Hot Water Cylinder Position (edited: as preheater using wood stove with combi!)

[CrazyFool]

Hi guys

Just trying to plan the central heating system in my new house. I'll have a combi gas boiler.
A standard Central Heating system should look like the attached, correct me if I'm wrong.
Is it much of a problem if the hot water cylinder is in the middle of the circuit ie. after 2 rads?
I'm thinking of installing a wood stove next to the cylinder and heating that up using biomass.

Thanks in advance

CF

 

[JohnD]

No, because it isn't on the same flow circuit as the rads.

 

[CrazyFool]

No, because it isn't on the same flow circuit as the rads.

Thanks buddy. I think I understand. Would something like this work?
I would have an automated valve that bypasses the hot water cylinder if the water inside is not hot. ie. because the wood stove hasn't been fired.

 

[oldbutnotdead]

A lot of modern boilers use the return temperature to control modulation so running a preheater may give issues.
Bypassing the cylinder- you might get away with just one 2 port valve in the bypass pipework.
Woodburner etc- if you are paying for the fuel it's unlikely you'll see any ROI.
You'll also need a heatloss rad on the primary pipework from the woodburner.
Any particular reason you want a combi?- if you go for a thermal store rather than a standard cylinder you can have mains pressure hot water via plate hex or large extra coil on the store- both will deliver far more litres/min of hot than a combi.
And do check the cold water dynamic pressure and flow rate before you start playing- you want 20 plus litre/min at 2 bar dynamic for decent performance from a combi or the above setup.

 

[JohnD]

I'll have a combi gas boiler.

A combi with a cylinder?

Unusual but you can if you want.

Which of them will supply the bath taps?

 

[JohnD]

Thanks buddy. I think I understand. Would something like this work?
I would have an automated valve that bypasses the hot water cylinder if the water inside is not hot. ie. because the wood stove hasn't been fired.

No, that picture doesn't make sense. You appear to have radiator water filling the cylinder.

Please tell us why you want to invent new ways to build a heating system.

 

[oldbutnotdead]

No, that picture doesn't make sense. You appear to have radiator water filling the cylinder.

Think he wants to use cylinder as a preheater.

 

[JohnD]

Does he?

 

[oldbutnotdead]

Does he?

Its what the 2nd sketch looks like

 

[CrazyFool]

A lot of modern boilers use the return temperature to control modulation so running a preheater may give issues.
Bypassing the cylinder- you might get away with just one 2 port valve in the bypass pipework.
Woodburner etc- if you are paying for the fuel it's unlikely you'll see any ROI.
You'll also need a heatloss rad on the primary pipework from the woodburner.
Any particular reason you want a combi?- if you go for a thermal store rather than a standard cylinder you can have mains pressure hot water via plate hex or large extra coil on the store- both will deliver far more litres/min of hot than a combi.
And do check the cold water dynamic pressure and flow rate before you start playing- you want 20 plus litre/min at 2 bar dynamic for decent performance from a combi or the above setup.

A combi with a cylinder?

Unusual but you can if you want.

Which of them will supply the bath taps?

Many thanks for the replys fellas.
Apologies, I meant preheater, not a cylinder!
I don't want to send pressurised central heating water through the wood stove. So thought a tank of hot water (or something like a sand battery) would be better/safer.

Why a combi boiler... thats what I already have and its brand new.
Bath taps etc would continue to be supplied by the combi.

Agree with your comments about sending hot water down the return, if the combi panics and switches the pump off then its game over. This is all riding on the hope that the conbi will continue pumping without the flame. Will have an experiment tomorrow.

Thanks again

CF

 

[Madrab]

More than a few reasons you wouldn't want to do what your 2nd sketch alludes too. If you are serious of course.

If your CH system is sealed then you wouldn't have a standard vented copper HW cylinder as part of that sealed system as in sketch 2. It would potentially be dangerous as it's not designed to be at pressures of >2 bar or more.

You may have found out that the combi boilers pump will not run for any length of time (bar over run) without the boiler firing/running.

You also don't want the return temp to the boiler any warmer than 50odd degrees nominal, otherwise the boiler will not condense and be much less efficient.

There are more reasons but those are show stoppers to start with.

 

[flameport]

Just trying to plan the central heating system in my new house.

Why are you considering 19th and 20th century options of wood burner and a gas boiler in a new property?

 

[CrazyFool]

Thanks for all the comments guys

Why are you considering 19th and 20th century options of wood burner and a gas boiler in a new property?

19th century option because I already have a burner and an unlimited supply of wood.
20th century option because its already installed in near brand new condition.
As soon as either go kaput, I will be investing in 21st century tech

You may have found out that the combi boilers pump will not run for any length of time (bar over run) without the boiler firing/running.

A lot of modern boilers use the return temperature to control modulation so running a preheater may give issues.

Just experimented with my boiler. blasted it to 80c, once the circuit got up to temp I dialled the boiler back down to 30c. The pump continued to run, no gas was being delivered to the combi as the boiler was still at temperature. I watched the boiler for atleast half an hour before retiring the experiment. What I don't know is if the combi was operating the pump at a lower speed. I was just going off sound. The sound of the boiler and the sound of the water gushing through the rads sounded the same.

You'll also need a heatloss rad on the primary pipework from the woodburner.
Any particular reason you want a combi?- if you go for a thermal store rather than a standard cylinder you can have mains pressure hot water via plate hex or large extra coil on the store- both will deliver far more litres/min of hot than a combi.

This is actually a great idea. I was toying with the idea of some sort of thermal mass/heat battery/sand battery as opposed to a hot water tank.
It'll be technically easier to install, safer, less risk of leaks/explosions, and it can act as a heatloss rad that you describe seeing as eg. Sand can be safely heated to hundreds of degrees.
Do i just run a coil of the central heating return pipe through the thermal mass?

You also don't want the return temp to the boiler any warmer than 50odd degrees nominal, otherwise the boiler will not condense and be much less efficient.

Agree, I usually have my boiler temp set to 33c, this is enough to keep my house warm while maximising the condensing effect. However, if I'm heating my system using free wood then condensing efficiency is not a concern. If and when gas is being used (eg. Early mornings before I've loaded the wood burner), the boiler would still be set to 33c so I would still benefit from the condensing effect under these conditions.

I need some advice about a pressure relief valve. There is already one built in to the combi. I would like an additional one somewhere near the wood burner. Where is the best place to have it?

I'm also thinking about another safety mechanism as I don't want to rely on the pressure relief valve.
My underfloor heating is already on smart valves. I can set all the underfloor heating to come on if return temperatures near the boiler approach 80c. Failing that, I can get a smart hot water tap to open, that should absorb enough heat and send it down the drain.
If my safety net is being triggered too often, I could increase the size of the thermal mass.

What do you guys think?

I also need you guys to keep me on the straight and narrow as far as the regs go. I'm not well versed in that area.

Thanks again

CF

 

[JonathanM]

I don't want to send pressurised central heating water through the wood stove. So thought a tank of hot water (or something like a sand battery) would be better/safer.

Could you use a normal (water based) thermal store, but instead of having the combi connected directly to it, use one where the feed from the combi passes through a heat exchanger in the store. That way you won't have a problem with pressure in the rest of the system.

EDIT: basically inverting the setup in your diagram. The stove would heat the thermal store directly and the combi would use the heat exchanger/coil.

 

[oldbutnotdead]

Is your heating system open vent or pressurised? If pressurised it is unwise to use a standard cylinder as in your sketch- the coil will be fine running at 2 or 3 bar, the cylinder itself is unlikely to be rated for that sort of pressure. Most thermal stores run open vent - much cheaper to manufacture
Keep the woodburner side open vent- you can run woodburners pressurised but you need a lot of control gear to keep it safe.
Your preheat idea is actually rather good- it means the energy in low temperature (40° or less) water left from the previous nights' burn is used and you don't have the overhead of heating the cylinder from cold with the gas boiler.
Downside is you will always be using gas for dhw, if you have unlimited timber for the woodburner and the time to stoke the thing it'll be worth looking at alternatives for dhw. As far as i know some combis tolerate preheated water coming into the dhw side but not all.
Sand or other dry mass heat stores- no idea, YouTube might be your friend there.
You should not rely on smart anything to dump excess heat- a wet system including an uncontrolled heat source (woodburner) must be designed so that it can operate safely under conditions of electrical supply failure whilst running at full thermal output.
You'll need an overtemperature automatic valve on the flow from the woodburner- the output from this valve (which will discharge at 95 °C) needs to be safely disposed of (no plastic pipe). If this valve operates, the volume of near boiling water expelled will be replaced by cooler water from the f & e tank.
By the way, any pipework, tankage, valves connected to the woodburner must be metal (copper usually). F & E tank must be metal or non-thermosetting plastic (fibreglass works, standard pvc tanks don't). Ballvalve and float for filling tank must be metal (brass usually).
One additional problem with your preheat sketch- if the cylinder temp does get above 90° then your boiler will be exposed to temperatures it is not designed to deal with. Mind, if the power is off the combi pump won't be running.....
There's a lot going on