Oil boiler monitoring (Heat meter & oil meter)


I’m planning out a monitoring project for our oil boiler (A Firebird Combipac C35 - Yes that’s 35kW … for some reason) and I have some questions around heat meters and the emonHP. I know that this topic has been raised in previous threads, but I’m interested in proper measurement rather than assuming the output based on boiler run-time.

Our boiler is a combi but it has been converted to a system boiler as we have an unvented cylinder with solar thermal heating. For some reason the original owners of this house had both a combi and a cylinder with some crazy combi mixing valve, but we removed that and it’s now using a standard S-Plan configuration. Sadly the boiler is turned up to max as the radiator sizing within this house is all over the place, if we have a lower flow temperature then some rooms simply don’t heat up - No point in upgrading radiators though as we’ll be going for underfloor throughout once some building work has been completed.

Anyway my first question is around heat meter sizing. There are two Kamstrup Multical 403 heat meters on the site (Need one with a metal body as it’s a high temperature loop and would likely be located near to the boiler) but there are two ratings available: QP2.5 m3/hr (<11kW) and QP3.5 m3/hr (<17kW)

From what I can see the main difference is the size of the flow meter, the larger one has 1" BSP connections and the smaller has 3/4" BSP connections. They appear to be rated based on their pressure drop and how that relates to heat pump sizing, rather than there being a hard limit of ‘X’ kW - Is this correct?

The boiler says:
If we assume that 1kg of water is 1 litre, then that means that the boiler requires a maximum flow rate of 2.874 cubic meters per hour which is well within the ratings of the larger heat meter. Does this mean that it’s suitable for the boiler? Is there any disadvantage with going for the larger heat meter, if we then want to re-use it in the future for a much smaller heat pump or modulating Sapphire boiler? Also how well would it work with the standard 22mm pipework on the boiler as there will be a step, does it need ‘X’ distance of straight 28mm pipework before any changes in size?

For the oil flow meter I was looking at the Aqua Metro CONTOIL VZO 4. It has a pulse output and goes down to 0.4lph - I’d prefer less in case we decide to go for a Sapphire boiler in the future (Which goes down to 6kW) but that was the lowest I could find with a pulse output. Suggestions very much welcome for oil flow meters with a lower minimum flow rate and a pulse output.

I was going to ask about interfacing such a pulse output with the emonHP, but then I realised that the
Kamstrup Multical 403 heat meter has an optional interface board which includes two pulse inputs - So that could be the interface between the oil flow meter and the monitor via M-Bus. Can emonHub support those extra pulse inputs and be used to generate a boiler efficiency graph?

Can the emonHP track multiple heat meters over the same M-Bus? I’d also be interested in including a heat meter for the solar thermal system.

I’d probably use the current sensors to then monitor the signals to the two S-Plan zone valves, so that I can track how efficiency and oil usage relates to heating vs DHW.

I’m not interested in monitoring general electrical usage as I have an IotaWatt for that.

Any suggestions for my project would be appreciated.

No harm using the larger meter for the heat pump later or for a smaller boiler. You’re not trying to measure very low flow rates that may be lower than the meters lower limits.

22 mm and 28 mm to 1" female BSP are standard fittings fig Screwfix set al. Easy to go from those to a 1" to 1 1/4" meter union. The 1" meter is not actually a 1" meter at 3.5 M3/HR. It’s got 1 1/4" connections. The matching meter unions that your connect to your pipework are 1" male threads though.

No official need for straight pipe lengths before or after for these small meters. Avoid putting next to pumps or partially closed valves though.

Thank you very much that’s fantastic. I was worried about the step in pipe size creating a lot of turbulence right at the flow meter but if they already have an adapter out of the factory it can’t be a problem. Not requiring any specific straight lengths will make installation a lot easier, I was wondering where’d I’d actually put it!

Very interesting. I have an oil system too which I am planning to monitor, but not in as much detail. (Non-modulating Grant eco vortex 12-16kw set at 14kW in a small detached)

I am just doing flow and return temps and a burner on/off signal so I can attempt to estimate oil usage (plus I have a rectangular tank and a linear sight gauge so can measure tank level accurately). So far I have just tweaked it to condense properly with a flow at just under 65deg, return just over 50deg and all the rads balanced on temperature.

The sizing of your boiler is far more reasonable than ours! I might have to ask Firebird about the smallest nozzle that we can fit as the manual suggests it can only go down to 26kW but they have another version with the same burner and blast tube which is “only” 20kW.

I’d be very interested to see how accurate the on-off measurement is given that you have a more appropriate tank shape for this kind of measurement. A float-based fuel sender from a vehicle might work quite well in your tank, the linear tube kind.

It would be interesting to see how your efficiency has changed with it condensing if you have data from before your adjustment. I’ve always wondered about this for an oil boiler as so many oil boilers are not condensing. (Ours certainly isn’t!) Oil boiler manufacturers certainly haven’t prioritised it given the steel construction of them and the “40C minimum return temperature” which they almost all state.

I’m curious to know what you’re looking to find out from your monitoring? I’m looking to find out some information about the house (Accurate heat loss, accurate oil usage) along with answering some questions I’ve had about oil boilers in general. (How much does condensing / flow temperature affect efficiency, does outdoor temperature affect efficiency, how efficient is an oil boiler because everyone tells me it’s very low (~70%) but the manufacturers claim much higher, is cycling an issue, should I use the now-unused 40l combi tank within the boiler as a buffer along side a PWM pump and try to switch to pure weather compensation and with steady state heating to reduce cycling?) I guess for me it’s a play thing, I just want to learn and answer some nagging questions!

I don’t have any “before” data unfortunately, we have only lived in the house since May. I had to renovate the place before we moved in, which involved replacing most of the radiators and pipework (it’s a 70s house and had old downstairs c/h pipes in the concrete floor. I replaced them all with drops from upstairs.)

The monitoring I am doing is to confirm my heat loss and rad size calcs and get a feel for how the fabric performs. Plus get the system running as efficiently and cheaply as possible.

Plan was to get lots of data to inform an ASHP install if the existing boiler terminally failed, but it is only 3 years old so that isn’t likely in the timespan I plan to live in the house.

Ah I see, shame there’s no baseline data but at least you have a very modern setup which should be consistent and reliable for years to come. I think the tank level monitoring should be as good as anything for your usage.

The issue with confirming heat loss from oil usage is that you need to make an assumption on the boiler efficiency, which is hard to know. If you can graph the oil level / usage along with the average outdoor temperature each day, you could possibly work that out based on how the usage changes against outdoor temperature - A sort of reverse weather compensation calculation on the basis that the heat loss is directly proportional to the difference between the indoor and outdoor temperature.

That of course still doesn’t count the variable of DHW, but I think it would give you a nice ballpark.