2000 sf balloon framed house, and we're trying to bring that down with air sealing.

    Fast forward to the heating season and the system is pretty unbearable. The heat pump is set as primary heat down to 40 degrees F, and then the hydroair system takes over. The blower speed is turned down as low as it can go, and it still sounds like a windstorm outside everytime it turns on. The high velocity system just makes it feel drafty with the cool air temp from the heat pump, though the boiler is not much better. At just below 40 degrees F, the boiler heats supply water to cose to 160 degrees F, which I assume means it's never able to condense because I suspect the return water temperature isn't more than 20 degrees lower. The system does modulate temperature, but I've never observed it below 130-something, and I wonder if it's just cooling down then. The system also turns on multiple times per hour for 5-10 min at a time max, then stops, to try to maintain temperature. I don't know if this means the boiler is close to short cycling. When it was 25 degrees outside last week, the hydroair just never turned off for hours continuously, but at least the exiting air temperature was warm enough to not be uncomfortable.

    In a nutshell, all of my fears about the new system compared to the old have come true. The one thing the system does well is maintain even temperature everywhere, and I suspect that it will cool the house well and the aspiration/air movement will help with summer comfort.

    Am I crazy to want to put back the CI radiators and use the new boiler for what it should be good at doing? We gutted the low height cast iron piping in the basement for future head height, but all the CI piping to the second floor is still in place. In an ideal world, I'd hope the system could be repiped with oxygen barrier pex for home runs and zoning by floor. Maybe even switch out some of the CI rads for Runtal panel radiators and a bathroom towel rack. I'm not sure if these two different hydronic systems would be compatible, or if we'd just have to abandon the Unico heating coil and repipe the boiler for radiators. If none of this is possible, any advice to get the most out of the current system? I've set the indoor temp to 72 degrees with minimal setbacks to try to maintain overall comfort (audio and temperature).

    This is definitely a costly mistake, but I'm taking the long term view for comfort. Thanks for your thoughts, very much appreciated.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

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