Underfloor Heating and Oil Boilers

[DaveWave]

We had underfloor heating fitted upstairs and downstairs late last year. It’s working fine as far as I can tell. However I’m not sure of best practice for UFH.

The installer told us that underfloor heating should be left on all day long at a low temperature and it would be fine because UFH only “sips” power to heat up.

We have no gas so we had a new oil boiler fitted at the same time as the UFH. However, as oil boilers generally don’t modulate like a gas boiler, surely the boiler is either on or off and whether it’s heating one room or five rooms, it’s burning the same amount of oil. Is my understanding wrong? It feels like oil is possibly a bad match for UFH?

Maybe wrongly but I’ve set the system up to switch on and off throughout the day (at different temperatures depending on the room) allowing warm up time.

I worry that it’s burning up a lot of oil. The set back temperature is set to between 16 and 18 degrees so certain rooms that fall below that are forcing the boiler to fire up during the night and again burn fuel. I can of course adjust the setback temperature to a lower temp but that might be defeating the object.

I hope this makes sense to someone.

 

[stem]

UFH is slower to heat up than radiators so generally needs to be on longer for it to warm the property. However, UFH doesn't require the water to be anywhere near as hot as for radiators, so doesn't require as much fuel and also with UFH the return water to the boiler is cooler which improves the efficiency of condensing boilers. [New oil boilers also condense] And the longer the heating is 'on' once the temperature is reached it will tick over at a low temperature. I suspect this is what the installer means by 'Sipping' fuel.

I'm more au fait with gas, but if oil boilers don't modulate, they will still need to regulate their heat output otherwise if they ran continually at full pelt the water they produced would eventually boil. I imagine your boiler has a thermostat on it to control this and save fuel.

Another benefit of UFH is that most users report that they feel comfortable with the room at a lower temperature than radiators as heat rises evenly from the floor up, which also saves energy, whereas often the floor level below radiators can be cold and the heat they produce somewhat localised.

 

[DaveWave]

Thanks for the reply. I think most of this fits in with my understanding of the underfloor heating and as I said, I'm happy with how the UFH works/feels currently. I'm just not sure were using it in the best way with the oil boiler. Its a condensing boiler with a separate hot water tank.

I'm more au fait with gas, but if oil boilers don't modulate, they will still need to regulate their heat output otherwise if they ran continually at full pelt the water they produced would eventually boil. I imagine your boiler has a thermostat on it to control this and save fuel.

I guess the above is what I need a better understanding of. So my current understanding is that whereas gas boilers modulate and can use less or more gas for a gentler or more fierce heat, oil boilers just burn oil at one rate. So whether I want to heat a room to 18 degrees or 30 degrees, they'll still suck the same amount of oil until that temperature is reached.

Again apologies for my basic understanding. Im trying to get my head round the way things are working.

 

[Exedon]

Underfloor will work fine with your boiler the lower return temperatures will help it run in condensing mode .
Grant recomend return temperatures off 55c or less.
Room backset temperatures should be a maximum of 4 C less than desired temperature.
Yes virtually all oil boilers don't modulate down but if demand lowers they fire less often.

 

[DaveWave]

Yes virtually all oil boilers don't modulate down but if demand lowers they fire less often.

Thanks for the reply.

Okay, forgive me as I just don't really get oil boilers at all...

So when demand is lower, the boiler can continue to heat the UFH water without firing up and burning oil?

 

[Exedon]

Oil boilers have significantly larger heat exchangers and hold much more water than equivalent gas boiler.
When boiler reaches its set temperature and heating demand is low it can take several minutes to cool enough to fire again bring itself up to temperature and shut down again.

 

[DaveWave]

When boiler reaches its set temperature and heating demand is low it can take several minutes to cool enough to fire again bring itself up to temperature and shut down again.

Ah okay so we're just talking about potentially a few minutes of use without firing?

I'm still trying to get my head around the economics of the oil. Like I said, with a gas boiler I get that it modulates to use a little or a lot of gas depending on the demand but I still don't see how an oil boiler can be as economical.

Let me try another example to explain what I'm getting at.

If I need to keep one room at 18 degrees for example and the temperature of the room drops to 17. My understanding is that a gas boiler will "sip" at the gas to gently heat but an oil boiler would just be chugging oil in that same situation.

Am I right? Or do I have it totally wrong?

 

[Johntheo5]

A oil boiler fires flat out or not at all but the average output will still match the heating demand, it just cycles on/off, for example if you have a 20kw boiler supplying a 10kw heating load then the boiler average on time will be 50% to use the same amount of fuel as a modulating gas boiler running continuously at 10kw output, or pretty close to it as cycling apparently doesn't have a huge effect on boiler efficiency. You might keep a eye on the boiler return temperature, if too low, this can cause corrosion and a short boiler life, I think Grant and the like specify something like 45C minimum. UFH return temperatures are often as low as 30/35C so some form of by pass is required, I would advise installing a strap on thermometer on the boiler return to monitor it, then, even a simple manual by pass valve can be set to give the required return temperature.

 

[DaveWave]

A oil boiler fires flat out or not at all but the average output will still match the heating demand, it just cycles on/off, for example if you have a 20kw boiler supplying a 10kw heating load then the boiler average on time will be 50% to use the same amount of fuel as a modulating gas boiler running continuously at 10kw output, or pretty close to it as cycling apparently doesn't have a huge effect on boiler efficiency.

Thanks very much for that. Really helpful response and helping me to get my head around the oil boiler. Thanks!