Gas Combi vs Condenser Boiler and BTU for Rads

A

avoca

Registered User
Messages
28
We're renovating our house and looking at heating systems. It's 2,500 sq ft with 3 bathrooms and trying to choose between a combi boiler and a high efficiency condenser boiler. We had a combi boiler in our last home but it was small with only one bathroom - we found it great, having constant hot water and no need for a hot water cylinder. However I'm finding it hard to get a solid answer from plumber or plumbing merchants about whether it would suit our new home. Apparently you can also get a condensing combi boiler. Our first fix is coming up and also looking at radiator options - Does anyone have rads that are sized for a flow temperature of 50 degress so that the boiler will work in condensing mode and apparently save money? Does it mean that you need to go for larger rads? Finally, I assume it is better to overspec on radiator sizes because you can always turn them down as opposed to them not giving enough heat output... Any advice is welcome, Thanks
 
Hi,

I think you have be mis informed on how a condensing boiler operates. The boiler will condense at any temperture. This could be a long post so bear with me..

Standard boilers would burn gas and the by product of the gas would be pumped out the flue at a very hot temperture.

Condensing boilers burn gas and instead of pumping the very hot byproducts out the flue (chimney) they recycle it around the boiler until all the latent heat is sucked out of it. the flue gas is between 50 and 60 degrees, dew pt is about 60 so hence all the condenstation. see here for more info [broken link removed]

Condensing boilers use more of the energy produced by the burning of gas/oil.
 
DavyJones,

Thanks for that... We had a think over the weekend and feel we would be interested in going with a Combi boiler again. However, I'm finding it hard to get information on whether it is suitable. We will have 2,300sq ft, 3 bathrooms (probably 2 showers max at a time) and 16 radiators. Does anyone have a combi in a larger home and how well does it work?
 
You could go for a condensing combi, however I think three bathrooms is a bit much to run off the combi.
You will need very good mains water pressure and there will be a huge water demand if even two of the bathrooms are in use as all your water hot and cold is coming from the mains supply.
There are bigger type of combi's that have a storage unit built in which may be more suitable for your needs.
Your plumber should advice, if you have one. If you were my client I would probably advice against a combi because of the likely demand problems that may occur.
 
Thanks again for that.... Was on to Dublin City Council and a water pressure test (2007) has shown that we have pressure in excess of 46psi and a flow rate of 36 litres per minute. We would have a max of 2 showers at any given time. We really liked the combi in our old house and if I could find out if it would work would be very keen to go with one again.
 
Check this out ttp://www.inspiredheating.co.uk/acatalog/VAILLANT_ECOTEC_PLUS_POWER_STORE_CONDENSING_COMBI_BOILERS.html

Just a random boiler company, they will all do something similar. With a bit of research you should get something suitable for your needs.
 
Davy,
Just reading the thread, have recently done the Vaillant boiler service/installation course (amoung others) and have to say, it's a bit more expensive but seems to be a great boiler, very well built compared to some. There combi with the large (200tlr) water store would be good for a house fo that size (ie 3 bathrooms) but you should also consider installing solar to reduce your demand on gas to heat your water (reduces demand by up to 65% in Ireland) and not bother with the combi.

Also, as regards installing a condenser boiler, since March 08 it is a building requirement that your boiler is at least Band B, so limits to either a HE or a condenser, and would be a bit of a no brainer to go band B (HE) and not spend the extra €200 or so for a condenser.
 
Thanks for your reply... we did look at the solar option but were advised that as our roof is facing west it would take a good bit longer to make the investment back. We've decided instead to insulate well and get an efficient heating system. Why would you not bother with the combi? I like that you heat water as you need it... and so aren't heating water you don't use and are never without. I have spoken to several heating suppliers and am getting mixed views on the suitablity - some say that there could be problems with flow i.e. If having two showers at once that the pressure could decrease... I would really appreciate any advice on this. The valliant store one looks like a good option.
 
For prices on Vaillant, look a not sure if they will sell to non-trade, but will give you a good indication of what your plumber will pay for it (don't tell him I told you though!)

I meant, wouldn't bother with the combi of I could install solar, would just use conventional boiler to boost solar heated water when temp is low.

As regards looking for advice, vaillant have only just re-entered the irish market and have a new sales force on the road, giving the product a big push, bet if you rang Vaillant (not sure the number) bet you could get one of the sales team onsite to advise on best route, and as they sell solar and all the rest too could be worth your time.
 
Did you have a chance to read the link?
Your supply is nearly 3.2 bar which is really high so a boiler like the one mentioned would do it. The problem with everything being mains fed is that any outlet that is being used will affect pressure I.E washing machine, kitchen sink etc.
if you fit thermostatic showers you should be fine. The Combi has many advantages as you already know, but here are some you may not know. no storage tank in attic, no hot press, and you only heat the water you need, so all in all much cheaper to install than normal system and cheaper to run.

With regard to the change in the regs, its the same old story of no enforcement and stanard boilers are still very handy to come across. I have had some landlords refusing to pay more for HE boilers.
 
Thanks for your reply. The Vaillant link you sent said it's suitable for high demand which I hope means that we could have 2 showers on the go and not have a decrease in pressure. I will look into the thermostatic shower - I know in our last house we had to get a shower to work with a combi boiler and it was great.
Re the rads - found a publication on the SEI website that says that the rads do need to be sized afor a a condenser boiler I've had mixed reports from suppliers and waiting on radiator company to come back with BTU Calculation and rad measurements [broken link removed]
Does anyone have any info on this?
 
A rough guide for calculating rad sizes is W x L x H x 5 for each room. use feet and you will get BTU's. As for low temperture rads there are aluminium ones that are bigger and very costly. The advantage of low temperture is running costs. Condensing boilers will also condense with standard normal sized rads. I don't think we have the climate for low temperature installations. We can get a cold snap in our "summer" and low temperature systems need time to come up to heat.

thermostatic shower is where you can set the temperature and if pressure drops that set temperature is maintained. A must if you have children.
 
why in the world would the calculation for rad be any different for a condenser boiler? the only difference is the 'return' water is 'pre-heated' by the latent heat left in the flue gases as compared to a standard boiler,
this is what gives a higher efficency (ie, standrd boiler flue gases are 200+ deg, and condenser is 55ish deg when condensing) other than that the plumbing is identical!!!! once radiators are sized and piping calculated it relates back to the kw (or btu) output of the boiler.......anything else is just bulls**t
 
opps, forgot that the design temp differential should be set at 20deg for condensers rather that 11 deg for conventional boilers........least someone pick me up on that!
 

I completely agree (see post 2) sometimes I feel like I missed something at boiler school when I hear people on about it. To be fair to Avoca, it is written in black and white in the SEI website.
 
O.k.... thanks for your help but am getting really confused.... Can I take it therefore that the rads are to be sized for the room i.e. calculate BTU and get company to size rads accordingly. I take it from what you've said that they will condense at any temperature. I was under the impression that they are more energy efficient when set at a lower temperature. What's with the information on the SEI site?
DGOBS - what do you mean by the design temperature differential of 20 degrees for a condenser?
Thanks again for all your help.
 
Yes the flow return design temp for a condenser should be 20 deg (ie..if flow is 80, return should ideally be 60, give a better pickup for the latent heat allowing condensing to take place)

You are getting confused, your radiator temp if 20 or 50 or 80 deg doesnt really matter, its when your room gets to its set temp, only then will the boiler shut down (via your room stats)

Condensing is a term given to the 'boiler only' nothing whatsoever to do with your rads, and the 20 deg diff. is set durning the balancing of your heating system (make sure your plumber does this, most dont bother)

And just so your really confused, make sure you house is properly zoned for heat (and hot water if u do end up with a storage cylinder) ie, you can seperatley contol heat and hot water in zones, one example, during the day, why heat your bedrooms? they should be on a seperate timer channel to your living area and hot water
 
Davy, think if we met for a few pints we'd put the world to rights in one night!
 
DGOBS said:
Davy, think if we met for a few pints we'd put the world to rights in one night!
Click to expand...

Dunno, but I'm sure we'd give it a shot or two or three

With regard to UFH, I'd like to do more research on it. I always belived heating the vast amount of water was costly, However I can see that once it is heated it takes less energy to keep it warm likewise with aluminum, As opposed to heating radiators from cold maybe twice a day or more. However I don't think we have the climate to justify low temperature systems.

Acova, you need to have a competent installer met you onsite and disuss things. With a bit more information you'll know exactly what you want.
 
You must log in or register to reply here.
Menu
Log in
Register