Open Vent Boiler sizing...should I oversize?

[vinny048]

Currently got a Glowworm spacesaver 50 mk2 in a 3 bed semi, couldn't see the data badge but on a gas rate it came out at 12.7kW.

Looking at replacing and keeping tanks/cylinders, was looking at a Vaillant 15kw to replace the above but i'm not too sure whether to go for the 18 kw instead.

I have only fitted combi's in the past and we have always gone for a higher kW output for HW demand but on an open vent i'm clueless what to go for.

Was looking at heat loss calculators/boiler sizing and the boiler size they suggest, a 15kW would be enough but my concern is as the back of house is a north facing wall and having an open field behind us the back of the house is alot colder especially in winter.

As these are modulating boilers would it make much difference in having a 18kW instead of a 15kW?

Thanks
Vinny

 

[petit_pablo]

Buy the 18kw and it can be down rated if needed. Bit more flexibility then for future.

What difference will it make whether its open vented?

 

[mickyg]

does the existing boiler heat up the radiators sufficiently?
are you changing any radiators?

 

[vinny048]

I would say it is slow in the winter to heat up, all rads are convector panels and are all under 5 years old, been changing a few at a time.

Current boiler is only 12kW and 25 years old so i'm thinking a new boiler with a higher rating should be more than enough.

 

[Agile]

I assume this is your own house!

Most 3 bed semis work out at about 10-12 kW. Add 2 kW for HW and that makes 12-14kW so a 15 kW boiler would be fine.

Efficiency falls by fitting a larger boiler than necessary. Although 15 up to 18 is not that bad,

I have seen 32 kW OV boilers fitted in 3 bed semis and that really causes a problem because the minimum power is still more than the house needs and it gives cycling problems. On for one minute, off for three!

My advice is to use the 15 kW! That should be more than enough!

The existing boiler is rated at 15 kW output but is obviously running at rather less if the INPUT power is only 12.7 kW which if 70% efficient would only be 8.9 kW so no wonder it may be a little slow to heat up.

If the hot water coil is not balanced to the heating that will cause less heat into the CH while the waters heating.

If qualified you should check the burner pressure and see if thats low. If not then perhaps clean the jets to see if thats responsible for the lower than rated output.

Tony

 

[simond]

The smaller Vaillant 6 series all modulate down to a similar minimum Kw rate so if you want the larger boiler I can't see any disadvantage, other than;

-it may cost a little more than the ideal sized unit
-your gas pipe may be undersized for the larger unit

 

[gas4you]

Whatever you do don't go for a Vaillant 400 series. As Simon says, look at the 618

 

[vinny048]

I assume this is your own house!

Most 3 bed semis work out at about 10-12 kW. Add 2 kW for HW and that makes 12-14kW so a 15 kW boiler would be fine.

Efficiency falls by fitting a larger boiler than necessary. Although 15 up to 18 is not that bad,

I have seen 32 kW OV boilers fitted in 3 bed semis and that really causes a problem because the minimum power is still more than the house needs and it gives cycling problems. On for one minute, off for three!

My advice is to use the 15 kW! That should be more than enough!

The existing boiler is rated at 15 kW output but is obviously running at rather less if the INPUT power is only 12.7 kW which if 70% efficient would only be 8.9 kW so no wonder it may be a little slow to heat up.

If the hot water coil is not balanced to the heating that will cause less heat into the CH while the waters heating.

If qualified you should check the burner pressure and see if thats low. If not then perhaps clean the jets to see if thats responsible for the lower than rated output.

Tony

Cheers for the in depth response Tony, i did clean the jets recently, I have been working towards my ACS (assessment W/c 11may) so I am competant to carry out the work, all the boilers I have installed have been combi's.

Back to my own....I believe that the heat exchanger is heavily sludged up and this is probably one of the main reasons for lack of heat, fitted magnaclean yesterday and added sludge remover, also connected up immersion for h/w.

Vaillant 415 ordered this morning, should be delivered tuesday.

Hopefully I'll get it fitted this week

 

[David937]

Vaillant 415 ordered this morning, should be delivered tuesday.

Oh dear.

Whatever you do don't go for a Vaillant 400 series. As Simon says, look at the 618

 

[mogget]

Vaillant 415 ordered this morning, should be delivered tuesday.

Oh dear.

My Ultracom hxi keeps me warm enough, should I be worried?

 

[Agile]

Whatever you do don't go for a Vaillant 400 series. As Simon says, look at the 618

Or rather more suited for the heating load, a 615 !

Whilst I have to take some information from some of the people on here it seems the 615 is more closely based on the combi 800 series whereas there seem to be some further differences on the 400 series. The open vented boilers probably only count for a very few percent of combi sales volumes.

Nevertheless, you should be fine with your 415 as long as the system is clean and the flow rate is adequate.

Tony

 

[simond]

The 400 series is a Glowworm in a Vaillant jumper.