oil boiler - to combi or not to combi?

[Snowbeetle]

Hi,

 

I have posted before about bungalow barn conversion which we are dual-handed doing ourselves bit by bit. (started March 2018... )

 

Approaching first fixes stage and decisions need to be made. I've bene home schooling the kids since March which has eaten into my research time and I feel in a spin with it. 

We have UFH throughout, no rads. Building is very well insulated. We will have one bathroom with shower and bath and one other toilet. 

We initially thought combi boiler but now are wondering if water tank would be better as I have heard short cycling of boiler from UFH demands aren't great with combi boilers and that oil boilers don't keep up with long showers so well (we were used to gas before, which isn't an option now). 

 

I don't know how to get to the bottom of the choices because I don't know what questions to ask as much as anything. 

 

We are getting our mains pressure tested at the end of the week but suspect  it is low... don't know of that is relevant. 

Can anyone shed any light on why you would choose one or the other or where to go for a good summary of the relevant factors? 

TIA

[Temp]

In addition to a pressure test it's also useful to do a flow rate test. 

[ProDave]

If the house really is well insulated have you considered an ASHP and unvented hot water cylinder?  Probably similar cost to an oil boiler and tank, no need to accommodate the oil tank, and not subject to the very volatile prices of oil.

 

The Grant combi oil boilers seem okay. They are not really true combi boilers like with gas, because they keep a small store of hot water, I think to counter the slower response of an oil burner firing up.

[Snowbeetle]

5 minutes ago, Temp said:

In addition to a pressure test it's also useful to do a flow rate test. 

Thanks. Good point. 

[Snowbeetle]

3 minutes ago, ProDave said:

If the house really is well insulated have you considered an ASHP and unvented hot water cylinder?  Probably similar cost to an oil boiler and tank, no need to accommodate the oil tank, and not subject to the very volatile prices of oil.

 

The Grant combi oil boilers seem okay. They are not really true combi boilers like with gas, because they keep a small store of hot water, I think to counter the slower response of an oil burner firing up.

We did look into ASHP pump early in the build but a satisfactory location was tricky and we were swamped assimilating all the information we needed to for learning to build a house for the first time. In the end we opted for various reasons for oil and now, because service ducts etc have already been dug and positioned. pipes through walls etc etc I think that ship has maybe sailed. 

[Snowbeetle]

From what I can gather most (if not all) oil combi boilers have a small tank to act as a buffer, as hot water is demanded, to compensate for the lower burn rate compared to gas, which means it gradually (but slowly) means you get colder water as the buffer is depleted. 

That might not be a show stopper if that is adequate for a family of 4 but don't know if it is an actual good idea compered to the alternative, and this is my dilemma. 

I was leaning away from cylinders initially but as someone said the old view of them as drains on your energy are less accurate now that they are so well insulated these days... but are they more efficient. ?

I just don't know how to weigh it all up and think I am lacking the right jargon to dig for the answer I'm looking for. ?‍♂️

[JFDIY]

The other point that have conventional tanks a bad reputation was poor hot water pressure/flow especially on a shower. With a mains pressure (unvented) cylinder that issue goes away. 

 

Personally I have a cylinder and wouldn't have it any other way. You also have the back up emersion heater if the primary heat source fails.

[PeterW]

1 hour ago, Snowbeetle said:

From what I can gather most (if not all) oil combi boilers have a small tank to act as a buffer, as hot water is demanded, to compensate for the lower burn rate compared to gas, which means it gradually (but slowly) means you get colder water as the buffer is depleted. 

 

That isn't correct. Oil boilers hold this buffer as it takes time for the burner to come up to full temperature and they do not respond as quickly as gas. The buffer allows small amounts of hot water to be drawn off without firing the boiler and short cycling which causes issues with the burners. Once up to full power, an oil boiler will keep the hot water flow at full temperature as long as their is oil in the tank.