Refurb - setting up for heat pump in future?

[-crashd]

Hi all, 

 

As per my post here : 

 we're currently in the process of planning an extension/loft conversion/part-reno of a bungalow in Twickenham. We're keen to look at a heat pump as our heating solution but aware of some of the requirements (around insulation etc..) and some of the potential downfalls, including initial outlay. The house currently has a relatively new gas combi boiler.

 

One of my thoughts is to do the initial work to prepare for a heat pump - wet underfloor on the ground floor and appropriate piping and radiators on the upstairs - but retain the gas boiler setup for now to alleviate some of the financial investment, and in the hope that pump costs and efficiency may improve.

 

Is this a stupid idea, and if not, what else should we consider?

[JohnMo]

Some of the pipe diameters will be larger for a HP, not sure why but have seen it mentioned quite a few times.  Someone should be able to advise.

 

Your cylinder will need a larger coil for the HP.

 

You don't say what boiler you have currently, but it may be able to run weather compensation, if so with the HP set up for the rest of the system, you may be able to run in WC mode at heat pump flow temperature, to save on gas costs.

 

other things to consider

Plenty of insulation in the ground floor to limit downwards heat loss from UFH

 

Draft proofing, airtightness.

 

Look at a condition based ventilation system or MVHR.

https://www.aereco.co.uk/knowledge-centre/how-to-choose-between-a-humidity-controlled-mev-and-mvhr/

 

[SteamyTea]

Really just a case of designing for a properly sized heat pump, then connect it up via a buffer tank.

The buffer may need changing, but they are cheap items.

[billt]

5 hours ago, JohnMo said:

Some of the pipe diameters will be larger for a HP, not sure why but have seen it mentioned quite a few times.

 

It's because you want as low a temperature drop across the emitters as possible to keep the mean emitter temperature up, unlike a condensing boiler system where you want to control the flow so there's 20C or so drop across the emitters to keep the boiler return temperature in the condensing range. With a high flow temperature the emitters will still have a reasonably high output.

 

John Cantor's web site has a very useful simulator which lets you fiddle with flow rates and radiator sizings etc to get an idea of optimum radiator sizes and flow rates. https://heatpumps.co.uk/heating-simulator-for-radiators/

 

  

6 hours ago, -crashd said:

One of my thoughts is to do the initial work to prepare for a heat pump - wet underfloor on the ground floor and appropriate piping and radiators on the upstairs - but retain the gas boiler setup for now to alleviate some of the financial investment, and in the hope that pump costs and efficiency may improve.

 

Is this a stupid idea, and if not, what else should we consider?

 

It seems sensible to me; that's all disruptive work so it makes sense to do it when you're doing other disruptive work. installing the heat pump itself should be fairly straightforward and can be done at a later stage.

[-crashd]

7 hours ago, JohnMo said:

You don't say what boiler you have currently, but it may be able to run weather compensation, if so with the HP set up for the rest of the system, you may be able to run in WC mode at heat pump flow temperature, to save on gas costs.

 

@JohnMoThanks for the thoughts - there's an unvented Vaillant ecoTEC Plus 415  (https://www.vaillant.co.uk/for-installers/products/ecotec-plus-412-415-418-424-430-and-435-3328.html) and a hot water tank. I don't really know enough about heating and hot water options to know if we'd need to replace the boiler for this setup initially?

[Marvin]

Hi @-crashd

 

Not a stupid Idea.👍

 

In my humble opinion:

 

1. Decide the size and makeup of the finished building. Work out the thermal resistance of the finished building through all the types of wall floor and roof in the building (Don't panic there's help about to guide you). Decide what the lowest outside temperature you want the ASHP to work against and this will give you one part of the heating requirement.
2. Decide how you are going to refresh the air in the building and use calculations to work out the heat loss.  Use the same outside temperature that you use for heating to calculate the heat loss through replacement air to the building ( a MVHR system can save you 80%- 90% of the heat loss which is a big amount of energy!)    This will give you another part of the heating requirement.
3. Add to this the water heating requirement and you have the size of the output you require from an ASHP. (after this I added 25% for the reality of not everything being perfect)
4. These calculations will give you the whole house heating need depending on the outside temperature.

Radiator and UFH pipe calculations:

1.   Split the heating into areas or rooms depending on the design. Identify how much heat the under floor heating (UFH) or radiator (often referred to as emitters) needs to give off in the individual areas then use Delta T30 calculations to work out how much pipe you need for under floor heating and how big you need radiators. (Delta T30 on radiators is the information radiator manufactures supply to say how much heat you will get off one of their radiators if you use lower temperature water like when using an ASHP. A boiler would typically use Delta T50 information.) So in summary because your going to use lower temperature water, the radiators need to be bigger to supply the same amount of heat as a smaller radiator using higher temperature water.  

You now know what you need, apart from hot water tank size, a buffer tank, manifolds for UFH, and controls. The next thing is siting the ASHP outside and the siting of tanks and manifolds inside. If you look at the installation instructions of an ASHP you will see the site requirements. If you read the Planning Permission requirements you will see where you can install an ASHP. When you decide the position you can then consider the connections required:

• one flow and one return pipe usually 28mm.
• Electric power supply
• Temperature sensor cable between buffer tank and ASHP
• Temperature sensor cable between hot water tank and ASHP
• Communication cable between control system in building somewhere and the ASHP outside
• A way to deal with the condensation water produced by the ASHP
• Anything else I have forgotten which I'm sure someone will point out.

 

• The pipes between the ASHP and the hot water tank and or buffer tank will need to be 28mm flow and return. Main thing is to work out where you would put the ASHP and where the pipe would have to connect to, choose the route and prepare the hole through the thermal envelope of the building (outside wall in your case, I would guess) making an opening that will take two 100mm diameter pipes (this is to allow 28mm pipes with insulation). Fill the holes with insulation until required. Don't forget holes for the cables...

 

Check you will have enough power supply. Ours is only a small one.  The ASHP will require its own fuse which, depending, will need to be an MCB, RCB or RCBO. See ASHP instructions. The electric supply cable will have a bigger cross sectional area than you expect, and that will be because the voltage must not drop too much. See ASHP instructions.

 

The temperature sensor cables and the control cables need to be long enough to reach between the ASHP and tanks.

 

Condensation is not a big issue, but you don't want to end up with puddles! Our is on the flat roof to collected by the gutter.

 

I would suggest the following when considering the installation:

1.  The shorter the length of pipework from the ASHP to the inside of the thermal envelope the better the COP.
2. The greater the insulation to the external pipework the better the COP.
3. The shorter the pipework form the ASHP to the tank/tanks the better the COP.
4. The lower the set temperature for heating and hot water the better the COP.
5. The greater the insulation to the 28mm pipework inside the building the better the COP.
6. Ensure any hot water tanks or buffer tanks are inside the thermal envelope to ensure the best COP.

 

 Good luck

 

M

 

[TonyT]

If you can returb to have the boiler, cylinder on the ground floor and create a small plant room/utility cupboard, it will be easier to modify pipework/cabling in future.

 

plus one to all the other sensible suggestions!