Click here to Find a Contractor in your area.
In fairness to all, we don't discuss pricing on the Wall. Thanks for your cooperation.
Need to contact us? Visit

Issues with coal fired hydronic heating system


I have had a coal fired hydronic heating installed which I believe is performing below optimum. The installer believes that it is going as well as it can, but I am not convinced and based on my reading, I believe that the distribution system is incorrectly designed and plumbed. I am on my own to fix this so I am keen to learn more about hydronic heating. I have summarised the issues in an attachment and included some schematics and annotated photos of the system.




  • SWEISWEI Member Posts: 7,356
    edited May 2013
    At a glance

    I'm assuming the boiler is probably oversized for the load, but even if it isn't, it needs a buffer tank.  The three-way valve should be motorized and controlled by bot outdoor reset and minimum boiler return water temp.  A Taco iSeries-R will do the trick.  Pump 4 should only run when

    Your pressure tank needs to be moved to the supply side of the circulator, preferably both pumping away.

    Oh, and most codes prohibit PVC pipe use on domestic water inside the building envelope.

    I'm sure there's more, but that's a start...
  • SantilalSantilal Member Posts: 42
    Three way valve


    Thanks for the help.

    The boiler is probably oversized at times of low load, but according to the installer the boiler is undersized - I don't believe that!

    I omitted to state that the three way valve at the boiler is a three port mixing valve and it is currently set to hold the outlet temperature at 60degrees C.

    Thanks again for your feedback. All help is greatly appreciated.

  • SWEISWEI Member Posts: 7,356
    Time for some math

    What is your design day heat loss?  What is the boiler rating? will vary the water temperature in your loops based on the outdoor air temperature.  As it warms up outside, the water temperature will be reduced proportionally.  It will also allow a minimum boiler return temp setting of 135F or 120F.
  • SantilalSantilal Member Posts: 42
    Heat Loss


    Thanks again for your input. I am the home owner. My system was designed and installed by a plumber. When I say designed, I think he actually just scaled up or copied a system that he had installed elsewhere. I am not aware of any load calculations he made. So I am afraid I can't provide any design information. I know the boiler is rated at 30kw.

    If you look at the bolier loop, you will see that all the return flow is forced through the three port thermostatic mixing valve. It can't go anywhere else. I think this is why my boiler inlet temperature often sits at 40-50 degrees C, and not 60 degrees. According to my reading the distribution system should be connected to the boiler loop via closely spaced tees. This gives an alternative path for the return water if the return temperature and flow were such that it would drop the boiler input temperature too low.

    Incidentally, do you know why PVC pipes are not permitted in domestic water systems. I think it is allowed here in NZ.
  • SWEISWEI Member Posts: 7,356
    edited May 2013
    Heat loss calculation

    Is not all that complex, but I don't happen to have a link to a metric app.  We have a few Canadians here who might have a handy link,  Henry?

    Do you know the winter design temp for your area?  How large is your house?  When was it built?  Any changes to windows, insulation, etc. since then?  We can do a quick sanity check using those.

    Closely spaced tees are not necessarily required for your system, but the mixing valve is if you want to use lower temperature water than the boiler limit setting at any point during the year.  Controlling the secondary (loop) water temps based on seasonal demand will dramatically improve both comfort and efficiency.  A buffer tank on the primary loop is almost certainly needed.  This does not have to be expensive -- an electric tank style water heater will do the job in most cases.

    I'm not familiar with NZ codes, pardon my assumption.
  • SantilalSantilal Member Posts: 42
    Heat Loss

    Hi again

    The house is 338 square metres. All windows double glazed. All walls, ceiling and concrete slabs insulated. Exterior cladding is brick. House about 2.5 years old. Average outside winter temperature about 3 degrees C. Minimum is -10.

    I might have confused you about the three port valve. It is not there because I need lower temperature water, it is there to provide boiler protection. i.e. to prevent the boiler input from falling below 60 degrees C and hence avoid condensation. I have attached a hand sketch of how it is at the moment (top sketch) and how I think it should be (bottom sketch).

    Cheers Santilal
  • ZmanZman Member Posts: 3,478

    Hydronic System Issues

    The following is a list of issues that I have found.

    • The electricity consumptions appears to be excessive. Pumps P3 and P4 operate all the

    time. The fan operates when the boiler thermostat calls for heating. The auger runs 3mins

    on, 2mins off while the fan is on. Pumps P1 and P2 run when M1 and M2 are on. These are

    time-clock controlled. (This is not a major concern at present as it needs further


    The boiler and pumps being always on is killing your efficiency. A large buffer tank and  controls will solve this. You could use it as a DHW reverse indirect and put it where your proposing closely spaced tees

    • Should the pressure vessel be located on the HP side of the pump (to prevent cavitation).

    It will work better on the hot side with the circ "pumping away"

    • Should the connection to the distribution system be via closely spaced Tees. (The present

    arrangement forces all of the returning water into the boiler via the three port valve

    meaning that the boiler inlet temperature will not reach minimum temperature to prevent

    condensation at times of high load – i.e. no boiler protection.

    Agreed, the piping is suspect

    • How closely spaced do the tees need to be. Would a pipe from the return side to the hot

    side suffice (as shown on the schematic).

    If you are going to do it over, do it right. The tees should be in a straight pipe not on ells

    • I have not used the in-slab heating for some time. I can’t remember why I have switched it

    off but I think it was because the return water to the boiler was too low. The mixer for the

    in-slab heating was set to a water temperature of 30°C, and the return water was at about

    20°C. If correct mixing was occurring the return water to the boiler should have been much

    higher than that, because the return water from other devices would have been higher.

    Fixing the boiler protection piping should resolve this. Consider outdoor reset for the floors.

    • Check valve CV2 was added because hot was circulating from the hot port to the cold port of

    the three port valve when motorized valve M2 was closed. Initially, I could not explain why

    this was happening – my theory now is that both hot and cold ports would open to try to

    maintain the required temperature at the outlet port. But because there was no flow,

    (because M2 was closed) the hot water was bypassing to the return line via the cold port. Is

    this possible?

    I think P3 is deadheading and forcing the zone valve open.

    • If all the thermostatic valves on the radiators close, pump P1 will be deadheaded. Pumps P3

    and P4 could also be deadheaded (if no other circuits were calling for heat). Is this damaging

    for the pumps. How can this be avoided.

    Replace P3 with a ECM circulator I think a delta P like grundfos alpha would work well

    • If the radiator circuits are on, there appears to be no flow to the water heater. I suspect that

    this is because there is no pump in this circuit and the radiator circuit hogs all of the

    available flow. The way I have deduced this is a bit convoluted. I have a thermometer on

    the inlet and the outlet to the water heater heat exchanger. When the water in the hot

    water tank is cold, I noticed that the heat exchanger inlet water temperature was at the

    same temperature as the water in the hot water tank (around 20°C) . The inlet temperature

    should be something like 70°C. This could only occur if there was no or low flow in the heat

    exchanger circuit – the cold water in the tank was cooling down the near stationary water at

    the inlet. When the house and day warmed up and the radiators cycled off, I saw the inlet

    temperature rise back up to 70°C. From this I deduced that the radiator circuit was hogging

    the flow. This causes the water heating recovery to be delayed in the mornings, and we run

    out of hot water. This is a major issue for us.

    On second thought, why does P3 exist? If each of the branches off the main had it's own circ, and check valve there would be no need for P3

    • Are there too many pumps?

    • The pumps appear to effectively be in series – is this good? My understanding is that you

    use series pumps to increase the head, but there is no indication that increased head is

    required here.

    • Am I correct in my belief that the piping arrangement at the boiler will not provide boiler

    protection. (see attached drawing). It is not piped like the drawings in your book (John


    • The return water temperature is often around 40°C when the outlet temperature is 70°C.

    This suggests that correct mixing is not occurring or the flow is too low. With the piping

    arrangement at the boiler, this temperature does not rise much before entering the boiler.

    • I am constantly adding make-up water. Perhaps a litre per day. Is that common – how often

    Is it coming from the prv?

    should you have to add make-up water? There is no evidence of leaks.

    • I also have a feeling that the precharge in the pressure vessel is insufficient or leaking. What

    should the precharge be?

    The precharge should be the same as the system pressure. It needs to be sized correctly. The manufacture has a calculator.

    • The boiler is lower than the floor level of the house. Much of the piping is in the ceiling

    space of the house. Should there be a air-purging valve in the ceiling at the high point?

    It depends, do you have an air problem? If the pipe velocities are sized correctly you shoild not need it

    • The maximum flow in the in-slab circuits is about 2l/min or less when all in-slab systems are

    What size and how long are the loops.

    calling for heat. At these flows the heating in most rooms is barely perceptible. Maybe if

    the distribution system was improved I could get the flow rate up a bit?

    These are some of the issues that I am having. Some of them might be inter-related. I have taken

    these to my installer and he believes that all of the problems stem from the fact that the boiler is not

    achieving its rated output of 30kW. However, I often see the boiler switched off on its thermostat,

    even when the water heater or the radiators are calling for heat. This suggests to me that it is not a

    heat issue but a distribution issue. That is, the boiler is producing the heat, but the distribution

    system is not circulating the heat effectively. Unfortunately my installer believes the system is

    performing as optimally as possible so is not prepared to do much more. He has offered to install an

    additional boiler at a good price, but I am reluctant to do that until I am convinced that the

    distribution system is working properly. So I am faced with the problem of fixing it myself. That is

    why I am trying to learn as much as I can about hydronic heating.

    I feel that the system should be able to cope with at least the water heating and the radiators going

    together or with the water heating and the in-slab heating operating together. I am prepared to

    accept that there might not be enough capacity in the system for all three at once. I suspect the

    installer has not carried out any design calculations, but merely scaled a system that he might have

    got working for another client. Our house is a modern one with double glazing and thermal wall and

    ceiling insulation. There is also 50mm thick polystyrene under the concrete slab. It is hard to believe

    that 30kW is insufficient, even 20kW should be sufficient based on what other homes of a similar

    construction and size are using.

    The way to figure this out is to do a heat loss calc on the building and compare it to the output rating on the boiler. 30KW is a sizable boiler. It would likely heat a house twice your size in your climate. It is absolutely not undersized.

    Thank you.

    Santilal Parbhu

    Personally, I would use a reverse indirect as a buffer, hydrolic separator and DHW source. I would  give each branch it's own circ and reset controls. Problems solved.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • SantilalSantilal Member Posts: 42

    Thank you so much for this. Your response has been very comprehensive. I will try to digest it over the weekend.

    The pressure relief valve might be operating and causing loss of pressure. It's hard to know unless you are there at the time.

    Your suggestion of having a pump on each circuit is a good one. Will it still work if the pumps are in the ceiling and the boiler about 4.5m lower?

    Thanks so much for putting your time into this.


  • SantilalSantilal Member Posts: 42

    Thank you so much for this. Your response has been very comprehensive. I will try to digest it over the weekend.

    The pressure relief valve might be operating and causing loss of pressure. It's hard to know unless you are there at the time.

    Your suggestion of having a pump on each circuit is a good one. Will it still work if the pumps are in the ceiling and the boiler about 4.5m lower?

    Thanks so much for putting your time into this.


  • ZmanZman Member Posts: 3,478
    It will work.

    The location of the circulators is unimportant. Seigenthaler has written some great stuff on both buffer tanks and indirects. He also talks about the effect of boiler cycling on overall system efficiency. Your coal boiler an additional problem. Once the coal is in the fire you are committed to it even if the load is not there. This is what the buffer does it stores the energy until you can use it. Without the buffer it will overheat the boiler and /or burn inefficiently.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • SWEISWEI Member Posts: 7,356
    Three way valve

    when motorized and properly controlled can provide both boiler protection and outdoor reset control.

    Agree with Carl here - your design load could easily be half of 30 kW.

    Can you hire a competent hydronic installer?
  • RobGRobG Member Posts: 1,850
    Piping Diagram

    If someone has a good generic piping diagram his type of system and could post it for this gentleman I'm sure that would answer allot of his questions. As well, Santilal, if you could post more pictures of your system, helping you will be allot easier.

  • SantilalSantilal Member Posts: 42


    Thanks to everyone for your very hepful comments. I will spent the next few weeks re-designing the system and then try to get my plumber to re-plumb it even though he won;t agree with it.

    Zman suggested a reverse indirect. I don't know what this is. I believe an indirect is using a heat exchanger to heat hot water indirectly (directly being when the boiler heats the water, I presume). But what is a reverse indirect.

    Zman also suggested that the loss of system pressure could be from a PRV. I have just witnessed a PRV in the boiler venting, so I am confident this has not been set properly. I will find out from the installer what this should be set to.

    Finally does anyone know of a flow meter I could install. I want to calculate the heat output from my boiler by monitoring the flow and input and output temperatures on a real time basis. Ideally the flow meter would have a 4-20mA output so I can feed it directly into a PLC. I suspect though, that such a beast is going to be too expensive. Any suggestions?


  • ZmanZman Member Posts: 3,478
    Keep us posted

    The reverse indirect is a way to use an indirect tank backwards.The domestic water runs through the coil and the boiler water is in the tank. It is not necessary for your system to work properly as it looks like you already have an indirect.

    I would absolutely add a buffer tank and a way to hydrolicly separate the boiler loop from the system side. The attached file might help you visualize.(Thanks RobG). You would have to add the boiler protection piping on the boiler  and the mixing assembly on the zones. The indirect would be a heat exchanger coil inside the buffer tank.

    I do not know of an inexpensive flow meter. Normally you could just measure the delta T going past a point of known flow. The way your circs are set up that looks difficult. Another way to do it would be to measure the fuel going in,also difficult in your case.

    As far as convincing you contractor, don't bother, trying to use math and science with someone that already "knows" the boiler is undersized is not worthwhile.


    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!