new boiler for UF heating ?

[nicka99]

we are having a single storey extension built in place of an old conservatory. It used to have 2 big double radiators but all the heat was lost through the roof so was freezing in winter. There was no door between kitchen and conny so that was always freezing too so we decided to bite the bullet and get rid of it ! The extension will have underfloor heating about 22m sq m plus about 10 going in the current kitchen that will be turned into utility room.

we have gravity fed/vented central heating and hot water tank. The boiler was replaced last year with a 'Worcester CDI 30 regular' when we moved in because the old one was knackered. We didnt know at that time (was summer) that we would end up getting rid of the conservtory so didnt oversize the boiler with any extra capacity in mind.

The plumber (part of the building firm) suggests that the existing boiler is not powerful enough for the extra heating needed. he says we need 120000 BTU based on number radiators + UF heating and size of HW tank but the worcester is 105000. he also suggested to take out the old gravity fed setup and go to sealed system heating and megaflo as its better in the long run for the size of house (4 bed detached, 2 adults + 2 kids).

Obviously Im a bit reluctant to take out a 6 month old boiler for one with a bit more oomph as its a big waste of money if not needed. Im also concerned we dont have sufficient pressure & flow rate for the megaflo - garden sprinker is rubbish in the summer heatwave. Plumber said could be solved by an accumulator. Currently the HW tank (open vent, 180 litre) is boosted by pumps for the showers & bath at that seems fine (but pumps cheap, old & noisy so can get new ones).

other posts suggest the requirements of a boiler is more determined by its ability to heat hot water than it is by the central heating - if so, couldnt we just keep our 30 kw and the same HW tank and assume its also powerful enough for the extra central heating ? can UF heating be run by a gravity fed/open setup or does it need sealed system ? maybe we just install the UF heating and see how it goes through the winter and if the heating and or HW struggles then we upgrade the boiler ? can the central heating be sealed system but the HW remain gravity fed/open ?

any thoughts please as I dont want to spend money unnecessarily (who does!)

thanks,
Nick

 

[Agile]

I think you are being wrongly advised by someone short of work!

You need to work out the whole house heat loss! is your gas bill really over 3000 pa?

It might be worth paying a proper professional to come and assess the situation.

Tony

 

[DP]

To free up some capacity, you could run the HW zone at night when heating is not required.
You could also split the rooms into ‘sleeping’ and ‘daytime’ zones and now also U/F heating. Boiler would function admirably.

Though not clear, but if the HW cylinder is gravity zone, it should have been converted to fully pumped and cylinder be factory insulated and correctly zoned with motorised valve and cylinderstat.

Slab heating does not have to be from sealed system alone, but I could be wrong. As far as I was aware, only constraint is the need to temper the water that flows through the UF coil.

 

[bengasman]

Very unlikely that a 30 kW steamer is not big enough.
Simple to work out though. Add up the total length of existing rads; every metre is 1 kW, double rads 2 kW per metre. Add only the extra capacity needed for ufh section, you can ignore the demand for the cylinder as you can run that at hours where there is no large heating demand.
As tony said, it looks like someone is trying to create more work for himself.

 

[Agile]

The only boiler capacity that has to be added for hot water heating is just 2kW per household.

You probably have a cowboy who pretends that you add the rads and the maximum water reheat power the cylinder can absorb. Quite possibly he does not know any better?

Regardless of the reason I would not want him working on my property.

Tony

 

[bengasman]

The only boiler capacity that has to be added for hot water heating is just 2kW per household.

Why count 2 kW if you have an independent timer set to heat the water between 3 and 4 a.m. Tony?

 

[Agile]

because thats what the instructions say. You cannot assume in the future that anyone will time it seperately.

In any case 2 kW is so minimal that its of little relevance. But it can cover those nupties who think you always oversize boilers.

All that is needed is to NOT add the max absorption of the cylinder to the heating requirement which the real nupties do.

Tony

 

[nicka99]

I dont think they are out to rip us off. for other aspects of the build (so far) they have been very forgiving when coming across problems not to add any extra costs where others almost certainly would (based on past experience).

they have quoted £2500 total for a vaillant ecotec plus 637 and santon premier plus 210 supplied & installed (gas safe certified). seems very reasonable since we were quoted more than this a while ago for a megaflo install alone.

I recall last winter that when the hot water was on at same time as heating (4-5pm before kids bath) then the heating was not very warm until the HW switched off. In the morning we set the HW to come on before the heating.

House has new loft insulation but cavity walls are not insulated. had 2 people out about that and both have shyed away from doing it due to extensions that have been added over the years making it complicated to avoid cold spots. It takes a very long time to heat up (circa 2200 sq ft) despite the new Worcester boiler. Based on size of existing rads, total is 25kw (without hot water) so need to add on HW, UF heating for 32 ish sq metres plus 2 new rads going in where the old kitchen was which has no heating currently. To me it seems borderline !

 

[ajrobb]

I recall last winter that when the hot water was on at same time as heating (4-5pm before kids bath) then the heating was not very warm until the HW switched off. In the morning we set the HW to come on before the heating.

In cold weather you can switch HW to constant. Does the boiler run constantly in cold weather (boiler too small) or does it cycle while house is heating (rads too small)?

I have an extra large, TRV-controlled radiator in the lounge. This heats the room quickly (30 minutes) and then turns itself down. The rest of the house heats up slowly until the hall thermostat turns off the heating.

Given that you currently have heat loss from the kitchen through the open conservatory, an extension could reduce your heating rather than increase it.

Underfloor heating usually has a manifold and extra pump to feed cooler water.

 

[D_Hailsham]

Obviously I'm a bit reluctant to take out a 6 month old boiler for one with a bit more oomph as its a big waste of money if not needed.

2,200 sq ft is a very large house, so you may need 25kW rads. However, the existing radiators are not a reliable indication of your heating requirements, Do as Agile suggested and do a whole house calculation using the Boiler Size Calculator. Do it for the house and then add in the underfloor heating. You don't need to add in the new kitchen rads as they will be included in the house calculation (unless the kitchen is another extension!).

we dont have sufficient pressure & flow rate for the megaflo - garden sprinker is rubbish in the summer heatwave. Plumber said could be solved by an accumulator.

Spending your money again.

other posts suggest the requirements of a boiler is more determined by its ability to heat hot water than it is by the central heating

That's only true if you have a combi (instant hot water) boiler, not if you have a hot water cylinder.

can the central heating be sealed system but the HW remain gravity fed/open?

Yes

 

[bengasman]

because thats what the instructions say.


In any case 2 kW is so minimal that its of little relevance.

That's more or less my point.
A. It has no practical basis.
B. It is pointless