Advice on whole house heating -combi plus UFH

[stavros.london]

Hi all,

I've been using this forum for many years now and found it particularly helpful. I'm now facing an interesting challenge and I'm getting a bit of conflicting advice so thought to tap into the diynot community more directly this time.

I currently have a regular boiler with a vented hot water cylinder and a cold water tank in the loft, and radiators serving the whole property. As I'm about to fully renovate the house, I would like to move to a combi boiler as I prefer the on demand capabilities, as well as underfloor heating throughout (no radiators left anywhere).

I have a two storey, ~200m2, 4 bedroom, 2 shower + 1 bath property, but only occupied regularly by two people. So I'm sure a decent combi boiler would suffice. However, for the days when we have visitors, and as a backup just in case, I would also like to keep the vented cylinder in place. However, I'd like to control when it would heat the water.

My overal desired setup would look something like the following:
- Combi boiler to feed all hot water outlets in the house by default
- Underfloor heating throughout the house, also fed by the combi. Note there will be no radiators at all in the house.
- Hot water cylinder to be heated by the combi boiler only when a switch / valve is turned on (so not on a daily basis as it happens currently with the regular boiler)
- Bathtub and second shower to have two hot water inputs, one from the combi by default, the other from the hot water tank; which one to choose to be controlled by a relevant valve in the bath and shower.
- The hot water cylinder will eventually also be heated by solar thermal water

I'm attaching a diagram of how I imagine this would look like.



Some questions I have:
- is the above setup actually feasible? Especially with regards to the combi heating the cylinder
- will I be able to use the solar thermal + cylinder to supply or preheat the water for the underfloor heating in the future?
- is it possible to control the hot water input for the bath and 2nd shower as I describe above?
- anything else I may be missing?

Many thanks in advance for your help and advice!

 

[Dan Robinson]

Why would got want a cylinder and cos water cistern full of stagnant water for must if the year. Bonkers idea of you ask me. Best fit a small Unvented and system boiler.

 

[stavros.london]

Dan, that's a very good point about the stagnant water. Could I feed a hot water cylinder from the mains to overcome this?

However, the system boiler will not give me hot water on demand, and I don't like the idea of a limited supply of water that I may or may not use, thus leading to inefficiencies (unless I am missing something else). So I'd rather stick with a combi.

 

[picasso]

Your property has too many showers/ bath for a combi to cope with, if your going to go with a "different" system I would go unvented cylinder for bath and one shower and the combi outlet on your other shower, that way you got unlimited hot water back up if you need it, modern cylinder have very little heat losses.

 

[stavros.london]

Thanks picasso. A couple of things: while there are many showers and baths they will typically be used one at a time. The only exception may be when we have guests staying over. So, the setup you propose is more or less what I had in mind with the difference of using an unvented rather than a vented cylinder; but will I be able to control when to use / heat the unvented cylinder rather than it being used daily? I.e. can I easily use it as a backup to the combi?

And once I have solar thermal heating the cylinder as well, will I be able to have it supply the underfloor heating or is there some other arrangement for solar thermal and UFH?

Many thanks

 

[picasso]

You don't want the cylinder to have standing water for long periods of time, that's why I suggested having the unvented as the main supply, you can run the ufh off the combi using an s plan and you can install a cylinder with a solar coil .

 

[muggles]

Combi boilers are a compromise between high heat output for the hot water supply, and the heating requirement for the home. You home will probably need about 18kW to heat it effectively (rough guess) so a 40kW combi is going to be massively oversized, making it inefficient. Modern unvented cylinders are extremely well insulated, very cheap to run, and can be heated in as little as 20 minutes. You can then have a much smaller boiler which will be more efficient in real-world terms (bear in mind that quoted boiler efficiencies are in "perfect scenarios" and not real-world figures).

If you want to use your solar to heat your UFH you'll need a thermal store, which adds a level of complication and cost that will quite likely outweigh any savings made by doing it. Keep it simple - twin-coil unvented cylinder, system boiler and a decent control setup to maximise efficiency. I'd recommend an Intergas boiler connected to a Joule Cyclone unvented cylinder, which can be set up to provide a high flow temperature for super-fast reheating of your cylinder, and a low flow temperature for ultra-efficient heating of your UFH