Underfloor Heating and Oil Boiler

[Sandman]

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 (until recently) 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 hope this makes sense to someone.

[Originaltwist]

I suggest you sit by the boiler with a stop watch to see if it's short cycling. My neighbour's oil boiler goes 'whoooof' about every 3 minutes - crazy.

[Temp]

We have oil boiler and ufh.  I decided to fit a thermal store which acts as a buffer between the two.

 

The room stats control the flow of water from the store to the UFH and floor loops

 

The stat on the store controls the flow from the boiler to the store.

 

The two sides, in and out of the store are independent except for the master On/off switch.

 

When the store needs topping up the oil boiler runs flat out until the store is satisfied. No cycling. To get it to do this I had to turn the pump up to maximum and the boiler flow temperature dial up to maximum. This way the actual flow temperature never hits value set on the boiler temperature dial so it never cycles. 

 

The big question is it worth it? The store is big and has lots of pumps and mixers and leakes quite a bit of heat into the store room it's it. 

 

It's also not ideal should I want to switch to a heat pump. If I did that I'd probably use the store only for DHW and connect the UFH direct to the heat pump. This would allow the heat pump to run at lower temperatures.

 

It's a 300L store which means we always have that much hot water available for high flow rate mains pressure showers. They are fantastic.

 

Do what @Originaltwist suggests an see if you can detect any cycling. If it's short cycling a lot then perhaps looks at a buffer tank.

 

If it never stops cycling even on very cold days that suggests the boiler is too big. One option might be to fit smaller jets if available for your model. That can also improve efficiency a few % on some makes.

 

As for the "leave it on all day"...

 

I think UFH is best suited to households that have one person at home most days. Otherwise the system can spend ages heating the house only for you both to go out just as it gets warm.

 

If you are both out working I would set the stats to "set back" to say 16C about an hour _before_ you leave for work. Most likely the house will still be warm when you leave due to thermal inertia. See what happens. It might even be possible to do this even earlier if the house is well insulated. Then have the temperature set to say 20C about an hour before you get home. If it's not actually reached 20 when you get home set it to 20 a bit earlier.

If someone is home in the daytime then 18 might be better than 16. Depends how active they are.

 

We have ours set back to 18 or 16 C at night depending on the room.

 

[Sandman]

Thanks for the advice. I've not noticed lots of cycling but like you say, I'd need to sit next to it and monitor.

 

 

[John Carroll]

Cycling doesn't have a huge effect on efficiency, also you don't see too many complaining about the constant cycling with systems like Evohome which will still cycle a  boiler even when the system demand is greater than the minimum (gas) boiler output.

 

[ProDave]

While the burner of an oil boiler is only on or off, it is controlled by a thermostat that is setting the temperature of the water leaving the boiler.  So if all UFH zones are open, the flow temperature will cool down quickly once the burner stops so it won't be long before the burner starts again.  But if just one zone is open, the flow temperature declines slowly so it will be much longer before the burner comes on for it's next burts.

 

Hence power delivered and oil usage is directly proportional to the heat delivered.

 

In our old house we used to turn the heating on 2 hours before we normally got up so it was warm by the time we were up, and in the evening it turned off at least an hour before we went to bed.  That worked well for us.