Oil combo / immersion tank / gas - Help!!

[Snowbeetle]

Hi there, 

I am doing my first self-build project and am learning as I go. I am going round in circles online with a plumbing dilemma and think it might be because as much as anything I don't know the right questions to ask. ? 

We had early in the project decided that oil fired combination boiler would be the most efficient way to heat our UFH and provide hot water. Now we are reading further, and getting to the point where we will need to do plumbing first fix, we aren't so sure. We could still change our mind at this point so want to see if we are shooting half-cocked. 

Factors we are working with:
Bathroom will have ceiling with storage space above it so we could put an immersion tank up there. 

We have laid plastic coated copper piping specced for oil supply and reading up on oil piping it seems we could (?) swap that for propane if that was better. 
Build is a single storey barn conversion with water UFH pipes in the floor slab.

We intended to use oil because there is no mains gas here and it's cheaper than electricity (for now). 

We had a combi boiler in previous house on mains gas and it was brilliant for heating and hot water. We are now worried that the oil equivalent won't cope so well with demands and won't provide enough hot water at speeds required for showers etc. We have been told recently that oil combo boiler won't keep up with a series of showers and that an immersion tank would be better for hot water which would mean we would only need UFH heating for boiler in which case an propane gas one would be more efficient. 

Heating water using electricity is more pence per KwH, but if oil can't keep up maybe I should look at it. 

I am not at all informed enough on this to be able to sift through the mountains of (sometimes seemingly contradictory) information on all this and would be really grateful to get some collective wisdom and pointers so I can start making final decisions. 

Any advice greatly appreciated. 

 

 

[ProDave]

I have never been a fan of combi boilers though plenty are happy with them.

 

What people are suggesting to you is a "system boiler" that heats the heating and heats a hot water tank.  The fact people are confusing you calling it an "immersion tank"  While it does have an immersion heater, it is primarily heated by the boiler, so won't be any more expensive.

 

An Unvented cylinder would be my choice so you get mains pressure hot water and the tank should be able to run more than one shower at a time (which I doubt a combi boiler would manage)

[Snowbeetle]

5 minutes ago, ProDave said:

I have never been a fan of combi boilers though plenty are happy with them.

 

What people are suggesting to you is a "system boiler" that heats the heating and heats a hot water tank.  The fact people are confusing you calling it an "immersion tank"  While it does have an immersion heater, it is primarily heated by the boiler, so won't be any more expensive.

 

An Unvented cylinder would be my choice so you get mains pressure hot water and the tank should be able to run more than one shower at a time (which I doubt a combi boiler would manage)

Thanks ProDave, 


Is it all combi boilers you think are not good or just oil ones?

Thanks for clearing up the confusion over immersion / system boiler - that makes more sense now. 

I liked our old mains gas combi boiler because it kept up with hot water demands well and you never got the tank-water-gone-cold experience of which I have treasured memories from my youth. Does the system you lean towards avoid that problem somehow? Size of tank maybe? 

[Temp]

We went with a thermal store which stores energy for both the DHW and UFH. That's because I was concerned our oil boiler would short cycle on and off too much without one. The burner in an oil boiler always runs flat out when lit. It doesn't modulate up and down like a gas boiler. So if you have a powerful burner to provide loads of DHW in a combi it might cycle on and off too much if/when the heating load is very small. Short cycling is s bit like stop start driving in a car, it's less efficient than doing fewer longer burns. 

 

That said you can lose a lot of heat from a thermal store and it's associated pipes and pumps.

 

Who knows what the right approach is. We're happy as our TS gives loads of mains pressure DHW and the boiler doesn't cycle when just one loop is calling for heat. However our bills are still higher than I expected.

 

 

[Temp]

PS: If you wish you had mains gas for cooking consider a dual fuel stove with an LPG hob running on cylinders. Cylinder gas is too expensive for heating but fine for a hob. 

Edited February 26, 2020 by Nickfromwales
Typos

[Snowbeetle]

7 hours ago, Temp said:

We went with a thermal store which stores energy for both the DHW and UFH. That's because I was concerned our oil boiler would short cycle on and off too much without one. The burner in an oil boiler always runs flat out when lit. It doesn't modulate up and down like a gas boiler. So if you have a powerful burner to provide loads of DHW in a combi it might cycle on and off too much if/when the heating load is very small. Short cycling is s bit like stop start driving in a car, it's less efficient than doing fewer longer burns. 

 

That said you can lose a lot of heat from a thermal store and it's associated pipes and pumps.

 

Who knows what the right approach is. We're happy as our TS gives loads of mains pressure DHW and the boiler doesn't cycle when just one loop is calling for heat. However our bills are still higher than I expected.

 

 

Thanks Temp, that's good food for thought, always useful to hear the rationale behind someone else choices - real world example to help sort the wheat from the chaff and decide what applies to us. Ta. 

[mm289]

We had an oil combi in our farmhouse I fitted about 15 years ago. Supplying a 200sqm building with traditional radiators and 2 bathrooms. It was fine for showers and wasn't to bad for baths, you just got used to running the hot in first incase it struggled.

 

When we did our major renovation the last few years we swapped this out with a trad oil boiler feeding an unvented HW tank as @ProDave describes. This gave me a couple of advantages. As we had added a 3rd bathroom I wanted lots of HW capacity and so we had a HUGE unvented tank fitted, probably overkill but what the plumber recommended. Also having a tank meant we could add an immersion heater which is great for the times when i forget to order the oil in time and we then have no HW?

 

The system is working really well for us and with oil at the lowest price it has been for over 10 years it is actually quite cheap heating.

 

cheers,

 

MM

[bassanclan]

I know this is slightly off topic, but have you looked into log gasification boilers?

There was a very decent looking quarterly payment for using them. Ideal if you have the space and no access to mains gas.

[Snowbeetle]

1 hour ago, bassanclan said:

I know this is slightly off topic, but have you looked into log gasification boilers?

There was a very decent looking quarterly payment for using them. Ideal if you have the space and no access to mains gas.

No, I haven't looked into that - food for thought... I'll have a rummage around and see what that's all about. Ta. 

 

[Snowbeetle]

12 hours ago, mm289 said:

We had an oil combi in our farmhouse I fitted about 15 years ago. Supplying a 200sqm building with traditional radiators and 2 bathrooms. It was fine for showers and wasn't to bad for baths, you just got used to running the hot in first incase it struggled.

 

When we did our major renovation the last few years we swapped this out with a trad oil boiler feeding an unvented HW tank as @ProDave describes. This gave me a couple of advantages. As we had added a 3rd bathroom I wanted lots of HW capacity and so we had a HUGE unvented tank fitted, probably overkill but what the plumber recommended. Also having a tank meant we could add an immersion heater which is great for the times when i forget to order the oil in time and we then have no HW?

 

The system is working really well for us and with oil at the lowest price it has been for over 10 years it is actually quite cheap heating.

 

cheers,

 

MM

That's brilliant to hear. I think the first thing that I take away from it is that perhaps whatever I choose will do the job broadly speaking so the dilemma isn't majorly risky in terms of making a bad choice. The size of your first example is very close to the size of mine - we're 152sq m. So, that's good to hear, I think I'm mostly concerned about the oil boiler short cycling on and off when warming the UFH so not being very efficient, and not keeping up with shower demands. Maybe the showers aren't so much of a concern. ?

Your second choice is interesting, the idea that immersion is a useful back up is definitely an advantage, and the picture helps to visualise. We will only be one bathroom and a second toilet so probably wouldn't need to Go Large on any similar system which might make it easier.