Boiler advice

fezster

A boiler should be sized to the heat loss of the house. Just because you have more radiators or bigger ones, does not necessarily mean you need a bigger boiler.

The max output of the boiler is based on the dhw requirement (if you were getting a combi) or the total heat loss of the house when the temperature outside is -1 degrees (@southcoastrgi can confirm - it's been a while since I looked into this).

southcoastrgi

If you gave an average of 1500w per rad which will be over for some & under for others that's only 16.5 kw, you will of course lose heat if you go in the loft (really not something I recommend), during the winter your boiler will be switching on & off 24/7 just to stop itself freezing, also if you do go WB even more reason not to put it in the loft as it doesn't have a low water pressure switch 

330d

southcoastrgi said:

If you gave an average of 1500w per rad which will be over for some & under for others that's only 16.5 kw, you will of course lose heat if you go in the loft (really not something I recommend), during the winter your boiler will be switching on & off 24/7 just to stop itself freezing, also if you do go WB even more reason not to put it in the loft as it doesn't have a low water pressure switch 

Thank you for the info.

The other boiler I am looking at is the Baxi 800 platinum range. These have built in frost protection. 

southcoastrgi

All boilers have built in frost protection at around 7° but that is only a boiler protection, you should also have an external frost stat wired inline with a low limit pipe stat but even then the boiler will still be running, most lofts are well insulated & vented you might as well fit the boiler outside 

Jeepers_Creepers

Your current 15kW boiler would probably/possibly struggle to heat a 5-bedroom house as well as heating up the hot cylinder - it's usually good for 3 beds-ish.

As Coast says, tho', you won't need anything like 30kW to heat the extended house or heat up a cylinder. Think about it - will your house need 15 2kW electric fires?

Are you sure the '30kW' wasn't for a combi boiler? Normal boilers ('heat only') tend to be in the 15kw, 18kW, 24kW and rarely even higher, but the latter would be for large houses.  Combi boilers, on the other hand, seem to start at 28kW and then go up to 38kW or beyond.

Why? Because 'heat only' boilers do just that - they provide enough hot water to (a) heat your rads and (b) heat up your hot tank over a good half hour. Combis, on the other hand, use around 15kW to heat the rads when needed and then go WHHOOOOMPH! when you open a tap so it can heat your DHW instantly. A 38kW combi will usually only be chugging out 10-odd kW for your rads, and this will throttle back to 5kW to keep them hot. It'll will only EVER fire up close to 38kW to supply instant hot water.

I have a 3/4-bed house and a 30kW combi boiler. The 30kW is only ever called upon when a hot tap is opened - the boiler fires up with the burner fan spinning like a loon, a jet of plume issues from the flue pipe, and - aaaah - the tap chucks out a continuous flow of ouchly hot water. Then the tap is turned off and the boiler resumes heating my rads; the boiler calms right down, the flue issues a gentle waft of plume, and the output from the boiler is - I'm guessing - only around 5kW to keep the house warm. In fact a 30kW combi couldn't output 30kW to the rads even if it wanted to.

Another point of note is, a 'combi' does NOT supply hot water to the CH and DHW simultaneously. A combi fires up, when asked, to heat up your house. When you then open a tap/have a shower, the combi diverts its output to instantly heat your DHW, and no longer heats up your rads. Chilly? No. Because how long do you run a tap for - 1 minute? How long do you shower for - 5 to 10 minutes? Your house isn't going to cool down in that time. Once your tap is off, your rads go back on.

Combis are, in theory, a great solution for most houses. Their drawbacks, however, are (a) it can usually only supply one good hot outlet at a time - ie one shower at a time, and (b) they are more complex than standard 'heat only' boilers and are therefore more likely to go wrong - they have more moving parts inside, and finer channels.

For the (a) issue, you just have to accept that only one person can have a good shower at a time. In my house I have a 30kW combi, one shower cubicle and 2 further bathrooms with baths with shower attachments (hardly ever used). The household consists of me, wife, 2 teenage kids and (currently...) mum-in-law. It has simply not been a problem in practice that two folk 'need' a shower at the same time - everyone prefers the 'cubicle', so they just wait their turn. Usually, tho', we just happen to have different times that don't overlap - morns, afa's, eves, late eves, whatevs. If you find someone else is using the shower when you want one, you just go away for 5 minutes.

Yes, when someone runs a hot or cold tap during a shower, the flow rate is affected. But a decent thermostatic shower means you won't freeze or scald; you just get on with a reduced-flow shower for the whatever seconds it takes for that twit to turn the tap off again. 

For the (b) issue, you buy a respected brand with decent longevity. You also really must have your system cleaned out thoroughly, and make sure a good dose of inhibitor is included. I've had a few things go wrong with my GlowWorm HE combi - it's 16 years old :-)  - but the one repeated issue that got on my wick is a blocked P2P exchanger, and this was due to a slow build-up of sludge in its narrow channels from the old pipework it was connected to.

I'm very happy with my combi. Yes I know it's a compromise - only one shower, etc - but it's served us well and does what's asked of it most of the time. I'd still go combi if I had to replace it, but would go for a more powerful beast, say 38kW - it would be nice to have that reserve for when it's needed.

Your three options remain. For the third - the combi - if you went for a 38kW jobbie along with a mains booster accumulator in the garage, then I think you'd actually be 'happy' even running two showers at the same time if it were needed. Ie you wouldn't suffer anything like the reduction that I do, for example, and with effectively 19kW available for each shower's hot, each shower would still be a lot better than any electric shower for example. Worst case scenario, as far as I can see, is that two showers running would each still be twice as good as a leccy shower. With one shower running, the shower will be as good as you could possibly want - a very powerful deluge.  

A powerful combi in t'loft plus a Challis mains booster = ~£3k. Add labour, new gas pipe, bits.

J_B

I've not read the five pages of comments, sorry, but .....

we have a holiday rental apartment on the Welsh coast. It's a 2 bed, 2 bath place with a Worcester Greenstar 30i ERP combi boiler.

Nobody has complained that the showers weren't good enough, so, we guess that a 30kw boiler is good enough for two bathrooms.

Jeepers_Creepers

J_B said:

I've not read the five pages of comments, sorry, but .....

we have a holiday rental apartment on the Welsh coast. It's a 2 bed, 2 bath place with a Worcester Greenstar 30i ERP combi boiler.

Nobody has complained that the showers weren't good enough, so, we guess that a 30kw boiler is good enough for two bathrooms.

Good to have a real-life example.

The reality of a 30kW combi is, tho', that the only way it could run two showers simultaneously is if the supply to each shower was throttled back by half-closed isolating valves.That would mean, of course, that each shower on its own would not be able to give the lovely blast of water that a 30kW combi is capable of providing - it'll always be at half-throttle.

I expect, in your holiday home, it's a case of the two showers never having been run at the same time.

330d

Jeepers_Creepers said:

Your current 15kW boiler would probably/possibly struggle to heat a 5-bedroom house as well as heating up the hot cylinder - it's usually good for 3 beds-ish.

As Coast says, tho', you won't need anything like 30kW to heat the extended house or heat up a cylinder. Think about it - will your house need 15 2kW electric fires?

Are you sure the '30kW' wasn't for a combi boiler? Normal boilers ('heat only') tend to be in the 15kw, 18kW, 24kW and rarely even higher, but the latter would be for large houses.  Combi boilers, on the other hand, seem to start at 28kW and then go up to 38kW or beyond.

Why? Because 'heat only' boilers do just that - they provide enough hot water to (a) heat your rads and (b) heat up your hot tank over a good half hour. Combis, on the other hand, use around 15kW to heat the rads when needed and then go WHHOOOOMPH! when you open a tap so it can heat your DHW instantly. A 38kW combi will usually only be chugging out 10-odd kW for your rads, and this will throttle back to 5kW to keep them hot. It'll will only EVER fire up close to 38kW to supply instant hot water.

I have a 3/4-bed house and a 30kW combi boiler. The 30kW is only ever called upon when a hot tap is opened - the boiler fires up with the burner fan spinning like a loon, a jet of plume issues from the flue pipe, and - aaaah - the tap chucks out a continuous flow of ouchly hot water. Then the tap is turned off and the boiler resumes heating my rads; the boiler calms right down, the flue issues a gentle waft of plume, and the output from the boiler is - I'm guessing - only around 5kW to keep the house warm. In fact a 30kW combi couldn't output 30kW to the rads even if it wanted to.

Another point of note is, a 'combi' does NOT supply hot water to the CH and DHW simultaneously. A combi fires up, when asked, to heat up your house. When you then open a tap/have a shower, the combi diverts its output to instantly heat your DHW, and no longer heats up your rads. Chilly? No. Because how long do you run a tap for - 1 minute? How long do you shower for - 5 to 10 minutes? Your house isn't going to cool down in that time. Once your tap is off, your rads go back on.

Combis are, in theory, a great solution for most houses. Their drawbacks, however, are (a) it can usually only supply one good hot outlet at a time - ie one shower at a time, and (b) they are more complex than standard 'heat only' boilers and are therefore more likely to go wrong - they have more moving parts inside, and finer channels.

For the (a) issue, you just have to accept that only one person can have a good shower at a time. In my house I have a 30kW combi, one shower cubicle and 2 further bathrooms with baths with shower attachments (hardly ever used). The household consists of me, wife, 2 teenage kids and (currently...) mum-in-law. It has simply not been a problem in practice that two folk 'need' a shower at the same time - everyone prefers the 'cubicle', so they just wait their turn. Usually, tho', we just happen to have different times that don't overlap - morns, afa's, eves, late eves, whatevs. If you find someone else is using the shower when you want one, you just go away for 5 minutes.

Yes, when someone runs a hot or cold tap during a shower, the flow rate is affected. But a decent thermostatic shower means you won't freeze or scald; you just get on with a reduced-flow shower for the whatever seconds it takes for that twit to turn the tap off again. 

For the (b) issue, you buy a respected brand with decent longevity. You also really must have your system cleaned out thoroughly, and make sure a good dose of inhibitor is included. I've had a few things go wrong with my GlowWorm HE combi - it's 16 years old :-)  - but the one repeated issue that got on my wick is a blocked P2P exchanger, and this was due to a slow build-up of sludge in its narrow channels from the old pipework it was connected to.

I'm very happy with my combi. Yes I know it's a compromise - only one shower, etc - but it's served us well and does what's asked of it most of the time. I'd still go combi if I had to replace it, but would go for a more powerful beast, say 38kW - it would be nice to have that reserve for when it's needed.

Your three options remain. For the third - the combi - if you went for a 38kW jobbie along with a mains booster accumulator in the garage, then I think you'd actually be 'happy' even running two showers at the same time if it were needed. Ie you wouldn't suffer anything like the reduction that I do, for example, and with effectively 19kW available for each shower's hot, each shower would still be a lot better than any electric shower for example. Worst case scenario, as far as I can see, is that two showers running would each still be twice as good as a leccy shower. With one shower running, the shower will be as good as you could possibly want - a very powerful deluge.  

A powerful combi in t'loft plus a Challis mains booster = ~£3k. Add labour, new gas pipe, bits.

Do you have a link to the mains booster accumulator? There are quite a few different ones out there.

J_B said:

I've not read the five pages of comments, sorry, but .....

we have a holiday rental apartment on the Welsh coast. It's a 2 bed, 2 bath place with a Worcester Greenstar 30i ERP combi boiler.

Nobody has complained that the showers weren't good enough, so, we guess that a 30kw boiler is good enough for two bathrooms.

Thats interesting and also conflicting in terms of my experience. My auntie has a Vailant 35kw combi and there shower struggles when a tap  comes on. Same has also been said my other family members. 

But then again they dont have a mains booster.

bris

I have seen a lot of houses with a combi running is a similar layout. The fact is it's very rare for two people to be using both showers at once and so never a problem, so if this was the case here I wouldn't change it..

However since you already have an unvented system it's also pointless to change that. The roughly 4k cost to do so would take a very long time to recover changing to a combi.

So basically what I would say is both set ups are fine and the cost to change it just wouldn't be worth it.

330d

bris said:

I have seen a lot of houses with a combi running is a similar layout. The fact is it's very rare for two people to be using both showers at once and so never a problem, like this here then I wouldn/t change it.

However since you already have an unvented system it's also pointless to change that. The roughly 4k cost to do so would take a very long time to recover changing to a combi.

As mentioned previously, due to the extension the boiler and cylinder HAVE to be moved. So I am expecting a large-ish cost for this.

To me it makes sense to upgrade boiler at the same time.