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to size, measure radiators or measure heat loss?

The "trouble" will be the <I>possibility</I> of boiler or flue damage. The cost however is measured in wasted fuel. The boiler may last indefinitely, but excepting the most extreme climates you'll be lucky to get a seasonal average of much more than 55% of your fuel as usuable heat to the structure!

The radiators in a hot water system have ZERO care regarding the temperature at which they operate. Regardless of your control system, they will not get much hotter than needed to meet the current heat loss. And guess what? They are unlikely to EVER reach 180° average temperature.

I once ran my 2x sized cast iron boiler for about 10 hours straight in cool (not cold) weather in an attempt to hit a measured <I>supply</I> temp of 180°. It maxed at about 165° and the house was an absolute sauna.

With a hot water system, size the boiler to the heat loss! Take whatever measures seem prudent to protect the boiler from low return temperatures. A simple, fixed bypass will often suffice.

Not only are the radiators and the boiler typically oversized, but the circulator is often extremely oversized as well. This is particularly true with gravity conversions. In my area of the country at least, nearly all hot water systems with standing iron were originally gravity systems.

The "typical" gravity conversion will have a circulator able to move around 30 gallons per minute. Say your design loss is 80 mbh. 30 gpm @ 80,000 btu = 5 1/3° delta-t <I>at the highest load</I>. At a more typical loss of say 45 mbh, delta-t is down to 2 2/3°! These systems were designed for MUCH lower flow and MUCH higher delta-t! Add the fact that you're now driving this high-volume system with a comparatively tiny volume boiler and inefficiencies pile on top of inefficiencies!

My own gravity conversion system (1,049 sq.ft. EDR) now operates with less than 4 gpm--VERY similar to the original design. How? TRVs and a Vitodens using ONLY the small, built-in circulator.


  • Chuckles_4
    Chuckles_4 Member Posts: 43

    I learned from old posts here that steam boilers should be sized using the radiators, but hot water boilers should be sized using heat loss.

    But when thes two sizes are very different, e.g. when an old house has been insulated and so has lots of excess radiation, is it really OK to size a cast-iron hot water boiler using the heat loss?

    The problem is that if you have lots of radiation and a smaller boiler, the water will never get to 180F. For example, if the BTU output of the radiation with 150F water equals the BTU output of the boiler, the water will heat to 150F but no further. If the return is always less than 140F, the heat exchanger will corrode.

    Am I confused about this?
    STEVEN MARKS Member Posts: 154
    boiler size

    Always size hot water boilers by radiation. When replacing a hot water boiler with excess radiation (large cast iron) at a minumum install a by-pass to prevent low water return temps. Installing a primary/secondary pumping system would be best.

  • Constantin
    Constantin Member Posts: 3,796
    The boiler doesn't have to heat to 180°F...

    ... because the excess in radiation emitters will ensure that the emitter never has to come to that temperature. You'd be roasting in your own juices while the snow howls outside...

    For that matter, the heat loss vs. the thermal capacity of the boiler determines the temperatures the supply water can reach in cold conditions, not just the radiator surface. Radiator surface area only comes to play on the steam side of the business as far as boiler sizing is concerned.

    On a hot water system your only concern re: boiler sizing is actual heat loss... The bigger the emitters are in a hot water system, the lower the supply temperatures can be to keep a room comfortable. In our home, the supply temperatures are planned to range from 90°F to 110°F because we benefit from a good insulation package and radiant floor heat (i.e. a very big emitting surface).

    Return water temps of 140°F or less do not have to lead to corrosion if:
    • If you pipe a regular CI boiler primary-secondary, the inside of the boiler (where it counts) will never see the low temperatures that can lead to flue gas condensation. Pri-Sec piping in conjunction with Pump Logic, Variable-Speed Injection, or even 4-way valves are well-accepted means of achieving this.
    • You can buy a Vitola, G215, or a condensing boiler and install them to manufacturers specs. Then, you won't have to worry about return temperatures at all. Condensing boilers LOOOVE cold return temperatures and the two other CI boilers I mentioned aren't harmed by them either. These choices require more vigilance re: flue gas condensation in the flue, however.
  • Chuckles_4
    Chuckles_4 Member Posts: 43

    Ah, ok, primary-secondary.

    But I was thinking about boiler changeout in a typical old house hydronic system (oversized cast iron boiler, lots of radiators). Someone who got a smaller boiler as a drop-in replacement, without any piping changes, might be asking for trouble.
  • Follow

    the instructions in the manual and you should be OK.
  • J.C.A._3
    J.C.A._3 Member Posts: 2,981
    With CI Rads.....

    The more I look, the more I like 4-way mixers with outdoor reset connected to the radiation supply. Boiler limits can be adjusted to be above condensing in either the flue or the boiler, and you get the desired COMFORT from the radiation. What more could you ask for?

    After my recent journey to ISH, I'm seeing things in a different light. In March, everyplace we went into was comfortable and never was the temperature of the emitters anywhere near 180°. Throw in TRV's to control temps. room by room and you've got yourself one heck of a heating system.

    For steam, you've got to be able to get all the radiation full so the only way to do it is using the total EDR. Hot water can be a bit more forgiving but a building heatloss would be my choice above just measuring radiation. Chris
  • R. Kalia_6
    R. Kalia_6 Member Posts: 28

    I certainly agree about the circulators being oversized. Our house is a gravity conversion and has a series 100 circulator, which was apparently standard issue in those days but moves an awful lot of water when there is no head loss (and there isn't in gravity systems).

    Regarding actual water temperature never reaching 180, you make a good point, but this is related to your point about circulators. Since we have excess circulation, it protects the boiler: if our supply is like yours and never exceeds 165, the return is still over 160. Even if the stat turns it off at 145, the return is over 140.
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688

    if you change to the right size circulator, the delta-t increase won't just happen on the return side. Sure the return might be a bit cooler, but the supply will also be hotter. And the flow thru the boiler will be closer to the assumptions the boiler's designers made.

    Follow this link to a pair of charts and an article which will help you select the right circ for your system. If you have any doubts after reading it, ask me how I know it works ;-)

  • Chuckles_4
    Chuckles_4 Member Posts: 43

    I have a question about that table in your article.

    Our house is a gravity conversion and now has a Grundfos variable-speed pump. According to your table, I should be using the maximum speed of this pump. I ran it at minimum speed all last winter and got enough heat. We have a Munchkin so a cold return temperature is not an issue.

    So, am I damaging anything by using flow rates maybe half of those in your table? Gravity systems used even lower flow rates originally, right?

    I do understand that reducing the flow rate could change the balance between upstairs and downstairs, but the radiators upstairs are controlled by TRVs. I believe that at slower flow rates the upstairs should get more heat, due to natural convection (gravity) becoming more important, but the TRVs take care of that. So we are not seeing a problem in daily use...

    Many thanks for the help,
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    If you can get away with it

    there's no problem. It didn't work too well on my system or any of the others I tried it on, but that doesn't mean it wouldn't work elsewhere.

    But is this a primary-secondary setup, or does the supply from the Munchkin go straight into the system? If the former, is the Grundfos circulating the primary loop or the secondary (system, feeding radiators) loop?
  • Chuckles_4
    Chuckles_4 Member Posts: 43

    Primary/secondary, and I was talking about the pump on the secondary (radiator) loop.

    I'm not playing with the primary pump speed since the Munchkin needs a specified amount of flow.
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688

    Well, if it works well, no need to change it.
  • jerry scharf_3
    jerry scharf_3 Member Posts: 419
    with a condensing boiler

    The cooler the return temperature to the boiler, the better.

    The immediate thought is that slower pump on the load side would always be better, but you have to be careful. In a P/S piping setup, it is quite possible to have the water in the closely spaced Tees going backwards if the flow of the boiler side is higher than the load side. If this happens, the boiler return will see some of it's output. The best situation is the slowest pump speed that assures the tees never flow backwards. This will produce the widest delta T and the lowest return water temp to the boiler. With TRVs, the load side flow is variable, so you can't compute a single optimum speed.

    If you want to check this, you could get an electronic dual temp sensor that can compute differential and record min and max differential. Then you put one on the load return, one on the boiler return and watch for a negative (or positive depending on the sensor order) differential. Anything greater than a few degrees would indicate feedback of boiler supply water and a need to speed up the load circulator.

  • Chuckles_4
    Chuckles_4 Member Posts: 43

    The boiler's delta-T is determined by the primary (boiler) loop pump speed. You ar right that if the secondary is running slower than the primary, some water will just short-circuit between the tees and go back to the boiler. You are suggesting that this is a problem...why?

    If the secondary is drawing BTUs out of the primary loop, it shouldn't matter whether it is doing so with high flow rate and low deltaT or low flow rate and high deltaT. Energy is not wasted either way and the primary loop deltaT is not changed. Of course there is no advantage to slow flow rates either, except for the electicity saved by running the secondary pump slower. Am I missing something?
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